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We'll look at how shell script files should be structured and how to use 'shebangs' to define how a shell script will run.",source:"@site/docs/04-shell-scripting/18-shell-script-essentials/index.md",sourceDirName:"04-shell-scripting/18-shell-script-essentials",slug:"/part-3-manipulating-text/shell-script-essentials/",permalink:"/part-3-manipulating-text/shell-script-essentials/",draft:!1,editUrl:"https://github.com/dwmkerr/effective-shell/edit/main/docs/04-shell-scripting/18-shell-script-essentials/index.md",tags:[],version:"current",frontMatter:{title:"Shell Script Essentials",slug:"/part-3-manipulating-text/shell-script-essentials/"},sidebar:"sidebar",previous:{title:"Part 4 - Shell Scripting",permalink:"/part-4-shell-scripting/"},next:{title:"Variables, Reading Input, and Mathematics",permalink:"/part-3-manipulating-text/variables-reading-input-and-mathematics/"}},r={},h=[{value:"What is a Shell Script?",id:"what-is-a-shell-script",level:2},{value:"Creating a Basic Shell Script",id:"creating-a-basic-shell-script",level:2},{value:"The 'common' Command",id:"the-common-command",level:3},{value:"Creating a Simple Script",id:"creating-a-simple-script",level:3},{value:"Commentsindex",id:"commentsindex",level:2},{value:"Building and Testing the Script",id:"building-and-testing-the-script",level:2},{value:"Multi-line Commands",id:"multi-line-commands",level:2},{value:"Running a Shell Script",id:"running-a-shell-script",level:2},{value:"Using Shebangs",id:"using-shebangs",level:2},{value:"Shebangs - Dealing with Paths",id:"shebangs---dealing-with-paths",level:3},{value:"Sourcing Shell Scripts",id:"sourcing-shell-scripts",level:2},{value:"Dot Sourcingindex",id:"dot-sourcingindex",level:3},{value:"Installing Your Script",id:"installing-your-script",level:2},{value:"Summary",id:"summary",level:2},{value:"Appendix - How the Script Works",id:"appendix---how-the-script-works",level:3}],p={toc:h};function c(e){let{components:t,...n}=e;return(0,i.kt)("wrapper",(0,a.Z)({},p,n,{components:t,mdxType:"MDXLayout"}),(0,i.kt)("p",null,"In this chapter we're going to look at how to write shell scripts and the different ways we can execute them. We'll look at how shell script files should be structured and how to use 'shebangs' to define how a shell script will run."),(0,i.kt)("p",null,"We'll learn the essential techniques that will help you build your own scripts. Even if you are familiar with shell scripts I would suggest skimming this chapter to make sure you understand each of the concepts, particularly the later section where we talk about the ",(0,i.kt)("inlineCode",{parentName:"p"},"env")," command in shebangs."),(0,i.kt)("h2",{id:"what-is-a-shell-script"},"What is a Shell Script?"),(0,i.kt)("p",null,"A shell script is just a text file which contains a set of commands. As soon as you find yourself repeating the same sequence of commands in a shell, it might be worth saving these commands to a file and running the file instead."),(0,i.kt)("p",null,"Saving your commands to a file has a number of benefits. It saves time - you don't need to type the commands out each time you want to run them! You can use your favourite editor to build the script file, and you can add 'comments' to describe what you are trying to achieve (which will make it far easier to update the script over time). Files can also easily be shared - meaning you can copy these scripts to other machines or share them with others who might find them useful."),(0,i.kt)("h2",{id:"creating-a-basic-shell-script"},"Creating a Basic Shell Script"),(0,i.kt)("p",null,"Let's create a simple shell script that shows us our most commonly used shell commands."),(0,i.kt)("p",null,"Almost every command that is needed to build the script has been discussed in the book already, so it shouldn't be too unfamiliar. But I'll still break it down blow-by-blow to help us understand it."),(0,i.kt)("p",null,"As we go through this section of the book, we're going to extend this script and make it more useful!"),(0,i.kt)("h3",{id:"the-common-command"},"The 'common' Command"),(0,i.kt)("p",null,"We're going to create a new command, called 'common', that shows the most commonly used shell commands."),(0,i.kt)("p",null,"We should be able to do this using techniques we've seen so far. We'll do it like this:"),(0,i.kt)("ol",null,(0,i.kt)("li",{parentName:"ol"},"Read a large number of commands from the history"),(0,i.kt)("li",{parentName:"ol"},"Sort the commands, then count the number of duplicates"),(0,i.kt)("li",{parentName:"ol"},"Sort this list showing the most commonly run commands first"),(0,i.kt)("li",{parentName:"ol"},"Print the results to the screen.")),(0,i.kt)("p",null,"Let's get started!"),(0,i.kt)("h3",{id:"creating-a-simple-script"},"Creating a Simple Script"),(0,i.kt)("p",null,"It's going to take some trial and error to get our commands right. So let's start by creating a shell script, which we'll run again and again."),(0,i.kt)("p",null,"In your favourite editor, create a file called ",(0,i.kt)("inlineCode",{parentName:"p"},"common.v1.sh")," and put it somewhere on your system. As an example, I'm going to create a folder called ",(0,i.kt)("inlineCode",{parentName:"p"},"scripts")," in my home directory and create the ",(0,i.kt)("inlineCode",{parentName:"p"},"common.v1.sh")," file there:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"# Create a directory called 'scripts'.\n# Using the '-p' flag means we won't get an error if the folder exists.\nmkdir -p ~/scripts\n\n# Create the script file.\ntouch ~/scripts/common.v1.sh\n\n# Open the script file in my favourite editor.\nvi ~/scripts.sh\n")),(0,i.kt)("p",null,"I have called the script ",(0,i.kt)("inlineCode",{parentName:"p"},"common.v1.sh")," rather than ",(0,i.kt)("inlineCode",{parentName:"p"},"common.sh")," because in each chapter of this section we are going to improve upon the script and change the version number. So in later chapters we will create ",(0,i.kt)("inlineCode",{parentName:"p"},"common.v2.sh"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"common.v3.sh")," and so on."),(0,i.kt)("p",null,"These commands should be familiar. The ",(0,i.kt)("inlineCode",{parentName:"p"},"mkdir")," command creates a directory. The ",(0,i.kt)("inlineCode",{parentName:"p"},"-p")," (create parent directories if needed) flag stops the command from returning an error if the directory already exists."),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"touch")," command creates an empty file with the given name. Finally, I open the file in an editor. I am using Vim, but you can open this file in any editor you like. If you would like to see how to use Vim you can also jump to ",(0,i.kt)("a",{parentName:"p",href:"/part-6-advanced-techniques/a-vim-crash-course/"},"Chapter 32 - A Vim Crash Course"),"."),(0,i.kt)("p",null,"Before we run the script, let's quickly talk about ",(0,i.kt)("em",{parentName:"p"},"comments"),"."),(0,i.kt)("h2",{id:"commentsindex"},"Comments"),(0,i.kt)("p",null,"Comments are lines of text that you add to a script or program to help the reader understand what is going on. Comments are not interpreted by the shell \u2013 they are purely for the benefit of the reader."),(0,i.kt)("p",null,"Any text that follows a ",(0,i.kt)("inlineCode",{parentName:"p"},"#")," hash symbol is treated as a comment. Whether this is text you type into a shell, or text in a shell script, the shell will ignore anything after the hash and not try to interpret it."),(0,i.kt)("p",null,"Using comments effectively can be a huge time-saver \u2013 it is amazing how quickly you can forget what a certain piece of code means, or why you solved a problem in a particular way."),(0,i.kt)("p",null,"Let\u2019s look at some examples of comments."),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},'# This is a comment - we can use this to describe what we\'re trying to do.\n\necho "Hello Shell" # Comments can go at the end of a line...\n\n# You can also use a comment symbol to \'comment out\' a line:\n# echo "Goodbye Shell"\n')),(0,i.kt)("p",null,"There are three comments in this sample. The first comment takes up a whole line, the second comment is at the end of a line to add some explanation, and the third comment is some 'commented out' code \u2013 we've just put a hash in front of some commands so that they will not be executed."),(0,i.kt)("p",null,"From this point on we'll use comments a lot to explain what we are trying to accomplish with each section of a script. It is generally good practice to use comments to describe your ",(0,i.kt)("em",{parentName:"p"},"intent")," - why you are doing something. This is far more useful for the reader than ",(0,i.kt)("em",{parentName:"p"},"what")," you are doing. The 'what' should be clear from the commands - the 'why' is the thing readers will likely want to understand."),(0,i.kt)("p",null,"Here's an example of a bad comment:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"# Write the CSV file, reverse it, cut it, reverse it.\ncat ~/effective-shell/data/top100.csv | rev | cut -d',' -f1 | rev\n")),(0,i.kt)("p",null,"The comment just describes what the script is doing. But it doesn't explain ",(0,i.kt)("em",{parentName:"p"},"why"),". A better comment would be:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"# Get the last field of each line in the csv file.\ncat ~/effective-shell/data/top100.csv | rev | cut -d',' -f1 | rev\n")),(0,i.kt)("p",null,"If you ",(0,i.kt)("em",{parentName:"p"},"don't")," come from a programming background, you might think that many of these comments are a little obvious. But as you write more and more code, you'll realise that something that seemed obvious when you wrote it a while ago can look surprisingly baffling even just a few days later!"),(0,i.kt)("h2",{id:"building-and-testing-the-script"},"Building and Testing the Script"),(0,i.kt)("p",null,"Add the following commands to the ",(0,i.kt)("inlineCode",{parentName:"p"},"common.v1.sh")," file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"# Write the title of our command.\necho \"common commands:\"\n\n# Show the most commonly used commands.\ntail ~/.bash_history -n 1000 | sort | uniq -c | sed 's/^ *//' | sort -n -r | head -n 10\n")),(0,i.kt)("p",null,"This is a short script, but there is quite a lot going on. Let's look at it blow-by-blow:"),(0,i.kt)("ol",null,(0,i.kt)("li",{parentName:"ol"},"First we take the last 1000 lines of the ",(0,i.kt)("inlineCode",{parentName:"li"},"~/.bash_history")," file using the ",(0,i.kt)("inlineCode",{parentName:"li"},"tail")," command",(0,i.kt)("sup",{parentName:"li",id:"fnref-1"},(0,i.kt)("a",{parentName:"sup",href:"#fn-1",className:"footnote-ref"},"1"))),(0,i.kt)("li",{parentName:"ol"},"Then we sort the commands. This will put all of the duplicates next to each other"),(0,i.kt)("li",{parentName:"ol"},"Then we remove all duplicates and use the ",(0,i.kt)("inlineCode",{parentName:"li"},"-c")," (",(0,i.kt)("em",{parentName:"li"},"show count"),") flag to count the duplicates"),(0,i.kt)("li",{parentName:"ol"},"Then we remove the leading spaces from the output (which we need to do so that we can sort properly)"),(0,i.kt)("li",{parentName:"ol"},"Then we sort ",(0,i.kt)("em",{parentName:"li"},"numerically")," and in reverse - the highest count first"),(0,i.kt)("li",{parentName:"ol"},"Finally, we limit the results to the first ten items")),(0,i.kt)("p",null,"If you need a refresher on the shell history, ",(0,i.kt)("inlineCode",{parentName:"p"},"sort")," or ",(0,i.kt)("inlineCode",{parentName:"p"},"uniq")," the check the ",(0,i.kt)("a",{parentName:"p",href:"/part-3-manipulating-text/slice-and-dice-text/"},"Slice and Dice Text")," chapter. If the ",(0,i.kt)("inlineCode",{parentName:"p"},"sed")," command doesn't look familiar then check the ",(0,i.kt)("a",{parentName:"p",href:"/part-3-manipulating-text/advanced-text-manipulation/"},"Advanced Text Manipulation with Sed")," chapter."),(0,i.kt)("p",null,"If you want to see a more detailed breakdown of how the script works, check ",(0,i.kt)("a",{parentName:"p",href:"#appendix-how-the-script-works"},"Appendix - How the Script Works"),". But this is not necessary for you to follow the content in this chapter."),(0,i.kt)("p",null,"Now save the file. In your shell, run the following command to execute the file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"sh ~/scripts/common.v1.sh\n")),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"sh")," (shell) command starts a new shell. When we pass the path of a shell script, the shell command will run the script and then exit. The output you see will look something like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"common commands:\n463 vi\n267 gc\n238 ga .\n212 ls\n169 gpo\n122 make dev\n112 gl\n97 gcm\n96 gpr\n")),(0,i.kt)("p",null,"You can see my most common commands are short aliases for Git commands (the ones that start with 'g'), opening Vim, running a makefile command and a few others."),(0,i.kt)("p",null,"We now have a basic shell script. Let's look at a few different ways we can run the script."),(0,i.kt)("h2",{id:"multi-line-commands"},"Multi-line Commands"),(0,i.kt)("p",null,"You can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"\\")," backslash character to create a 'continuation' that tells the shell it needs to join lines up. This allows you to break long commands into multiple lines."),(0,i.kt)("p",null,"As an example, we could re-write our pipeline command to look like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"# Show the most commonly used commands.\ntail ~/.bash_history -n 1000 \\\n | sort \\\n | uniq -c \\\n | sed 's/^ *//' \\\n | sort -n -r \\\n | head -n 10\n")),(0,i.kt)("p",null,"This will probably look very familiar to anyone with a background in functional programming!"),(0,i.kt)("p",null,"Be careful when you split lines up - the continuation character ",(0,i.kt)("em",{parentName:"p"},"must")," be the last character on the line. If you add something after it (such as a comment) then the command will fail."),(0,i.kt)("h2",{id:"running-a-shell-script"},"Running a Shell Script"),(0,i.kt)("p",null,"There are a few different ways we can run shell scripts."),(0,i.kt)("p",null,"The first is to run a shell program and pass the script as a parameter. This is what we did in the earlier example. Here's another example of how we could run the script we created:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"bash ~/scripts/common.v1.sh\n")),(0,i.kt)("p",null,"This is a perfectly valid technique. Now let's see the other ways we can run a script."),(0,i.kt)("p",null,"The next way we can run a script is to make it 'executable'. This means we change the file permissions of the script file, adding the 'executable bit'. This tells the systems we can run the file. We use the ",(0,i.kt)("inlineCode",{parentName:"p"},"chmod")," (",(0,i.kt)("em",{parentName:"p"},"change file mode"),") command to do this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"chmod +x ~/scripts/common.v1.sh\n")),(0,i.kt)("p",null,"If the ",(0,i.kt)("inlineCode",{parentName:"p"},"chmod")," command looks unfamiliar then check the ",(0,i.kt)("a",{parentName:"p",href:"/part-2-core-skills/understanding-commands"},"Understanding Commands")," chapter. Now that the file has been made executable, we can simply enter the path to the file and run it, as if it was any other command:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"~/scripts/common.v1.sh\n")),(0,i.kt)("p",null,"There is a problem with this approach though. How this file is executed is going to vary depending on how your system is set up",(0,i.kt)("sup",{parentName:"p",id:"fnref-2"},(0,i.kt)("a",{parentName:"sup",href:"#fn-2",className:"footnote-ref"},"2")),". For example, if you are using Bash, then the script will run in a new instance of the Bash shell. However, if you are using the Z shell, then the script will most likely run in the ",(0,i.kt)("inlineCode",{parentName:"p"},"sh")," program (and depending on your system, this program might just be a link to ",(0,i.kt)("em",{parentName:"p"},"another")," type of shell)."),(0,i.kt)("p",null,"We want to avoid any ambiguity and be explicit about ",(0,i.kt)("em",{parentName:"p"},"what")," program should run our script. We can do this using a special construct called a ",(0,i.kt)("em",{parentName:"p"},"shebang"),"."),(0,i.kt)("h2",{id:"using-shebangs"},"Using Shebangs"),(0,i.kt)("p",null,"A ",(0,i.kt)("em",{parentName:"p"},"shebang")," is a special set of symbols at the beginning of a file that tells the system what program should be used to run the file."),(0,i.kt)("p",null,"If we were to add a shebang to our ",(0,i.kt)("inlineCode",{parentName:"p"},"common.v1.sh")," file, it would look like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"#!/usr/bin/sh\n\n# Write the title of our command.\necho \"common commands:\"\n\n# Show the most commonly used commands.\ntail ~/.bash_history -n 1000 | sort | uniq -c | sed 's/^ *//' | sort -n -r | head -n 10\n")),(0,i.kt)("p",null,"The shebang is the two characters - ",(0,i.kt)("inlineCode",{parentName:"p"},"#!"),". The name 'shebang' comes from the names of the symbols. The first symbol is a 'sharp' symbol (sometimes it is called a hash, it depends a little on context). The second symbol is an exclamation point. In programming the exclamation point is sometimes called the 'bang' symbol. When we put the two together, we get 'sharp bang', which is shortened to 'shebang'."),(0,i.kt)("p",null,"Immediately after the shebang you write the full path to the program which should be used to open the file."),(0,i.kt)("p",null,"For example, if you wanted to write a script that is run in Python, you could do this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-python"},"#!/usr/bin/python3\n\nprint('Hello from Python')\n")),(0,i.kt)("p",null,"If we wanted to explicitly use the Bash shell to run a script, we might use a shebang like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},'#!/usr/bin/bash\n\necho "Hello from Bash"\n')),(0,i.kt)("p",null,"What about Node.js? Easy!"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-js"},'#!/usr/bin/node\n\nconsole.log("Hello from Node.js");\n')),(0,i.kt)("h3",{id:"shebangs---dealing-with-paths"},"Shebangs - Dealing with Paths"),(0,i.kt)("p",null,"When we use a shebang we need to provide the full path to the executable that runs the script."),(0,i.kt)("p",null,"For example, if we want to use Ruby to run a script we could write a script like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-ruby"},"#!/usr/bin/ruby\n\nputs 'Hello from Ruby'\n")),(0,i.kt)("p",null,"But there is a problem here. This will only work if you have the Ruby program installed in the location specified after the shebang (i.e. ",(0,i.kt)("inlineCode",{parentName:"p"},"/usr/bin/ruby"),"). If you do not have the Ruby program in this location the script will fail to run."),(0,i.kt)("p",null,"How can we know where a specific program is installed?"),(0,i.kt)("p",null,"There is a common trick for dealing with this issue. We can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"env")," (",(0,i.kt)("em",{parentName:"p"},"set environment and execute command"),") command to run a command and it will work out the path for us."),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"env")," command is often used to show environment variables, but you can also use it to execute an arbitrary command (often with a modified environment). One handy feature of the ",(0,i.kt)("inlineCode",{parentName:"p"},"env")," command is that it looks through the ",(0,i.kt)("inlineCode",{parentName:"p"},"$PATH")," variable to find the path of the command to execute."),(0,i.kt)("p",null,"You can see this by running a command like the below:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ env python3\nPython 3.8.5 (default, Jan 27 2021, 15:41:15)\n[GCC 9.3.0] on linux\nType "help", "copyright", "credits" or "license" for more information.\n>>>\n')),(0,i.kt)("p",null,"We've used the ",(0,i.kt)("inlineCode",{parentName:"p"},"env")," command to run the ",(0,i.kt)("inlineCode",{parentName:"p"},"python3")," command - and it worked out the correct path for us."),(0,i.kt)("p",null,"To use ",(0,i.kt)("inlineCode",{parentName:"p"},"env")," in a shebang, specify the full path to ",(0,i.kt)("inlineCode",{parentName:"p"},"env")," (which should be the same on all Unix-like systems) and then provide the name of the command to run:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'#!/usr/bin/env bash\n\necho "Hello from Bash"\n')),(0,i.kt)("p",null,"Or another example:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"#!/usr/bin/env ruby\n\nputs 'Hello from Ruby'\n")),(0,i.kt)("p",null,"Using a shebang to specify the exact command to run, and then using the ",(0,i.kt)("inlineCode",{parentName:"p"},"env")," command to allow the ",(0,i.kt)("inlineCode",{parentName:"p"},"$PATH")," to be searched is generally the safest and most portable way to specify how a shell script should run."),(0,i.kt)("h2",{id:"sourcing-shell-scripts"},"Sourcing Shell Scripts"),(0,i.kt)("p",null,"We have discussed how to ",(0,i.kt)("em",{parentName:"p"},"run")," shell scripts. You can also use the ",(0,i.kt)("em",{parentName:"p"},"source")," (",(0,i.kt)("em",{parentName:"p"},"execute commands from a file"),") command"," to load the contents of a file into the ",(0,i.kt)("em",{parentName:"p"},"current")," shell."),(0,i.kt)("p",null,"Remember that when we run a shell script, a new shell is created as a child process of the current shell. This means that if you change something in the environment, such as a variable, it will not affect the environment of the shell that ran the script."),(0,i.kt)("p",null,"Let's see an example. We'll create a script called ",(0,i.kt)("em",{parentName:"p"},"set_editor.sh")," that sets the ",(0,i.kt)("inlineCode",{parentName:"p"},"EDITOR")," environment variable to ",(0,i.kt)("inlineCode",{parentName:"p"},"nano"),". The script's contents are below (can also find it in the samples at ",(0,i.kt)("em",{parentName:"p"},"~/effective-shell/scripts/set_editor.sh"),"):"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},'EDITOR=nano\necho "Editor changed to: $EDITOR"\n')),(0,i.kt)("p",null,"Let's run this script and see what editor looks like before and after:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ echo $EDITOR\nvim\n$ ~/effective-shell/scripts/set_editor.sh\nEditor changed to: nano\n$ echo $EDITOR\nvim\n")),(0,i.kt)("p",null,"Notice that although we changed the ",(0,i.kt)("inlineCode",{parentName:"p"},"EDITOR")," environment variable in our script, the change has not persisted in the current shell. This is because each shell (and in fact, each process) gets its own ",(0,i.kt)("em",{parentName:"p"},"copy")," of the environment."),(0,i.kt)("p",null,"If we want to run the commands in the file in the context of the current shell, we can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"source")," command to load the file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ echo $EDITOR\nvim\n$ source ~/effective-shell/scripts/set_editor.sh\nEditor changed to: nano\n$ echo $EDITOR\nnano\n")),(0,i.kt)("p",null,"Our existing environment has been changed. When we use ",(0,i.kt)("inlineCode",{parentName:"p"},"source"),", the commands in the file are executed in the current shell, rather than in a new shell."),(0,i.kt)("p",null,"We can see this even more clearly if we use the ",(0,i.kt)("em",{parentName:"p"},"showpstree.sh")," file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ ~/effective-shell/scripts/show-info.sh\nbash\n \u2514\u2500sh /home/ubuntu/effective-shell/scripts/showpstree.sh\n \u2514\u2500pstree -l -a -s 2240\n")),(0,i.kt)("p",null,"This script shows the current 'process tree', using the ",(0,i.kt)("inlineCode",{parentName:"p"},"pstree")," (",(0,i.kt)("em",{parentName:"p"},"show process tree"),") command. We can see that the ",(0,i.kt)("inlineCode",{parentName:"p"},"pstree")," command was run as a child of the ",(0,i.kt)("inlineCode",{parentName:"p"},"sh")," program. This program was run with the script path, by the shell I was using, ",(0,i.kt)("inlineCode",{parentName:"p"},"bash"),". This is a nice visualisation of what is going on - our ",(0,i.kt)("inlineCode",{parentName:"p"},"bash")," shell has run the ",(0,i.kt)("em",{parentName:"p"},"showpstree.sh")," script in a child shell."),(0,i.kt)("p",null,"If we source the same file, we'll see that we do ",(0,i.kt)("em",{parentName:"p"},"not")," create a new shell:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ source ~/effective-shell/scripts/show-info.sh\nbash\n \u2514\u2500pstree -l -a -s 2169\n")),(0,i.kt)("h3",{id:"dot-sourcingindex"},"Dot Sourcing"),(0,i.kt)("p",null,"There is a slightly more concise syntax to source a script - the ",(0,i.kt)("em",{parentName:"p"},"dot sourcing")," notation. When the shell sees a ",(0,i.kt)("inlineCode",{parentName:"p"},".")," dot character, it will source the file that follows:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ . ~/effective-shell/scripts/show-info.sh\nbash\n \u2514\u2500pstree -l -a -s 2169\n")),(0,i.kt)("p",null,"You may encounter this syntax as you look at things like shell configuration files, which we will discuss in ",(0,i.kt)("a",{parentName:"p",href:"../../part-5-building-your-toolkit/configuring-the-shell"},"Chapter 24 - Configuring the Shell"),"."),(0,i.kt)("h2",{id:"installing-your-script"},"Installing Your Script"),(0,i.kt)("p",null,"Before we finish with our shell script fundamentals, we'll take a look at one final commonly used pattern to run shell scripts - installing them as a local binary."),(0,i.kt)("p",null,"Our ",(0,i.kt)("inlineCode",{parentName:"p"},"common.v1.sh")," script (with the added shebang) looks like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"#!/usr/bin/env sh\n\n# Write the title of our command.\necho \"common commands:\"\n\n# Show the most commonly used commands.\ntail ~/.bash_history -n 1000 | sort | uniq -c | sed 's/^ *//' | sort -n -r | head -n 10\n")),(0,i.kt)("p",null,"If we have made the script executable with the ",(0,i.kt)("inlineCode",{parentName:"p"},"chmod")," command, then we can run the script by simply typing the location of the script in the shell:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ ~/scripts/common.v1.sh\ncommon commands:\n97 gcm\n96 gpr\n...\n")),(0,i.kt)("p",null,"If we want to 'install' this script as a local command which we can run easily, we can create a ",(0,i.kt)("em",{parentName:"p"},"symbolic link")," to the shell script in our ",(0,i.kt)("inlineCode",{parentName:"p"},"/usr/local/bin")," folder:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"ln -s ~/scripts/common.v1.sh /usr/local/bin/common\n")),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"ln")," (",(0,i.kt)("em",{parentName:"p"},"create link"),") command creates a link (which is like a shortcut in Windows and other desktop systems) in our ",(0,i.kt)("inlineCode",{parentName:"p"},"/usr/local/bin")," folder, with the name ",(0,i.kt)("inlineCode",{parentName:"p"},"common"),", which points to the script we have written. We can now run the ",(0,i.kt)("inlineCode",{parentName:"p"},"common")," command without specifying its path:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"$ common\ncommon commands:\n97 gcm\n96 gpr\n...\n")),(0,i.kt)("p",null,"This works because when the shell sees a command, it searches through the folders in the $PATH environment variable to find out where the command is. And the ",(0,i.kt)("inlineCode",{parentName:"p"},"/usr/local/bin")," folder is in this list of paths."),(0,i.kt)("p",null,"Why do we use the ",(0,i.kt)("inlineCode",{parentName:"p"},"/usr/local/bin")," folder rather than the ",(0,i.kt)("inlineCode",{parentName:"p"},"/usr/bin")," folder? This is just a convention. In general, the ",(0,i.kt)("inlineCode",{parentName:"p"},"/usr/bin")," folder is for commands which are installed with package manager tools like ",(0,i.kt)("inlineCode",{parentName:"p"},"apt")," or Homebrew (on MacOS). The ",(0,i.kt)("inlineCode",{parentName:"p"},"/usr/local/bin")," folder is used for commands which you create for yourself on your local machine and manage yourself",(0,i.kt)("sup",{parentName:"p",id:"fnref-3"},(0,i.kt)("a",{parentName:"sup",href:"#fn-3",className:"footnote-ref"},"3")),"."),(0,i.kt)("h2",{id:"summary"},"Summary"),(0,i.kt)("p",null,"In this chapter we've covered quite a few of the fundamentals of shell scripts:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},"How to create a shell script"),(0,i.kt)("li",{parentName:"ul"},"How comments work in shell scripts"),(0,i.kt)("li",{parentName:"ul"},"How to handle long lines with continuations"),(0,i.kt)("li",{parentName:"ul"},"How to run a shell script"),(0,i.kt)("li",{parentName:"ul"},"How to make a shell script executable"),(0,i.kt)("li",{parentName:"ul"},"How shebangs work"),(0,i.kt)("li",{parentName:"ul"},"How to use the ",(0,i.kt)("inlineCode",{parentName:"li"},"env")," command to make our shebangs more portable"),(0,i.kt)("li",{parentName:"ul"},"How to 'install' scripts for the current user")),(0,i.kt)("p",null,"In the next chapter we'll look at how to add logic to our shell scripts."),(0,i.kt)("hr",null),(0,i.kt)("h3",{id:"appendix---how-the-script-works"},"Appendix - How the Script Works"),(0,i.kt)("p",null,"This section briefly covers how the ",(0,i.kt)("inlineCode",{parentName:"p"},"common.v1.sh")," script works. Assuming we have a history that looks like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"vi README.md\ngit status\ngit checkout main\ngit status\nrestart-shell\ngit status\nopen .\nvi README.md\nopen .\n")),(0,i.kt)("p",null,"First we sort, putting duplicate lines next to each other:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"git checkout main\ngit status\ngit status\ngit status\nopen .\nopen .\nrestart-shell\nvi README.md\nvi README.md\n")),(0,i.kt)("p",null,"Then we use ",(0,i.kt)("inlineCode",{parentName:"p"},"uniq")," to remove duplicate adjacent lines, passing the ",(0,i.kt)("inlineCode",{parentName:"p"},"-c")," flag to include a count:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"}," 1 git checkout main\n 3 git status\n 2 open .\n 1 restart-shell\n 2 vi README.md\n")),(0,i.kt)("p",null,"Now we remove the leading whitespace:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"1 git checkout main\n3 git status\n2 open .\n1 restart-shell\n2 vi README.md\n")),(0,i.kt)("p",null,"Finally we sort numerically (by using the ",(0,i.kt)("inlineCode",{parentName:"p"},"-n")," flag) and in descending order (by using the ",(0,i.kt)("inlineCode",{parentName:"p"},"-r")," flag):"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"3 git status\n2 vi README.md\n2 open .\n1 restart-shell\n1 git checkout main\n")),(0,i.kt)("p",null,"Why the numeric sort? If we didn't sort numerically and instead performed the default lexographic sort and have more than single digit results, the output would look like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"1 git checkout main\n1 restart-shell\n13 git status\n2 open .\n2 vi README.md\n")),(0,i.kt)("p",null,"This is a lexographic sort - the line starting with 13 comes after the line starting with 2. We want to sort by the value of the number."),(0,i.kt)("div",{className:"footnotes"},(0,i.kt)("hr",{parentName:"div"}),(0,i.kt)("ol",{parentName:"div"},(0,i.kt)("li",{parentName:"ol",id:"fn-1"},"The path to the shell history file is normally available in the ",(0,i.kt)("inlineCode",{parentName:"li"},"$HISTFILE")," environment variable. However, in a non-interactive shell this variable is not set (and when we run a shell script, it is run in a non-interactive shell). We'll see more about interactive and non-interactive shells later, this is just a note in case you are wondering why we don't use the ",(0,i.kt)("inlineCode",{parentName:"li"},"$HISTFILE")," variable or ",(0,i.kt)("inlineCode",{parentName:"li"},"history")," command!",(0,i.kt)("a",{parentName:"li",href:"#fnref-1",className:"footnote-backref"},"\u21a9")),(0,i.kt)("li",{parentName:"ol",id:"fn-2"},"Try putting the command ",(0,i.kt)("inlineCode",{parentName:"li"},"pstree -p $$")," in a shell script and running the script - you'll see exactly what process is run.",(0,i.kt)("a",{parentName:"li",href:"#fnref-2",className:"footnote-backref"},"\u21a9")),(0,i.kt)("li",{parentName:"ol",id:"fn-3"},"If you want to know more about these folders and the conventions behind them then check back soon, I am going to be adding an entire section on Linux Fundamentals, and one of the chapters will specifically be on the Linux Filesystem. This will cover 'The Linux Filesystem Hierarchy Standard' which defines how folders like this should be used.",(0,i.kt)("a",{parentName:"li",href:"#fnref-3",className:"footnote-backref"},"\u21a9")))))}c.isMDXComponent=!0}}]); \ No newline at end of file diff --git a/pr-preview/pr-346/assets/js/8e52dcb9.d883960e.js b/pr-preview/pr-346/assets/js/8e52dcb9.d883960e.js new file mode 100644 index 00000000..852d3d96 --- /dev/null +++ b/pr-preview/pr-346/assets/js/8e52dcb9.d883960e.js @@ -0,0 +1 @@ +"use strict";(self.webpackChunkeffective_shell=self.webpackChunkeffective_shell||[]).push([[2616],{3905:(e,t,n)=>{n.d(t,{Zo:()=>d,kt:()=>u});var a=n(7294);function i(e,t,n){return t in e?Object.defineProperty(e,t,{value:n,enumerable:!0,configurable:!0,writable:!0}):e[t]=n,e}function l(e,t){var n=Object.keys(e);if(Object.getOwnPropertySymbols){var a=Object.getOwnPropertySymbols(e);t&&(a=a.filter((function(t){return Object.getOwnPropertyDescriptor(e,t).enumerable}))),n.push.apply(n,a)}return n}function o(e){for(var t=1;t<arguments.length;t++){var n=null!=arguments[t]?arguments[t]:{};t%2?l(Object(n),!0).forEach((function(t){i(e,t,n[t])})):Object.getOwnPropertyDescriptors?Object.defineProperties(e,Object.getOwnPropertyDescriptors(n)):l(Object(n)).forEach((function(t){Object.defineProperty(e,t,Object.getOwnPropertyDescriptor(n,t))}))}return e}function r(e,t){if(null==e)return{};var n,a,i=function(e,t){if(null==e)return{};var n,a,i={},l=Object.keys(e);for(a=0;a<l.length;a++)n=l[a],t.indexOf(n)>=0||(i[n]=e[n]);return i}(e,t);if(Object.getOwnPropertySymbols){var l=Object.getOwnPropertySymbols(e);for(a=0;a<l.length;a++)n=l[a],t.indexOf(n)>=0||Object.prototype.propertyIsEnumerable.call(e,n)&&(i[n]=e[n])}return i}var s=a.createContext({}),p=function(e){var t=a.useContext(s),n=t;return e&&(n="function"==typeof e?e(t):o(o({},t),e)),n},d=function(e){var t=p(e.components);return a.createElement(s.Provider,{value:t},e.children)},m="mdxType",c={inlineCode:"code",wrapper:function(e){var t=e.children;return a.createElement(a.Fragment,{},t)}},h=a.forwardRef((function(e,t){var n=e.components,i=e.mdxType,l=e.originalType,s=e.parentName,d=r(e,["components","mdxType","originalType","parentName"]),m=p(n),h=i,u=m["".concat(s,".").concat(h)]||m[h]||c[h]||l;return n?a.createElement(u,o(o({ref:t},d),{},{components:n})):a.createElement(u,o({ref:t},d))}));function u(e,t){var n=arguments,i=t&&t.mdxType;if("string"==typeof e||i){var l=n.length,o=new Array(l);o[0]=h;var r={};for(var s in t)hasOwnProperty.call(t,s)&&(r[s]=t[s]);r.originalType=e,r[m]="string"==typeof e?e:i,o[1]=r;for(var p=2;p<l;p++)o[p]=n[p];return a.createElement.apply(null,o)}return a.createElement.apply(null,n)}h.displayName="MDXCreateElement"},7125:(e,t,n)=>{n.r(t),n.d(t,{assets:()=>s,contentTitle:()=>o,default:()=>m,frontMatter:()=>l,metadata:()=>r,toc:()=>p});var a=n(7462),i=(n(7294),n(3905));const l={title:"Managing Your Files",slug:"/part-1-transitioning-to-the-shell/managing-your-files/"},o=void 0,r={unversionedId:"transitioning-to-the-shell/managing-your-files/index",id:"transitioning-to-the-shell/managing-your-files/index",title:"Managing Your Files",description:"Downloading, unzipping, copying, moving, renaming and deleting files in a graphical user interface is normally fairly intuitive. Now we'll learn how to perform the same operations in a shell. Once you can organise your files, you are well on your way to being able to use the shell more effectively for day to day tasks.",source:"@site/docs/01-transitioning-to-the-shell/03-managing-your-files/index.md",sourceDirName:"01-transitioning-to-the-shell/03-managing-your-files",slug:"/part-1-transitioning-to-the-shell/managing-your-files/",permalink:"/part-1-transitioning-to-the-shell/managing-your-files/",draft:!1,editUrl:"https://github.com/dwmkerr/effective-shell/edit/main/docs/01-transitioning-to-the-shell/03-managing-your-files/index.md",tags:[],version:"current",frontMatter:{title:"Managing Your Files",slug:"/part-1-transitioning-to-the-shell/managing-your-files/"},sidebar:"sidebar",previous:{title:"Navigating Your System",permalink:"/part-1-transitioning-to-the-shell/navigating-your-system/"},next:{title:"Becoming a Clipboard Gymnast",permalink:"/part-1-transitioning-to-the-shell/become-a-clipboard-gymnast/"}},s={},p=[{value:"Creating a Playground",id:"creating-a-playground",level:2},{value:"Finding out about files",id:"finding-out-about-files",level:2},{value:"Extracting the Zip",id:"extracting-the-zip",level:2},{value:"Deleting Files",id:"deleting-files",level:2},{value:"Examining the Contents of a Folder",id:"examining-the-contents-of-a-folder",level:2},{value:"Copying a File",id:"copying-a-file",level:2},{value:"Saving Some Keystrokes",id:"saving-some-keystrokes",level:2},{value:"Renaming or Moving Files",id:"renaming-or-moving-files",level:2},{value:"Creating a New Folder",id:"creating-a-new-folder",level:2},{value:"Copying or Moving Multiple Files with Wildcards",id:"copying-or-moving-multiple-files-with-wildcards",level:2},{value:"Deleting a Folder",id:"deleting-a-folder",level:2},{value:"Looking at Text Files",id:"looking-at-text-files",level:2},{value:"Zipping up Files",id:"zipping-up-files",level:2},{value:"Summary",id:"summary",level:2}],d={toc:p};function m(e){let{components:t,...l}=e;return(0,i.kt)("wrapper",(0,a.Z)({},d,l,{components:t,mdxType:"MDXLayout"}),(0,i.kt)("p",null,"Downloading, unzipping, copying, moving, renaming and deleting files in a graphical user interface is normally fairly intuitive. Now we'll learn how to perform the same operations in a shell. Once you can organise your files, you are well on your way to being able to use the shell more effectively for day to day tasks."),(0,i.kt)("p",null,"Now that we know how to organise the files in our computer, we'll take a look at how to download files, create new files, preview the contents of files, open files, copy, move and delete files."),(0,i.kt)("p",null,"This chapter will introduce the ",(0,i.kt)("inlineCode",{parentName:"p"},"wget"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"unzip"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"cp"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"mv"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"rm"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"mkdir"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"rmdir"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"cat")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"zip")," commands. We'll also briefly look at ",(0,i.kt)("em",{parentName:"p"},"wildcards")," and ",(0,i.kt)("em",{parentName:"p"},"redirection"),"."),(0,i.kt)("h2",{id:"creating-a-playground"},"Creating a Playground"),(0,i.kt)("p",null,"Before we start copying, deleting, moving and renaming files, we should create a 'playground' area we can work in. We don't want to test all of this on our own personal files until we know exactly what we're doing! "),(0,i.kt)("p",null,"To help with this, I've created a zipped up 'samples' which has a lot of files in it which we can use to play with. Now the file itself is available on the ",(0,i.kt)("a",{parentName:"p",href:"https://effective-shell.com"},"effective-shell.com")," website, right here:"),(0,i.kt)("p",null,(0,i.kt)("a",{parentName:"p",href:"https://effective-shell.com/downloads/effective-shell-samples.zip"},"effective-shell.com/downloads/effective-shell-samples.zip")),(0,i.kt)("p",null,"We ",(0,i.kt)("em",{parentName:"p"},"could")," open up a web browser, download the file, unzip it and then start from there, but this book is all about how to deal with every day tasks in your shell, so let's skip the browser and do it in the shell instead!"),(0,i.kt)("p",null,"Open your shell - if you've not yet got set up with your shell, that's OK, just check ",(0,i.kt)("a",{parentName:"p",href:"/part-1-transitioning-to-the-shell/getting-started/"},"Chapter 1 - Getting Started"),"."),(0,i.kt)("p",null,"Now that you have your shell open, we can run the ",(0,i.kt)("inlineCode",{parentName:"p"},"wget")," command (",(0,i.kt)("em",{parentName:"p"},"Web Get"),") to download the zip file. Let's download it to our Home folder. If you are not sure what the Home folder is, check ",(0,i.kt)("a",{parentName:"p",href:"/part-1-transitioning-to-the-shell/navigating-your-system/"},"Chapter 2- Navigating Your System"),"."),(0,i.kt)("p",null,"First, we'll move to our home directory, then download the file."),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cd\nwget https://effective-shell.com/downloads/effective-shell-samples.zip\n")),(0,i.kt)("p",null,"You'll see something like this:"),(0,i.kt)("img",{alt:"Screenshot: wget",src:n(2434).Z,width:"800px"}),(0,i.kt)("p",null,"When you call the ",(0,i.kt)("inlineCode",{parentName:"p"},"wget")," command, you can give it any web address and it'll download it to your current folder. It also shows the progress of the download interactively (particularly useful if it's a big file!)."),(0,i.kt)("p",null,"As an aside, if we were not in our home folder when we called the ",(0,i.kt)("inlineCode",{parentName:"p"},"wget")," command, we'd download the file to wherever we are currently working in. If we wanted to be explicit about where we download the file, we can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"-O")," (",(0,i.kt)("em",{parentName:"p"},"Output File"),") flag to say explicitly where we want to download the file."),(0,i.kt)("p",null,"As an example, if were not in the home folder, but wanted to download there, we'd just call:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cd\nwget -O ~/playground.zip https://effective-shell.com/downloads/effective-shell-samples.zip\n")),(0,i.kt)("p",null,"Now that we've downloaded the file, let's look at our home directory now, with a quick call to ",(0,i.kt)("inlineCode",{parentName:"p"},"ls ~"),":"),(0,i.kt)("img",{alt:"Screenshot: ls home",src:n(9074).Z,width:"800px"}),(0,i.kt)("p",null,"Cool - we have the zip file downloaded! Now we need to work out how to unzip it so we can get to the files in the zip archive."),(0,i.kt)("h2",{id:"finding-out-about-files"},"Finding out about files"),(0,i.kt)("p",null,"One of the interesting things you can do in a shell is ask it to tell you more about a file. This can be useful if we've got a file, and we're not sure what it might be. Let's try it out now:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"file ~/effective-shell-samples.zip\n")),(0,i.kt)("img",{alt:"Screenshot: file",src:n(6470).Z,width:"800px"}),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"file")," command is showing us we have a zip file - now it's time to unzip it!"),(0,i.kt)("h2",{id:"extracting-the-zip"},"Extracting the Zip"),(0,i.kt)("p",null,"Right now we have a zip file. We need to extract it, unpack the files so that we can play with them. Again, in a system with a graphical user interface, this is easy, generally you just double click on it. But we're going to use the shell for this!"),(0,i.kt)("p",null,"Run the command:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"unzip ~/effective-shell-samples.zip\n")),(0,i.kt)("p",null,"Now let's look at what we've got with the ",(0,i.kt)("inlineCode",{parentName:"p"},"ls")," command:"),(0,i.kt)("img",{alt:"Screenshot: unzip",src:n(868).Z,width:"800px"}),(0,i.kt)("p",null,"Excellent - we've now got a ",(0,i.kt)("em",{parentName:"p"},"folder")," which contains all of the files in the zip archive."),(0,i.kt)("h2",{id:"deleting-files"},"Deleting Files"),(0,i.kt)("p",null,"Now that we've downloaded and unzipped the file, we don't need the zipped version any more. So let's delete this file."),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"rm")," (",(0,i.kt)("em",{parentName:"p"},"Remove"),") command can be used to delete a file. If we run:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"rm ~/effective-shell-samples.zip\nls | grep samples\n")),(0,i.kt)("p",null,"Then we'll see the following:"),(0,i.kt)("img",{alt:"Screenshot: rm",src:n(7257).Z,width:"800px"}),(0,i.kt)("p",null,"Notice that the zip file is gone - just the folder is left."),(0,i.kt)("p",null,"By the way - be really careful with the ",(0,i.kt)("inlineCode",{parentName:"p"},"rm")," command. Unlike in a graphical interface, it won't put files you delete into a recycle bin, they are blatted forever! In a later chapter we'll see some ways to change this behaviour for your local machine, but always remember ",(0,i.kt)("inlineCode",{parentName:"p"},"rm")," is a little risky!"),(0,i.kt)("p",null,"However one thing it ",(0,i.kt)("em",{parentName:"p"},"will")," do to try and help you not make mistakes is let you know if you are trying to delete a ",(0,i.kt)("em",{parentName:"p"},"folder"),", not a file."),(0,i.kt)("p",null,"Run the following command to try and delete the unzipped folder:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"rm ~/effective-shell-samples\n")),(0,i.kt)("img",{alt:"Screenshot: rm error directory",src:n(7318).Z,width:"800px"}),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"rm")," command has not succeeded in this case - it's warning us that we're not deleting a file, but a whole directory."),(0,i.kt)("p",null,"Now we can get around this by adding the ",(0,i.kt)("inlineCode",{parentName:"p"},"-r")," flag, which means 'recursive' - i.e. not just the folder but everything in it. But use this with caution!"),(0,i.kt)("h2",{id:"examining-the-contents-of-a-folder"},"Examining the Contents of a Folder"),(0,i.kt)("p",null,"Let's take a look at what is in the samples. By the way, the output you see on your computer might have a few more files in it as I might have added some after writing this article!"),(0,i.kt)("p",null,"In a graphical user interface, we'd open the folders and look at the files. In the shell, we can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"tree")," command to show the contents of a folder."),(0,i.kt)("p",null,"Now the ",(0,i.kt)("inlineCode",{parentName:"p"},"tree")," command is ",(0,i.kt)("em",{parentName:"p"},"not")," installed by default on all systems. So if you are on a Mac, run:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"brew install tree\n")),(0,i.kt)("p",null,"If you are on Linux, you will likely already have it. If you don't, use your distributions package manager to get it (e.g. ",(0,i.kt)("inlineCode",{parentName:"p"},"apt-get install -y tree"),")."),(0,i.kt)("p",null,"Using a non-universal command is generally ",(0,i.kt)("em",{parentName:"p"},"not")," our goal in this book, but in these early stages while we are transitioning from the graphical user interface, the ",(0,i.kt)("inlineCode",{parentName:"p"},"tree")," command can be really helpful. Later on we'll see how to use the more universal ",(0,i.kt)("inlineCode",{parentName:"p"},"find")," command to give a similar output."),(0,i.kt)("p",null,"Try it out with:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"tree ~/effective-shell-samples\n")),(0,i.kt)("img",{alt:"Screenshot: tree",src:n(4).Z,width:"800px"}),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"tree")," command shows you all of the folders and files in a location. If we are unsure what one of the files is, we can ask the shell to give us more info. For example, I could find out more about the ",(0,i.kt)("inlineCode",{parentName:"p"},"loas-gch.JPG")," file by running:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"file ~/effective-shell-samples/pictures/loas-gch.JPG\n")),(0,i.kt)("img",{alt:"Screenshot: file info for JPEG file",src:n(6392).Z,width:"800px"}),(0,i.kt)("p",null,"Note that the ",(0,i.kt)("inlineCode",{parentName:"p"},"file")," command is already showing it is a bit more clever. It knows that the file is a ",(0,i.kt)("inlineCode",{parentName:"p"},"JPEG")," file (a picture), but is giving other details as well."),(0,i.kt)("h2",{id:"copying-a-file"},"Copying a File"),(0,i.kt)("p",null,"Let's say we really love that photo, and we want to make a copy of it. We can do that easily by using the ",(0,i.kt)("inlineCode",{parentName:"p"},"cp")," (_Copy) command:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cp ~/effective-shell-samples/pictures/laos-gch.JPG ~/effective-shell-playground/pictures/laos-gch-copy.JPG\n")),(0,i.kt)("p",null,"This makes a copy of the file - if you are not sure if it has worked, just run:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"tree ~/effective-shell-samples\n")),(0,i.kt)("img",{alt:"Screenshot: cp command",src:n(3).Z,width:"800px"}),(0,i.kt)("p",null,"We can see we've made a copy."),(0,i.kt)("h2",{id:"saving-some-keystrokes"},"Saving Some Keystrokes"),(0,i.kt)("p",null,"Wow, it's painful putting ",(0,i.kt)("inlineCode",{parentName:"p"},"~/effective-shell-samples")," before everything! From ",(0,i.kt)("a",{parentName:"p",href:"/part-1-transitioning-to-the-shell/navigating-your-system/"},"Chapter 2- Navigating Your System")," we already know how to change directory, so let's do that now:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cd ~/effective-shell-samples\n")),(0,i.kt)("p",null,"Remember - ",(0,i.kt)("inlineCode",{parentName:"p"},"cd")," is ",(0,i.kt)("em",{parentName:"p"},"change directory"),". Excellent - until we tell our shell otherwise, this our new working directory."),(0,i.kt)("h2",{id:"renaming-or-moving-files"},"Renaming or Moving Files"),(0,i.kt)("p",null,"You might have noticed that the photos have different endings - one of them ends in ",(0,i.kt)("inlineCode",{parentName:"p"},".JPG"),". Let's rename it so that it has the ending ",(0,i.kt)("inlineCode",{parentName:"p"},".jpeg")," to be consistent with the others."),(0,i.kt)("p",null,"To do this, we use the ",(0,i.kt)("inlineCode",{parentName:"p"},"mv")," (",(0,i.kt)("em",{parentName:"p"},"Move"),") command. When it comes down to it, moving a file or renaming a file amount to the same kind of operation, so one command can do both."),(0,i.kt)("p",null,"Rename the copy we made of the photo by running:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"mv pictures/loas-gch-copy.JPG pictures/loas-gch-copy.jpeg\n")),(0,i.kt)("p",null,"Let's run ",(0,i.kt)("inlineCode",{parentName:"p"},"tree")," to see what happened. Remember - now that our working folder is the playground, we don't even need to tell ",(0,i.kt)("inlineCode",{parentName:"p"},"tree")," where to look, if we give it no arguments it'll assume we're looking at the working directory:"),(0,i.kt)("img",{alt:"Screenshot: mv command",src:n(5714).Z,width:"800px"}),(0,i.kt)("p",null,"Much nicer! Now our copied file has been moved to have a new name. It's in the same folder still, but you can use ",(0,i.kt)("inlineCode",{parentName:"p"},"mv")," to also change what folder a file is in."),(0,i.kt)("h2",{id:"creating-a-new-folder"},"Creating a New Folder"),(0,i.kt)("p",null,"Perhaps we're not happy with the name ",(0,i.kt)("inlineCode",{parentName:"p"},"pictures")," for our folder we've been playing with, maybe we'd prefer to have them all in a folder called ",(0,i.kt)("inlineCode",{parentName:"p"},"photos"),"?"),(0,i.kt)("p",null,"Probably the first thing we'd do in a graphical environment is create a new folder - so let's do thee same here!"),(0,i.kt)("p",null,"Run the commands:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"mkdir photos\ntree\n")),(0,i.kt)("p",null,"And we should see:"),(0,i.kt)("img",{alt:"Screenshot: mkdir command",src:n(5704).Z,width:"800px"}),(0,i.kt)("p",null,"We've use the ",(0,i.kt)("inlineCode",{parentName:"p"},"mkdir")," command, which is short for ",(0,i.kt)("em",{parentName:"p"},"Make Directory"),". This is how we create a new folder in the shell."),(0,i.kt)("p",null,"Now let's say we wanted to be ",(0,i.kt)("em",{parentName:"p"},"really")," organised, and create a photos folder by year and topic, perhaps ",(0,i.kt)("inlineCode",{parentName:"p"},"2019/outdoors/pictures"),". In a graphical user interface, we'd have to create each folder one at a time. In the shell, it's easy!"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"mkdir -p 2019/outdoors/pictures\ntree\n")),(0,i.kt)("p",null,"Let's see how it looks:"),(0,i.kt)("img",{alt:"Screenshot: mkdirp command",src:n(8960).Z,width:"800px"}),(0,i.kt)("p",null,"All we had to do was add the ",(0,i.kt)("inlineCode",{parentName:"p"},"-p")," flag (which means \"make the parent folder if it doesn't already exist) and we can create a whole set of subfolders. Now we're starting to see why knowing the shell can be powerful - if you know you have this trick up your sleeve you can be doing things like re-organising files ",(0,i.kt)("em",{parentName:"p"},"more effectively")," in a shell than in your graphical user interface!"),(0,i.kt)("h2",{id:"copying-or-moving-multiple-files-with-wildcards"},"Copying or Moving Multiple Files with Wildcards"),(0,i.kt)("p",null,"Let's copy the photos that we have in the ",(0,i.kt)("inlineCode",{parentName:"p"},"pictures")," folder into the ",(0,i.kt)("inlineCode",{parentName:"p"},"photos/2019/outdoor/climbing")," folder."),(0,i.kt)("p",null,"When we run the ",(0,i.kt)("inlineCode",{parentName:"p"},"cp")," or ",(0,i.kt)("inlineCode",{parentName:"p"},"mv")," command, we can use a ",(0,i.kt)("em",{parentName:"p"},"wildcard")," to specify the files we are copying and moving. A wildcard is a simple pattern which can be used to select multiple files. Here's how we can copy the photos over:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"cp pictures/* photos/2019/outdoor/climbing\n")),(0,i.kt)("p",null,"Here's how it works for\nNow we need to copy over our files from the ",(0,i.kt)("inlineCode",{parentName:"p"},"pictures")," folder to the ",(0,i.kt)("inlineCode",{parentName:"p"},"2019/outdoor/photos")," folder. We'll use exactly the command we used before to copy a file - ",(0,i.kt)("inlineCode",{parentName:"p"},"cp"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"$ cp pictures/* photos/2019/outdoors/climbing/\n\n$ tree photos\nphotos\n\u251c\u2500\u2500 2019\n\u2502\xa0\xa0 \u2514\u2500\u2500 outdoors\n\u2502\xa0\xa0 \u2514\u2500\u2500 climbing\n\u2502\xa0\xa0 \u251c\u2500\u2500 laos-gch-copy.jpeg\n\u2502\xa0\xa0 \u251c\u2500\u2500 laos-gch.JPG\n\u2502\xa0\xa0 \u2514\u2500\u2500 nepal-mardi-himal.jpeg\n\u2514\u2500\u2500 2020\n \u2514\u2500\u2500 outdoors\n \u2514\u2500\u2500 climbing\n\n6 directories, 3 files\n")),(0,i.kt)("p",null,"Here we've used the ",(0,i.kt)("em",{parentName:"p"},"wildcard")," symbol, which is ",(0,i.kt)("inlineCode",{parentName:"p"},"*"),', to say "everything in the folder". Many commands can take wildcards as inputs. We\'ll see much more about them later!'),(0,i.kt)("h2",{id:"deleting-a-folder"},"Deleting a Folder"),(0,i.kt)("p",null,"Now that we have our more organise ",(0,i.kt)("inlineCode",{parentName:"p"},"2019/outdoors/photos")," folder, we don't need the ",(0,i.kt)("inlineCode",{parentName:"p"},"photos")," folder we created. So let's delete it! Remember how ",(0,i.kt)("inlineCode",{parentName:"p"},"rm")," removes a file, and ",(0,i.kt)("inlineCode",{parentName:"p"},"mkdir")," creates a folder? Well ",(0,i.kt)("inlineCode",{parentName:"p"},"rmdir")," will remove a folder!"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"rmdir photos\ntree\n")),(0,i.kt)("img",{alt:"Screenshot: rmdir command",src:n(6729).Z,width:"800px"}),(0,i.kt)("p",null,"As an important sidenote, just how ",(0,i.kt)("inlineCode",{parentName:"p"},"rm")," doesn't move files to your recycle bin, so you cannot undo the operation, ",(0,i.kt)("inlineCode",{parentName:"p"},"rmdir")," works the same way. So if we try to remove a directory which has things in it, such as the ",(0,i.kt)("inlineCode",{parentName:"p"},"pictures")," directory, it will fail:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"rmdir pictures\n")),(0,i.kt)("img",{alt:"Screenshot: rmdir fail command",src:n(4195).Z,width:"800px"}),(0,i.kt)("p",null,"In this case, it is actually easier to just call ",(0,i.kt)("inlineCode",{parentName:"p"},"rm -r pictures"),". Why is that? Well it's just like we saw in the earlier example - ",(0,i.kt)("inlineCode",{parentName:"p"},"rm")," can delete files or directories. And if the directory is not empty, we just add the ",(0,i.kt)("inlineCode",{parentName:"p"},"-r")," (",(0,i.kt)("em",{parentName:"p"},"Recursive"),") flag to tell it to delete the directory and everything it contains."),(0,i.kt)("h2",{id:"looking-at-text-files"},"Looking at Text Files"),(0,i.kt)("p",null,"Run ",(0,i.kt)("inlineCode",{parentName:"p"},"tree")," and you'll see we have a ",(0,i.kt)("inlineCode",{parentName:"p"},"quotes")," folder:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"tree\n")),(0,i.kt)("img",{alt:"Screenshot: tree of new quotes folder",src:n(6227).Z,width:"800px"}),(0,i.kt)("p",null,"We're going to use the ",(0,i.kt)("inlineCode",{parentName:"p"},"cat")," (",(0,i.kt)("em",{parentName:"p"},"Concatenate"),") command to look at the Ursula Le Guin quote. Run the following command:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cat quotes/ursula-le-guin.txt\n")),(0,i.kt)("img",{alt:"Screenshot: cat",src:n(1718).Z,width:"800px"}),(0,i.kt)("p",null,"In the screenshot we snuck in a quick ",(0,i.kt)("inlineCode",{parentName:"p"},"file")," call to see what the shell thinks the file is."),(0,i.kt)("p",null,"Why ",(0,i.kt)("em",{parentName:"p"},"Concatenate"),"? We're just showing the text in the terminal, not concatenating (i.e. joining) anything! Well the reason is that the ",(0,i.kt)("inlineCode",{parentName:"p"},"cat")," command ",(0,i.kt)("em",{parentName:"p"},"does")," concatenate files (i.e. puts them together), it's just that we only gave it one file, so it had nothing to join it to. By default, ",(0,i.kt)("inlineCode",{parentName:"p"},"cat")," writes the output to the screen, so this is one of the most common ways you'll see to quickly look at the contents of a file."),(0,i.kt)("p",null,"We'll see a ",(0,i.kt)("em",{parentName:"p"},"lot")," more about how this works later, and how you can then take that output and put it somewhere else. But for now, let's finish with a couple of tricks."),(0,i.kt)("p",null,"First, let's just ",(0,i.kt)("inlineCode",{parentName:"p"},"cat")," the whole folder:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cat quotes/*\n")),(0,i.kt)("img",{alt:"Screenshot: cat wildcard",src:n(9391).Z,width:"800px"}),(0,i.kt)("p",null,"There we see the ",(0,i.kt)("inlineCode",{parentName:"p"},"*")," wildcard again. We could be more specific and use something like ",(0,i.kt)("inlineCode",{parentName:"p"},"cat quotes/*.txt")," to only show files ending in ",(0,i.kt)("inlineCode",{parentName:"p"},".txt"),"."),(0,i.kt)("p",null,"Notice how the output from all of the files has been ",(0,i.kt)("em",{parentName:"p"},"concatenated together")," into a single output? That's where the ",(0,i.kt)("inlineCode",{parentName:"p"},"cat")," name comes from - it ",(0,i.kt)("em",{parentName:"p"},"concatenates"),", i.e. ",(0,i.kt)("em",{parentName:"p"},"joins")," files."),(0,i.kt)("p",null,"As one last trick, let's use this output but instead of showing it on the screen, put it into a single ",(0,i.kt)("inlineCode",{parentName:"p"},"all-quotes.txt")," file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cat quotes/* > quotes/all-quotes.txt\ntree\ncat quotes/all-quotes.txt\n")),(0,i.kt)("img",{alt:"Screenshot: cat redirect",src:n(1415).Z,width:"800px"}),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},">")," part of this is called a ",(0,i.kt)("em",{parentName:"p"},"redirect operator")," - in short it's telling the shell not to write to the screen, ",(0,i.kt)("em",{parentName:"p"},"but to write to a file"),". We've concatenated all of the individual quotes and made a single file from them."),(0,i.kt)("p",null,"We'll look at wildcards and redirection in a lot more detail as we continue through the book!"),(0,i.kt)("h2",{id:"zipping-up-files"},"Zipping up Files"),(0,i.kt)("p",null,"Let' say that we want to zip up the new ",(0,i.kt)("inlineCode",{parentName:"p"},"2019/outdoors/pictures")," folder. We've already seen the ",(0,i.kt)("inlineCode",{parentName:"p"},"unzip")," command, let's see how to use the ",(0,i.kt)("inlineCode",{parentName:"p"},"zip")," command to zip up a folder:"),(0,i.kt)("p",null,"Run the command below:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"zip -r 2019-outdoor-pictures.zip 2019\n")),(0,i.kt)("p",null,"This is how it will look - there's a ",(0,i.kt)("inlineCode",{parentName:"p"},"tree")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"ls")," command before and after so we can see what's happening!"),(0,i.kt)("img",{alt:"Screenshot: zip",src:n(7860).Z,width:"800px"}),(0,i.kt)("p",null,"Great! We've created a zip. Let's dissect the command a bit:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"zip")," just means call the ",(0,i.kt)("inlineCode",{parentName:"li"},"zip")," executable"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"-r")," means ",(0,i.kt)("em",{parentName:"li"},"recursive")," we don't just want to zip the 2019 folder, we want to zip everything inside it as well!"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"2019-outdoor-pictures.zip")," is the name of the file we want to create, we put this first..."),(0,i.kt)("li",{parentName:"ul"},"...because everything which follows (e.g. ",(0,i.kt)("inlineCode",{parentName:"li"},"2019"),") is going to be zipped, and we can specify many files and folders if we want")),(0,i.kt)("h2",{id:"summary"},"Summary"),(0,i.kt)("p",null,"In this chapter we introduced the following: "),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},"The 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",(0,i.kt)("inlineCode",{parentName:"li"},"rm")," command won't delete a folder which has files in it, unless you tell it to by adding the ",(0,i.kt)("inlineCode",{parentName:"li"},"-r")," (",(0,i.kt)("em",{parentName:"li"},"recursive"),") flag."),(0,i.kt)("li",{parentName:"ul"},"The ",(0,i.kt)("inlineCode",{parentName:"li"},"tree")," command can show the files and folders in a given directory, or the current directory by default."),(0,i.kt)("li",{parentName:"ul"},"The ",(0,i.kt)("inlineCode",{parentName:"li"},"cp")," (",(0,i.kt)("em",{parentName:"li"},"copy"),") command can copy a file."),(0,i.kt)("li",{parentName:"ul"},"The ",(0,i.kt)("inlineCode",{parentName:"li"},"cp")," can also be given wildcards like ",(0,i.kt)("inlineCode",{parentName:"li"},"*")," to copy many files."),(0,i.kt)("li",{parentName:"ul"},"The ",(0,i.kt)("inlineCode",{parentName:"li"},"mv")," (",(0,i.kt)("em",{parentName:"li"},"move"),") command can move or rename a file."),(0,i.kt)("li",{parentName:"ul"},"The 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effectively.",source:"@site/docs/02-core-skills/07-thinking-in-pipelines/index.md",sourceDirName:"02-core-skills/07-thinking-in-pipelines",slug:"/part-2-core-skills/thinking-in-pipelines/",permalink:"/part-2-core-skills/thinking-in-pipelines/",draft:!1,editUrl:"https://github.com/dwmkerr/effective-shell/edit/main/docs/02-core-skills/07-thinking-in-pipelines/index.md",tags:[],version:"current",frontMatter:{title:"Thinking in Pipelines",slug:"/part-2-core-skills/thinking-in-pipelines/"},sidebar:"sidebar",previous:{title:"Part 2 - Core Skills",permalink:"/part-2-core-skill/"},next:{title:"Fly on the Command Line",permalink:"/part-2-core-skills/fly-on-the-command-line"}},s={},p=[{value:"Input and Output",id:"input-and-output",level:2},{value:"Standard Input, Output and Error",id:"standard-input-output-and-error",level:2},{value:"A Pipeline in Action",id:"a-pipeline-in-action",level:2},{value:"Common Patterns - Standard Input",id:"common-patterns---standard-input",level:2},{value:"Common Patterns - Standard Output",id:"common-patterns---standard-output",level:2},{value:"Common Patterns - Standard Error",id:"common-patterns---standard-error",level:2},{value:"One Last Trick - The T Pipe",id:"one-last-trick---the-t-pipe",level:2},{value:"Thinking in Pipelines",id:"thinking-in-pipelines",level:2},{value:"Summary",id:"summary",level:2}],h={toc:p};function d(e){let{components:t,...r}=e;return(0,i.kt)("wrapper",(0,a.Z)({},h,r,{components:t,mdxType:"MDXLayout"}),(0,i.kt)("p",null,"Understanding the concept of ",(0,i.kt)("em",{parentName:"p"},"pipelines")," in the shell, as well as how input and output work for command line programs is critical to be able to use the shell effectively."),(0,i.kt)("p",null,"In this chapter, we'll look at the ways programs handle input and output, then we'll look at how we can chain multiple commands together with pipelines. We'll also look at some really common ways to use pipelines which should hopefully make your life easier!"),(0,i.kt)("p",null,"When you understand these concepts, it will open up a new world in terms of what you can do with in the shell. We'll briefly touch on the 'The Unix Philosophy', which is a concept which allows us to perform highly complex tasks by composing together small, simple components."),(0,i.kt)("h2",{id:"input-and-output"},"Input and Output"),(0,i.kt)("p",null,"Many of the programs we have seen so far follow a very similar pattern:"),(0,i.kt)("img",{src:n(7007).Z,alt:"Diagram: Input -> Program -> Output",width:"480px"}),(0,i.kt)("p",null,"In fact, when you get down to the details, there are very few programs which ",(0,i.kt)("em",{parentName:"p"},"don't")," do something like this! As a more concrete example, we can look at the ",(0,i.kt)("inlineCode",{parentName:"p"},"sort")," program - which sorts the input in alphabetic order:"),(0,i.kt)("img",{src:n(1023).Z,alt:"Diagram: Sort",width:"480px"}),(0,i.kt)("p",null,"We can easily see this in action by just running ",(0,i.kt)("inlineCode",{parentName:"p"},"sort")," in a shell. Start the ",(0,i.kt)("inlineCode",{parentName:"p"},"sort")," program, enter some text, then press ",(0,i.kt)("inlineCode",{parentName:"p"},"Ctrl+D"),". ",(0,i.kt)("inlineCode",{parentName:"p"},"Ctrl+D")," (which is normally written as ",(0,i.kt)("inlineCode",{parentName:"p"},"^D")," is a special ",(0,i.kt)("em",{parentName:"p"},"control character")," which means 'end of transmission' - in this case we use it to tell ",(0,i.kt)("inlineCode",{parentName:"p"},"sort")," we've finished writing text. If you were to use ",(0,i.kt)("inlineCode",{parentName:"p"},"^C")," (which is the ",(0,i.kt)("em",{parentName:"p"},"interrupt")," command) it would closes the ",(0,i.kt)("inlineCode",{parentName:"p"},"sort")," program instead)."),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ sort\nDogs\nchase\ncats\nand\ncats\nchase\nmice\n")),(0,i.kt)("p",null,"Once you've entered the text you want to sort, hit ",(0,i.kt)("inlineCode",{parentName:"p"},"^D")," and you'll see the sorted output:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"Dogs\nand\ncats\ncats\nchase\nchase\nmice\n")),(0,i.kt)("p",null,"So by default, the ",(0,i.kt)("inlineCode",{parentName:"p"},"sort")," command is reading ",(0,i.kt)("em",{parentName:"p"},"input")," from the keyboard (until we send it a special message saying we're done), then writing the ",(0,i.kt)("em",{parentName:"p"},"output")," to the terminal."),(0,i.kt)("p",null,"In fact, ",(0,i.kt)("inlineCode",{parentName:"p"},"sort")," is using two special ",(0,i.kt)("em",{parentName:"p"},"files")," - ",(0,i.kt)("inlineCode",{parentName:"p"},"stdin")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," - but what does this mean?"),(0,i.kt)("h2",{id:"standard-input-output-and-error"},"Standard Input, Output and Error"),(0,i.kt)("p",null,"Every program has access to three 'special' files, ",(0,i.kt)("inlineCode",{parentName:"p"},"stdin"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr"),":"),(0,i.kt)("img",{src:n(6699).Z,alt:"Diagram: stdin, stdout, stderr",width:"480px"}),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"stdin")," is short for 'standard input' - it's where ",(0,i.kt)("em",{parentName:"li"},"many")," programs read their input from"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"stdout")," is short for 'standard output' - it's where ",(0,i.kt)("em",{parentName:"li"},"many")," programs write their output to"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"stderr")," is short for 'standard error' - it's where ",(0,i.kt)("em",{parentName:"li"},"some")," programs write error messages to")),(0,i.kt)("p",null,"Why do I say 'many' and 'some'? Well the reason is that while this is ",(0,i.kt)("em",{parentName:"p"},"convention"),", it is not adhered to universally. Anyone who writes a program is free to choose how they read input and write output, so some programs might not follow these conventions. In Chapter 29 we'll look at how to write tools which follow these conventions, as well as others which are useful."),(0,i.kt)("p",null,"Each of these files has a special number which is shown in grey in the diagram. This is known as the ",(0,i.kt)("em",{parentName:"p"},"file descriptor")," and we'll see it later on. Each of these files also has a special location in the system which you can access directly - you can see these files by running ",(0,i.kt)("inlineCode",{parentName:"p"},"ls -al /dev/std*"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ ls -al /dev/std*\nlr-xr-xr-x 1 root wheel 0 Jan 1 1970 /dev/stderr -> fd/2\nlr-xr-xr-x 1 root wheel 0 Jan 1 1970 /dev/stdin -> fd/0\nlr-xr-xr-x 1 root wheel 0 Jan 1 1970 /dev/stdout -> fd/1\n")),(0,i.kt)("p",null,"If you are not familiar with ",(0,i.kt)("inlineCode",{parentName:"p"},"ls")," (the ",(0,i.kt)("em",{parentName:"p"},"list directory contents")," command) then check ",(0,i.kt)("a",{parentName:"p",href:"/part-1-transitioning-to-the-shell/navigating-your-system/"},"Chapter 2 - Navigating Your System"),". The first part of the output isn't too important - but we can see we have three files in the special ",(0,i.kt)("inlineCode",{parentName:"p"},"/dev/")," (short for ",(0,i.kt)("em",{parentName:"p"},"device")," folder). We can also see the associated file descriptors."),(0,i.kt)("p",null,"As an aside - this is a really fundamental thing we'll see again and again in Unix and Linux - almost everything can be represented as a file. This is a core concept and one we'll touch on regularly."),(0,i.kt)("p",null,"When you are running programs in a shell, the shell attaches your keyboard to the program's standard input",(0,i.kt)("sup",{parentName:"p",id:"fnref-1"},(0,i.kt)("a",{parentName:"sup",href:"#fn-1",className:"footnote-ref"},"1")),", and attaches the standard output and standard error to the terminal display:"),(0,i.kt)("img",{src:n(4956).Z,alt:"Diagram: Shell, Keyboard, Terminal",width:"640px"}),(0,i.kt)("p",null,"This means when we're in a shell, we can type on the keyboard, which goes to the input of the program and then as the program outputs information and errors they show up on the screen."),(0,i.kt)("p",null,"We can already see the beginnings of a ",(0,i.kt)("strong",{parentName:"p"},"pipeline")," here. There's a clear flow of data from the keyboard, through the ",(0,i.kt)("inlineCode",{parentName:"p"},"stdin")," file, through the program, then through the output files, then to the display."),(0,i.kt)("p",null,"Looking at some real programs in action will hopefully make this clearer!"),(0,i.kt)("h2",{id:"a-pipeline-in-action"},"A Pipeline in Action"),(0,i.kt)("p",null,"Do you remember the ",(0,i.kt)("inlineCode",{parentName:"p"},"cat")," command? It's the one which writes the contents of a file to the screen. For example:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ cat ~/effective-shell/text/simpsons-characters.txt\nArtie Ziff\nKirk Van Houten\nTimothy Lovejoy\nArtie Ziff\nNick Riviera\nSeymore Skinner\nHank Scorpio\nTimothy Lovejoy\nJohn Frink\nCletus Spuckler\nRuth Powers\nArtie Ziff\nAgnes Skinner\nHelen Lovejoy\n")),(0,i.kt)("p",null,"We saw in ",(0,i.kt)("a",{parentName:"p",href:"/part-1-transitioning-to-the-shell/become-a-clipboard-gymnast/"},"Chapter 4 - Becoming a Clipboard Gymnast")," that we could ",(0,i.kt)("strong",{parentName:"p"},"pipe")," the output of this command into the ",(0,i.kt)("inlineCode",{parentName:"p"},"sort")," command to order it and then into the ",(0,i.kt)("inlineCode",{parentName:"p"},"uniq")," command to remove duplicates, like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ cat ~/effective-shell/text/simpsons-characters.txt | sort | uniq\nAgnes Skinner\nArtie Ziff\nCletus Spuckler\nHank Scorpio\nHelen Lovejoy\nJohn Frink\nKirk Van Houten\nNick Riviera\nRuth Powers\nSeymore Skinner\nTimothy Lovejoy\n")),(0,i.kt)("p",null,"The ",(0,i.kt)("strong",{parentName:"p"},"pipe")," operator (which is the vertical pipe symbol or ",(0,i.kt)("inlineCode",{parentName:"p"},"|"),") has a very specific meaning in the shell - it attaches the ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," of the first program to the ",(0,i.kt)("inlineCode",{parentName:"p"},"stdin")," of the second. This means we can now visualise the entire pipeline and see exactly what is going on:"),(0,i.kt)("img",{src:n(586).Z,alt:"Diagram: cat-sort-uniq pipeline",width:"1024px"}),(0,i.kt)("p",null,"That's it! If you can follow what is going on here then you have the key information you need to know to understand how pipelines work. The pipe operator just connects the output of one program to the input of another. A pipeline is just a set of connected programs. Easy!"),(0,i.kt)("p",null,"We could do the same thing by writing the output of each step as a file, then reading that file with the next step, but that would mean we'd have a lot of intermediate files to clean up (and if we're processing a ",(0,i.kt)("em",{parentName:"p"},"big")," file, it also uses a lot of space). Pipelines let us create complex sequences of operations which work well even on very large files."),(0,i.kt)("p",null,"Now we'll look at ",(0,i.kt)("inlineCode",{parentName:"p"},"stdin"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr")," in a little more detail. We'll be seeing these special streams a lot as we go through the book. Knowing more about them is really going to help you when working in the shell or with Linux-like systems."),(0,i.kt)("h2",{id:"common-patterns---standard-input"},"Common Patterns - Standard Input"),(0,i.kt)("p",null,"Let's have a quick look at some of the common things we might see as sources of inputs for other programs. Each one illustrates an interesting point about how the shell or the standard input stream works."),(0,i.kt)("img",{src:n(3657).Z,alt:"Diagram: Input Examples",width:"1024px"}),(0,i.kt)("p",null,"This list is by no means exhaustive, in fact with a bit of tinkering you can make almost anything the input to anything else, but let's check each example."),(0,i.kt)("p",null,"These examples will use some new programs to transform the output - don't worry about the details of them, each will be described as we go through the book!"),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"The Shell")),(0,i.kt)("p",null,"You might just use code in the shell as input, for example:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ echo \"I am in the $PWD folder\" | sed 's/folder/directory/'\nI am in the /Users/dwmkerr/repos/github/dwmkerr/effective-shell directory\n")),(0,i.kt)("p",null,"Here we've just used ",(0,i.kt)("inlineCode",{parentName:"p"},"echo")," to write out message including a variable and then used the ",(0,i.kt)("inlineCode",{parentName:"p"},"sed")," (",(0,i.kt)("em",{parentName:"p"},"stream editor"),") program to replace the word ",(0,i.kt)("inlineCode",{parentName:"p"},"folder")," with ",(0,i.kt)("inlineCode",{parentName:"p"},"directory"),". We'll get a lot of practice with ",(0,i.kt)("inlineCode",{parentName:"p"},"sed")," as we go through this book!"),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"Files")),(0,i.kt)("p",null,"We've already seen a few examples of using ",(0,i.kt)("inlineCode",{parentName:"p"},"cat")," to write a file to ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout"),"."),(0,i.kt)("p",null,"A lot of the time we don't need to use ",(0,i.kt)("inlineCode",{parentName:"p"},"cat")," many programs accept the path of a file as a parameter, meaning we can just tell the program to open the file directly. For example, we could count the number of lines in a file like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ cat ~/effective-shell/text/simpsons-characters.txt | wc -l\n\n 14\n")),(0,i.kt)("p",null,"Or more simply, like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ wc -l ~/effective-shell/text/simpsons-characters.txt\n 14 /Users/dwmkerr/effective-shell/text/simpsons-characters.txt\n")),(0,i.kt)("p",null,"In this case, we've passed the file path as an argument to the ",(0,i.kt)("inlineCode",{parentName:"p"},"wc")," (",(0,i.kt)("em",{parentName:"p"},"word, line, character and byte count"),") program. But be aware - not all programs use the same convention or parameter names!"),(0,i.kt)("p",null,"Now here's a cool trick. Type ",(0,i.kt)("inlineCode",{parentName:"p"},"rev < /dev/stdin"),", then enter some text and hit ",(0,i.kt)("inlineCode",{parentName:"p"},"^D")," or ",(0,i.kt)("inlineCode",{parentName:"p"},"^C")," when done. You should see something like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ rev < /dev/stdin\nRed Rum\nmuR deR\n")),(0,i.kt)("p",null,"What's going on here? Remember we mentioned that ",(0,i.kt)("inlineCode",{parentName:"p"},"stdin")," is a special stream which represents input and that it lives at ",(0,i.kt)("inlineCode",{parentName:"p"},"/dev/stdin"),"? This little trick uses ",(0,i.kt)("em",{parentName:"p"},"redirection")," to redirect the ",(0,i.kt)("inlineCode",{parentName:"p"},"stdin")," file to the ",(0,i.kt)("inlineCode",{parentName:"p"},"rev")," (",(0,i.kt)("em",{parentName:"p"},"reverse"),") command."),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"<")," operator redirects the standard input of a program to come from the given file. We could also have written ",(0,i.kt)("inlineCode",{parentName:"p"},"cat /dev/stdin | rev"),". Or just enter ",(0,i.kt)("inlineCode",{parentName:"p"},"rev")," and type in the input we want to reverse!"),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"The Clipboard")),(0,i.kt)("p",null,"In ",(0,i.kt)("a",{parentName:"p",href:"/part-1-transitioning-to-the-shell/become-a-clipboard-gymnast/"},"Chapter 4 - Becoming a Clipboard Gymnast")," we saw a trick to remove formatting from text in the clipboard. Here's a similar trick to reverse the contents of the clipboard:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ pbpaste | rev | pbcopy\n")),(0,i.kt)("p",null,"This pipeline pastes the contents of the clipboard to ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout"),", which is piped to ",(0,i.kt)("inlineCode",{parentName:"p"},"rev")," (reversing the text) and then pipes the output to ",(0,i.kt)("inlineCode",{parentName:"p"},"pbcopy"),", which copies the results to the clipboard",(0,i.kt)("sup",{parentName:"p",id:"fnref-2"},(0,i.kt)("a",{parentName:"sup",href:"#fn-2",className:"footnote-ref"},"2")),"."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"Filtered Input")),(0,i.kt)("p",null,"This is a trick a friend shared with me. He works with data scientists and whenever he shows them this command they love it!"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ head -n 100 100GBFile.csv > 100linefile.csv\n")),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"head")," (",(0,i.kt)("em",{parentName:"p"},"display first lines of a file"),") command in this case just grabs the first 100 lines of a file and puts it straight into a smaller, more manageable file. We'll see what the ",(0,i.kt)("inlineCode",{parentName:"p"},">")," symbol (the ",(0,i.kt)("em",{parentName:"p"},"redirection")," symbol) means in the section lower down on Standard Output."),(0,i.kt)("p",null,"You can also use ",(0,i.kt)("inlineCode",{parentName:"p"},"tail")," in the same way to get the ",(0,i.kt)("em",{parentName:"p"},"last")," lines from a file. And if you are a more advanced user, you might use something like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ grep -C 5 error /var/log/mylogfile.txt | less\n")),(0,i.kt)("p",null,"We'll see all of these commands as we go through the book, but this very cool trick uses the ",(0,i.kt)("inlineCode",{parentName:"p"},"grep")," (",(0,i.kt)("em",{parentName:"p"},"file pattern searcher"),") command to search for the text ",(0,i.kt)("inlineCode",{parentName:"p"},"error")," in the file ",(0,i.kt)("inlineCode",{parentName:"p"},"/var/log/mylogfile.txt"),", shows five lines of ",(0,i.kt)("em",{parentName:"p"},"context")," (",(0,i.kt)("inlineCode",{parentName:"p"},"-C 5"),"), which are the lines before and after the match, then puts the result into your pager! We'll see the pager just below. We'll do a lot of ",(0,i.kt)("inlineCode",{parentName:"p"},"grep"),"-ing as we go through the book so don't worry if this looks a little confusing for now."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"Many More!")),(0,i.kt)("p",null,"We've only scratched the surface - almost any program will write to the standard output, meaning it can be the input for any pipeline you can imagine!"),(0,i.kt)("h2",{id:"common-patterns---standard-output"},"Common Patterns - Standard Output"),(0,i.kt)("p",null,"Now let's look at some of the things we can do with the standard output:"),(0,i.kt)("img",{src:n(4146).Z,alt:"Diagram: Output Examples",width:"1024px"}),(0,i.kt)("p",null,"Some of these outputs are things we've seen before, but let's do a quick revision."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"Display")),(0,i.kt)("p",null,"This is what we've been doing a lot of so far. When you are working with the shell ",(0,i.kt)("em",{parentName:"p"},"interactively")," this makes a lot of sense."),(0,i.kt)("p",null,"If you have jobs which run in the ",(0,i.kt)("em",{parentName:"p"},"background")," (or on a timer, such as backup jobs which run nightly), you might not actually have a terminal attached to the program to see the output, in which case you'll likely write to a file."),(0,i.kt)("p",null,"What about if you have a ",(0,i.kt)("em",{parentName:"p"},"lot")," of output? It can be quite inconvenient to have to scroll through the terminal (or impossible, depending on the system you are on). In this case use a ",(0,i.kt)("em",{parentName:"p"},"pager"),". A pager is a program which makes it possible to interactively ",(0,i.kt)("em",{parentName:"p"},"page")," through output in the shell, scrolling up and down, searching and so on."),(0,i.kt)("p",null,"Try this out as an example:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"ls /usr/bin /usr/local/bin /usr/sbin | less\n")),(0,i.kt)("p",null,"You'll see something like this:"),(0,i.kt)("img",{src:n(1377).Z,alt:"Screenshot: Less Example",width:"800px"}),(0,i.kt)("p",null,"This long list of files would be hard to search through if it was printed directly to the shell, but in the pager we can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"d")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"u")," keys to go ",(0,i.kt)("em",{parentName:"p"},"down")," and ",(0,i.kt)("em",{parentName:"p"},"up"),", or the ",(0,i.kt)("inlineCode",{parentName:"p"},"/")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"?")," keys to search forwards or backwards."),(0,i.kt)("p",null,"Piping into your pager is a really useful trick - you can read more about pagers in ",(0,i.kt)("a",{parentName:"p",href:"/part-1-transitioning-to-the-shell/getting-help/"},"Chapter 5 - Getting Help"),"."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"File")),(0,i.kt)("p",null,"The shell has a built in operator which will pipe the standard output of a program and write it to a file. It is the ",(0,i.kt)("inlineCode",{parentName:"p"},">")," or ",(0,i.kt)("em",{parentName:"p"},"redirection")," operator:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ echo "Here\'s some data" > some_file.txt\n')),(0,i.kt)("p",null,"It's as easy as that! Note that this will ",(0,i.kt)("em",{parentName:"p"},"overwrite")," anything already in the file."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"Append")),(0,i.kt)("p",null,"What if you don't want to overwrite a file, but instead just add a new line? The ",(0,i.kt)("inlineCode",{parentName:"p"},">>")," or ",(0,i.kt)("em",{parentName:"p"},"append redirection")," operator:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ echo "Tuesday was good" >> diary.txt\n$ echo "Wednesday was better!" >> diary.txt\n$ echo "Thursday suuucks" >> diary.txt\n$ cat diary.txt\nTuesday was good\nWednesday was better!\nThursday suuucks\n')),(0,i.kt)("p",null,"This example writes each line in turn to the ",(0,i.kt)("inlineCode",{parentName:"p"},"diary.txt")," file, appending the text to the end of the file (and creating it if it doesn't already exist)."),(0,i.kt)("p",null,"Appending to a file is extremely useful for circumstances where you might want to build or update a log of events over time."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"Pipe")),(0,i.kt)("p",null,"This is what we've spent most of this chapter looking at - to simply pipe the standard output to the standard input of another program!"),(0,i.kt)("p",null,"In this case, the output of our program becomes the input of the next one in the pipeline."),(0,i.kt)("h2",{id:"common-patterns---standard-error"},"Common Patterns - Standard Error"),(0,i.kt)("p",null,"We haven't actually seen ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr")," in action yet. Let's see how it works."),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ mkdir ~/effective-shell/new-folder\n$ mkdir ~/effective-shell/new-folder\nmkdir: /home/dwmkerr/effective-shell/new-folder: File exists\n")),(0,i.kt)("p",null,"In the first call to ",(0,i.kt)("inlineCode",{parentName:"p"},"mkdir"),", the folder is created successfully. In the second call, we get an error. Now let's try and use this output and make it louder - making all of the text uppercase."),(0,i.kt)("p",null,"There are lots of ways to make text uppercase in the shell, let's use the ",(0,i.kt)("inlineCode",{parentName:"p"},"tr")," (",(0,i.kt)("em",{parentName:"p"},"translate characters"),") program. Here's an example of how it works:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ echo 'Be quiet, this is a library!' | tr '[:lower:]' '[:upper:]'\nBE QUIET, THIS IS A LIBRARY!\n")),(0,i.kt)("p",null,"Now let's use it to shout out our error message:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ mkdir ~/effective-shell/new-folder | tr '[:lower:]' '[:upper:]'\nmkdir: /home/dwmkerr/effective-shell/new-folder: File exists\n")),(0,i.kt)("p",null,"In this case the output has not been made uppercase. What's going on?"),(0,i.kt)("p",null,"To understand, let's quickly review the three streams:"),(0,i.kt)("img",{src:n(6699).Z,alt:"Diagram: stdin/stdout/stderr",width:"1024px"}),(0,i.kt)("p",null,"When we are in the shell, the shell automatically writes the ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr")," stream to the screen. But the shell's ",(0,i.kt)("em",{parentName:"p"},"pipe")," operator pipes ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," only - it is ",(0,i.kt)("em",{parentName:"p"},"not")," piping our error output. And the ",(0,i.kt)("inlineCode",{parentName:"p"},"mkdir")," command is writing this error message to ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr"),"."),(0,i.kt)("p",null,"The command we ran before:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"mkdir ~/effective-shell/new-folder | tr '[:lower:]' '[:upper:]'\n")),(0,i.kt)("p",null,"Actually looks like this:"),(0,i.kt)("img",{src:n(930).Z,alt:"Diagram: Standard Error",width:"1024px"}),(0,i.kt)("p",null,"The pipe ",(0,i.kt)("em",{parentName:"p"},"has")," piped the standard output to the ",(0,i.kt)("inlineCode",{parentName:"p"},"tr")," program. But there is no standard output - the error message was written to ",(0,i.kt)("em",{parentName:"p"},"standard error")," instead. The shell has still written it to the screen for us, but has not piped it to the ",(0,i.kt)("inlineCode",{parentName:"p"},"tr")," program."),(0,i.kt)("p",null,"So how do we deal with ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr"),"? Here are some common options:"),(0,i.kt)("img",{src:n(3523).Z,alt:"Diagram: Standard Error Options",width:"1024px"}),(0,i.kt)("p",null,"Now this might be the ",(0,i.kt)("strong",{parentName:"p"},"ah-ha!")," moment if you have done some shell scripting before - some of these obscure sequences like ",(0,i.kt)("inlineCode",{parentName:"p"},"2>&1")," might look familiar (even if it is just the thing you know you always have to Google to get right!)."),(0,i.kt)("p",null,"Let's take a quick look at some of these options."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"To Standard Output")),(0,i.kt)("p",null,"If we want to be able to pipe the error message to another command, we can use another redirection trick - we can redirect ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr")," to ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout"),"."),(0,i.kt)("p",null,"The characters ",(0,i.kt)("inlineCode",{parentName:"p"},"2>&1")," look really obscure - let's break it down:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},"Take the file with descriptor ",(0,i.kt)("inlineCode",{parentName:"li"},"2")," - which is ",(0,i.kt)("em",{parentName:"li"},"standard error")),(0,i.kt)("li",{parentName:"ul"},"Redirect it with the redirect symbol ",(0,i.kt)("inlineCode",{parentName:"li"},">")," - we saw this in the earlier section"),(0,i.kt)("li",{parentName:"ul"},"Redirect it into the file with descriptor (",(0,i.kt)("inlineCode",{parentName:"li"},"&"),") ",(0,i.kt)("inlineCode",{parentName:"li"},"1")," - which is ",(0,i.kt)("em",{parentName:"li"},"standard output"))),(0,i.kt)("p",null,"Remember, there are three 'magic' files each process has access to:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"stdin"),", the standard input, which has the file descriptor ",(0,i.kt)("inlineCode",{parentName:"li"},"0")),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"stdout"),", the standard output, which has the file descriptor ",(0,i.kt)("inlineCode",{parentName:"li"},"1")),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"stderr"),", the standard error, which has the file descriptor ",(0,i.kt)("inlineCode",{parentName:"li"},"2"))),(0,i.kt)("p",null,"File descriptors are just numbers the operating system uses to keep track of files. When a program opens a 'normal' file, it'll get a new file descriptor. Here's a little example:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"python <<EOF\nimport os\nfor r in range(3): print(os.open('/dev/random', os.O_RDONLY))\nEOF\n")),(0,i.kt)("p",null,"This code uses redirection (see how useful it is?) to pipe a small Python script into the Python program, which writes the results to ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout"),". You will probably see the following output:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"3\n4\n5\n")),(0,i.kt)("p",null,"It doesn't really matter whether you know Python or not (and there weird looking ",(0,i.kt)("inlineCode",{parentName:"p"},"EOF")," is a ",(0,i.kt)("em",{parentName:"p"},"heredoc")," which we have a whole chapter on later). The script is just a way of showing the file descriptors that the operating system gives me when I try to open three files (each time I open the same file, the magic ",(0,i.kt)("inlineCode",{parentName:"p"},"/dev/random")," file which just contains random data)."),(0,i.kt)("p",null,"The interesting thing is that the descriptors in my program start from ",(0,i.kt)("inlineCode",{parentName:"p"},"3")," and go upwards - that's because ",(0,i.kt)("inlineCode",{parentName:"p"},"0"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"1")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"2")," are already in use, for ",(0,i.kt)("inlineCode",{parentName:"p"},"stdin"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr"),"!"),(0,i.kt)("p",null,"So to make our error message go through the ",(0,i.kt)("inlineCode",{parentName:"p"},"tr")," command, we can redirect ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr")," to ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout"),", which means the error message will go to ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," and then be piped to ",(0,i.kt)("inlineCode",{parentName:"p"},"tr"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ mkdir ~/effective-shell/new-folder 2>&1 | tr '[:lower:]' '[:upper:]'\nMKDIR: /HOME/DWMKERR/PLAYGROUND/NEW-FOLDER: FILE EXISTS\n")),(0,i.kt)("p",null,"Visually, what is happening is this:"),(0,i.kt)("img",{src:n(9387).Z,alt:"Diagram: stderr redirect",width:"1024px"}),(0,i.kt)("p",null,"If you can wrap your head around this, the other options we showed for ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr")," might start to make a little more sense."),(0,i.kt)("p",null,"A nice trick to remember the slightly obscure ampersand ",(0,i.kt)("inlineCode",{parentName:"p"},"&")," which references a file descriptor - if you were to write this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cat some-file-that-might-not-exist 2>1\n")),(0,i.kt)("p",null,"What would happen is that the shell would write ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr")," to a ",(0,i.kt)("em",{parentName:"p"},"new file")," with the name ",(0,i.kt)("inlineCode",{parentName:"p"},"1"),"! Why don't we need an ampersand ",(0,i.kt)("em",{parentName:"p"},"before")," the ",(0,i.kt)("inlineCode",{parentName:"p"},">")," symbol, only for the file descriptor afterwards? This is just because the shell only supports redirecting file descriptors, so an additional ampersand would be superfluous."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"To a File")),(0,i.kt)("p",null,"Before, we redirected to ",(0,i.kt)("inlineCode",{parentName:"p"},"&2"),", which is 'the file with descriptor ",(0,i.kt)("inlineCode",{parentName:"p"},"2"),". We can also use a similar trick to redirect to any arbitrary file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"mkdir ~/effective-shell/new-folder 2>./errors.txt\n")),(0,i.kt)("p",null,"This command just redirects all of the errors (remember, ",(0,i.kt)("inlineCode",{parentName:"p"},"2")," is ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr"),") to a file called ",(0,i.kt)("inlineCode",{parentName:"p"},"./errors.txt"),"."),(0,i.kt)("p",null,"This is quite a common trick - run the program, but log the errors to a file for later review."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"To Nowhere")),(0,i.kt)("p",null,"What if we just don't want to see the errors at all? Well there's a special file called ",(0,i.kt)("inlineCode",{parentName:"p"},"/dev/null")," which we can use for this. When we write to this file, the operating system just discards the input. In fact, it exists for just this kind of purpose!"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"mkdir ~/effective-shell/new-folder 2>/dev/null\n")),(0,i.kt)("p",null,"This just redirects all errors to the black hole of ",(0,i.kt)("inlineCode",{parentName:"p"},"/dev/null")," - we won't see them on the screen or anywhere else. This is a common way to 'silence' errors",(0,i.kt)("sup",{parentName:"p",id:"fnref-3"},(0,i.kt)("a",{parentName:"sup",href:"#fn-3",className:"footnote-ref"},"3"))," in shell commands."),(0,i.kt)("p",null,"Notice how we're starting to see patterns? This is just redirection, the same tricks we saw for ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout"),", but we're explicitly redirecting ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr")," (file descriptor ",(0,i.kt)("inlineCode",{parentName:"p"},"2"),"). If we don't tell the shell ",(0,i.kt)("em",{parentName:"p"},"what")," to redirect, it assumes ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," by default."),(0,i.kt)("p",null,"So if we can redirect, can we append too?"),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"Append")),(0,i.kt)("p",null,"Yes! Just like we did with ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout"),", there's nothing stopping us ",(0,i.kt)("em",{parentName:"p"},"appending")," to a file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"mkdir ~/effective-shell/new-folder 2>>./all-errors.log\n")),(0,i.kt)("p",null,"Just like before, we use ",(0,i.kt)("inlineCode",{parentName:"p"},">>")," which means ",(0,i.kt)("em",{parentName:"p"},"append")," (rather than ",(0,i.kt)("em",{parentName:"p"},"overwrite or create"),")."),(0,i.kt)("p",null,(0,i.kt)("strong",{parentName:"p"},"All to a File")),(0,i.kt)("p",null,"This is a really important subtlety. If you want to write ",(0,i.kt)("em",{parentName:"p"},"both")," ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," and ",(0,i.kt)("inlineCode",{parentName:"p"},"stderr")," to a file, you might try this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"ls /usr/bin /nothing 2>&1 > all-output.txt\n")),(0,i.kt)("p",null,"If you run this command, you'll get ",(0,i.kt)("inlineCode",{parentName:"p"},"stdout")," written to ",(0,i.kt)("inlineCode",{parentName:"p"},"all-output.txt"),", but the error message ",(0,i.kt)("inlineCode",{parentName:"p"},"cannot access '/nothing'")," is written to the screen, not the file. Why is this?"),(0,i.kt)("p",null,"Bash (and most bash-like shells) process redirections from ",(0,i.kt)("em",{parentName:"p"},"left to right"),", and when we redirect we ",(0,i.kt)("em",{parentName:"p"},"duplicate")," the source. So breaking this down:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"2>&1")," - duplicate file descriptor ",(0,i.kt)("inlineCode",{parentName:"li"},"2")," (",(0,i.kt)("inlineCode",{parentName:"li"},"stderr"),") and write it to ",(0,i.kt)("inlineCode",{parentName:"li"},"1")," - which is ",(0,i.kt)("em",{parentName:"li"},"currently the terminal"),"!"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"> all-output.txt")," - duplicate file descriptor ",(0,i.kt)("inlineCode",{parentName:"li"},"1")," (",(0,i.kt)("inlineCode",{parentName:"li"},"stdout"),") and write it to a file called ",(0,i.kt)("inlineCode",{parentName:"li"},"all-output.txt"))),(0,i.kt)("p",null,"To write ",(0,i.kt)("em",{parentName:"p"},"everything")," to the file, try do this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"ls /usr/bin /nothing > all-output.txt 2>&1\n")),(0,i.kt)("p",null,"This will work. Breaking it down:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},"Redirect ",(0,i.kt)("inlineCode",{parentName:"li"},"stdout")," to the file ",(0,i.kt)("inlineCode",{parentName:"li"},"all-output.txt")),(0,i.kt)("li",{parentName:"ul"},"Now redirect ",(0,i.kt)("inlineCode",{parentName:"li"},"stderr")," to ",(0,i.kt)("inlineCode",{parentName:"li"},"stdout")," - which by this point ",(0,i.kt)("em",{parentName:"li"},"has already been redirected to a file"))),(0,i.kt)("p",null,"This can be tough to remember so it's worth trying it out",(0,i.kt)("sup",{parentName:"p",id:"fnref-4"},(0,i.kt)("a",{parentName:"sup",href:"#fn-4",className:"footnote-ref"},"4")),". There are many variations you can play with and we'll see more as we go through the book."),(0,i.kt)("h2",{id:"one-last-trick---the-t-pipe"},"One Last Trick - The T Pipe"),(0,i.kt)("p",null,"This is a long chapter, but I can't talk about pipelines without briefly mentioning the T pipe. Check out this command:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cat ~/effective-shell/text/simpsons-characters.txt | sort | tee sorted.txt | uniq | grep '^A'\n")),(0,i.kt)("p",null,"This command sorts the list of Simpsons characters, removes duplicates and filters down to ones which start with the letter ",(0,i.kt)("inlineCode",{parentName:"p"},"A"),". And it has the ",(0,i.kt)("inlineCode",{parentName:"p"},"tee")," command in the middle. What does this do?"),(0,i.kt)("p",null,"Well the ",(0,i.kt)("inlineCode",{parentName:"p"},"tee")," command is like a T-pipe in plumbing - it lets the stream of data go in two directions! The ",(0,i.kt)("inlineCode",{parentName:"p"},"sorted.txt")," file contains the sets of characters ",(0,i.kt)("em",{parentName:"p"},"after")," the sort operation, but before the unique and filter operation. Visually, it does this:"),(0,i.kt)("img",{src:n(1658).Z,alt:"Diagram: Tee",width:"1024px"}),(0,i.kt)("p",null,"As soon as you visualise a T-pipe it's easy to remember this useful command! You might use it in more complex pipelines or other scenarios to write things to a file which would otherwise go straight to another program or just the display."),(0,i.kt)("h2",{id:"thinking-in-pipelines"},"Thinking in Pipelines"),(0,i.kt)("p",null,"Once you get comfortable with pipelines, a whole world of possibilities open up."),(0,i.kt)("p",null,"Just the day before I wrote this chapter, I had to find out how many unique data points were in a data file, which also included empty lines and comments, it took less than a minute to quickly build this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"cat data.dat | sort | uniq | grep -v '^#' | wc -l\n")),(0,i.kt)("p",null,"I didn't have to find a special program which does exactly what I needed",(0,i.kt)("sup",{parentName:"p",id:"fnref-5"},(0,i.kt)("a",{parentName:"sup",href:"#fn-5",className:"footnote-ref"},"5"))," - I just incrementally built a pipeline. Each section I added one by one, writing to the screen each time, until I had it working. The thought process was:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"cat data.dat")," - OK, first I need to write out the file"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"sort")," - now I can sort it, that'll put all the blank lines together"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"uniq")," - this'll remove all of those ",(0,i.kt)("em",{parentName:"li"},"duplicate")," blank lines, although it still leaves one blank one at the top!"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"grep -v '^#'")," - this should get rid of all the lines which start with ",(0,i.kt)("inlineCode",{parentName:"li"},"#")),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"wc -l")," - this'll count the number of lines I'm left with")),(0,i.kt)("p",null,"Now there's probably better ways, and this has an oddity which is that if there are blank lines it'll remove all but one of them (although that would be quick to fix), but it gave me my quick and dirty answer in less than a minute."),(0,i.kt)("p",null,"Of course, as things get more complex you might want to build scripts, or use a programming language, or other methods, but this ",(0,i.kt)("em",{parentName:"p"},"Unix Philosophy")," (which we'll talk about more as we continue) of having lots of small, simple programs which we can chain together can be immensely powerful."),(0,i.kt)("h2",{id:"summary"},"Summary"),(0,i.kt)("p",null,"We'll see pipelines again and again. The standard streams, redirection, pipelines and all of the tricks we've introduced in this chapter are fundamental not only to using the shell effectively, but really understanding how computer programs work."),(0,i.kt)("p",null,"Don't be worried if this feels like a lot to take in - we'll see more and more examples in later chapters which will help reinforce these concepts. If you find yourself struggling later you might want to quickly review this chapter, because we introduced a lot!"),(0,i.kt)("p",null,"In this chapter we looked at:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},"How each program has access to three 'standard' streams - one for input, one for output and one for reporting errors"),(0,i.kt)("li",{parentName:"ul"},"The standard input stream is available as a file at ",(0,i.kt)("inlineCode",{parentName:"li"},"/dev/stdin"),", is often called ",(0,i.kt)("inlineCode",{parentName:"li"},"stdin")," in programming languages, and always has the special file descriptor ",(0,i.kt)("inlineCode",{parentName:"li"},"0")),(0,i.kt)("li",{parentName:"ul"},"The standard output stream is available as a file at ",(0,i.kt)("inlineCode",{parentName:"li"},"/dev/stdout"),", is often called ",(0,i.kt)("inlineCode",{parentName:"li"},"stdout")," in programming languages, and always has the special file descriptor ",(0,i.kt)("inlineCode",{parentName:"li"},"1")),(0,i.kt)("li",{parentName:"ul"},"The standard error stream is available as a file at ",(0,i.kt)("inlineCode",{parentName:"li"},"/dev/stderr"),", is often called ",(0,i.kt)("inlineCode",{parentName:"li"},"stderr")," in programming languages, and always has the special file descriptor ",(0,i.kt)("inlineCode",{parentName:"li"},"2")),(0,i.kt)("li",{parentName:"ul"},"The ",(0,i.kt)("inlineCode",{parentName:"li"},"Ctrl+D")," sequence means 'end of transmission' - we can use it to signal that we have completed putting our input into ",(0,i.kt)("inlineCode",{parentName:"li"},"stdin"),"..."),(0,i.kt)("li",{parentName:"ul"},"...but the ",(0,i.kt)("inlineCode",{parentName:"li"},"Ctrl+C")," sequence means 'interrupt' and is normally used to force a program to close"),(0,i.kt)("li",{parentName:"ul"},"We can ",(0,i.kt)("em",{parentName:"li"},"pipe")," the output of one program to the input of another with the pipe ",(0,i.kt)("inlineCode",{parentName:"li"},"|")," symbol"),(0,i.kt)("li",{parentName:"ul"},"We can ",(0,i.kt)("em",{parentName:"li"},"redirect")," a file to the standard input of a program with the ",(0,i.kt)("inlineCode",{parentName:"li"},"<")," operator"),(0,i.kt)("li",{parentName:"ul"},"We can ",(0,i.kt)("em",{parentName:"li"},"redirect")," the standard output of a program to create or overwrite a file with the ",(0,i.kt)("inlineCode",{parentName:"li"},">")," operator"),(0,i.kt)("li",{parentName:"ul"},"We can ",(0,i.kt)("em",{parentName:"li"},"redirect")," the standard output of a program to create or append to a file with the ",(0,i.kt)("inlineCode",{parentName:"li"},">>")," operator"),(0,i.kt)("li",{parentName:"ul"},"We can redirect the standard error of a program to its standard output with ",(0,i.kt)("inlineCode",{parentName:"li"},"2>&1")),(0,i.kt)("li",{parentName:"ul"},"We can redirect the standard error of a program to another file (such as the 'null' file) with ",(0,i.kt)("inlineCode",{parentName:"li"},"2>/dev/null")),(0,i.kt)("li",{parentName:"ul"},"We can redirect the standard error of a program to create or append to a file, just like with standard output, using the ",(0,i.kt)("inlineCode",{parentName:"li"},">>")," operator")),(0,i.kt)("p",null,"We also briefly saw some commands:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"sort")," sorts text"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"sed")," can replace content in text"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"tr")," can replace parts of text"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"wc")," can count words or lines of text"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"tee")," takes the input stream and sends it straight to the output, but also to a file (like a T-pipe in plumbing)"),(0,i.kt)("li",{parentName:"ul"},(0,i.kt)("inlineCode",{parentName:"li"},"grep")," can filter lines")),(0,i.kt)("p",null,"These programs can do a lot more and are workhorses we'll see in more detail through the book."),(0,i.kt)("p",null,"There are a few chapters which are planned to come later which go into detail on some of the concepts we only briefly touched on:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},"Writing Good Programs - How to write programs which use ",(0,i.kt)("inlineCode",{parentName:"li"},"stdin"),", ",(0,i.kt)("inlineCode",{parentName:"li"},"stdout")," and ",(0,i.kt)("inlineCode",{parentName:"li"},"stderr")," sensibly"),(0,i.kt)("li",{parentName:"ul"},"The Unix Philosophy - Why we have so many small simple programs which we can pipe together"),(0,i.kt)("li",{parentName:"ul"},"Streams in Detail - How streams like ",(0,i.kt)("inlineCode",{parentName:"li"},"stdin")," actually work, especially with things like line endings, command sequences like ",(0,i.kt)("inlineCode",{parentName:"li"},"^D")," and so on"),(0,i.kt)("li",{parentName:"ul"},"Signals - A little more on Signals (such as ",(0,i.kt)("inlineCode",{parentName:"li"},"^C")," and ",(0,i.kt)("inlineCode",{parentName:"li"},"^D"),")")),(0,i.kt)("p",null,"When these chapters are published I'll update the links here. If you want to be updated when new chapters are published, you can ",(0,i.kt)("a",{parentName:"p",href:"https://effective-shell.com"},"Join the Mailing Lits on the Homepage"),"."),(0,i.kt)("div",{className:"footnotes"},(0,i.kt)("hr",{parentName:"div"}),(0,i.kt)("ol",{parentName:"div"},(0,i.kt)("li",{parentName:"ol",id:"fn-1"},"Technically there is another layer here, which is the ",(0,i.kt)("inlineCode",{parentName:"li"},"tty"),". You can see this by running ",(0,i.kt)("inlineCode",{parentName:"li"},"tty")," in the shell. We'll talk more about this in the ",(0,i.kt)("a",{parentName:"li",href:"/part-2-core-skills/what-is-a-shell"},"Interlude - What is a Shell")," section.",(0,i.kt)("a",{parentName:"li",href:"#fnref-1",className:"footnote-backref"},"\u21a9")),(0,i.kt)("li",{parentName:"ol",id:"fn-2"},"Check ",(0,i.kt)("a",{parentName:"li",href:"/part-1-transitioning-to-the-shell/become-a-clipboard-gymnast/"},"Chapter 4 - Becoming a Clipboard Gymnast")," for how to do this on a Linux or Windows machine.",(0,i.kt)("a",{parentName:"li",href:"#fnref-2",className:"footnote-backref"},"\u21a9")),(0,i.kt)("li",{parentName:"ol",id:"fn-3"},"Although always use tricks like this with caution! If we had a ",(0,i.kt)("em",{parentName:"li"},"different")," error, perhaps one we really do want to know about, we would lose the message in this case.",(0,i.kt)("a",{parentName:"li",href:"#fnref-3",className:"footnote-backref"},"\u21a9")),(0,i.kt)("li",{parentName:"ol",id:"fn-4"},"There is a very detailed explanation of this behaviour at ",(0,i.kt)("a",{parentName:"li",href:"https://linuxnewbieguide.org/21-and-understanding-other-shell-scripts-idioms/"},"https://linuxnewbieguide.org/21-and-understanding-other-shell-scripts-idioms/"),".",(0,i.kt)("a",{parentName:"li",href:"#fnref-4",className:"footnote-backref"},"\u21a9")),(0,i.kt)("li",{parentName:"ol",id:"fn-5"},"With the correct options, ",(0,i.kt)("inlineCode",{parentName:"li"},"sed")," could likely do this in a single operation, but I'd probably spend a lot longer Googling the right options for it!",(0,i.kt)("a",{parentName:"li",href:"#fnref-5",className:"footnote-backref"},"\u21a9")))))}d.isMDXComponent=!0},586:(e,t,n)=>{n.d(t,{Z:()=>a});const a=n.p+"assets/images/diagram-cat-sort-uniq-pipeline-8c8d76566f351b4b9b900dde52af86b3.png"},3657:(e,t,n)=>{n.d(t,{Z:()=>a});const a=n.p+"assets/images/diagram-input-examples-e014dd4998bee0b50a94849ad55b01ce.png"},7007:(e,t,n)=>{n.d(t,{Z:()=>a});const a=n.p+"assets/images/diagram-input-program-output-fb71da951178e72bf4504bd292743d1c.png"},4146:(e,t,n)=>{n.d(t,{Z:()=>a});const a=n.p+"assets/images/diagram-output-examples-27e30c4a4036b2591e10e8c4fca7dc73.png"},4956:(e,t,n)=>{n.d(t,{Z:()=>a});const a=n.p+"assets/images/diagram-shell-keyboard-terminal-0475940cdf40bbcc8a329c090aa9e76a.png"},1023:(e,t,n)=>{n.d(t,{Z:()=>a});const a=n.p+"assets/images/diagram-sort-040db6e92c70e60612c9f711e81c9612.png"},3523:(e,t,n)=>{n.d(t,{Z:()=>a});const 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o=n.length,i=new Array(o);i[0]=m;var l={};for(var s in t)hasOwnProperty.call(t,s)&&(l[s]=t[s]);l.originalType=e,l[c]="string"==typeof e?e:r,i[1]=l;for(var u=2;u<o;u++)i[u]=n[u];return a.createElement.apply(null,i)}return a.createElement.apply(null,n)}m.displayName="MDXCreateElement"},1773:(e,t,n)=>{n.r(t),n.d(t,{assets:()=>s,contentTitle:()=>i,default:()=>c,frontMatter:()=>o,metadata:()=>l,toc:()=>u});var a=n(7462),r=(n(7294),n(3905));const o={title:"Functions, Parameters and Error Handling",slug:"/part-4-shell-scripting/functions-parameters-and-error-handling"},i=void 0,l={unversionedId:"shell-scripting/functions-parameters-and-error-handling/index",id:"shell-scripting/functions-parameters-and-error-handling/index",title:"Functions, Parameters and Error Handling",description:"The shell allows you to create functions - a set of commands that you can call at any time. In this chapter we'll see how to create functions and how function parameters and script parameters are handled. We will also look at status codes for commands and scripts and error handling.",source:"@site/docs/04-shell-scripting/22-functions-parameters-and-error-handling/index.md",sourceDirName:"04-shell-scripting/22-functions-parameters-and-error-handling",slug:"/part-4-shell-scripting/functions-parameters-and-error-handling",permalink:"/part-4-shell-scripting/functions-parameters-and-error-handling",draft:!1,editUrl:"https://github.com/dwmkerr/effective-shell/edit/main/docs/04-shell-scripting/22-functions-parameters-and-error-handling/index.md",tags:[],version:"current",frontMatter:{title:"Functions, Parameters and Error Handling",slug:"/part-4-shell-scripting/functions-parameters-and-error-handling"},sidebar:"sidebar",previous:{title:"Loops and working with Files and Folders",permalink:"/part-4-shell-scripting/loops-and-working-with-files-and-folders"},next:{title:"Useful Patterns for Shell Scripts",permalink:"/part-4-shell-scripting/useful-patterns-for-shell-scripts"}},s={},u=[{value:"Creating a Function index ",id:"creating-a-function--index-",level:2},{value:"Variables in Functions",id:"variables-in-functions",level:2},{value:"Variable Scoping index",id:"variable-scoping-index",level:3},{value:"Passing Parameters to Functions",id:"passing-parameters-to-functions",level:2},{value:"Parameter Variables",id:"parameter-variables",level:3},{value:"Parameter Shifting",id:"parameter-shifting",level:3},{value:"Return Values",id:"return-values",level:2},{value:"Writing Results to Stdout",id:"writing-results-to-stdout",level:2},{value:"Dealing with Output in Commands",id:"dealing-with-output-in-commands",level:3},{value:"Returning Status Codes",id:"returning-status-codes",level:3},{value:"Error Handling",id:"error-handling",level:2},{value:"The Function Keyword",id:"the-function-keyword",level:2},{value:"Parameters and Status Codes for Scripts",id:"parameters-and-status-codes-for-scripts",level:2},{value:"Updating the 'common' Command",id:"updating-the-common-command",level:2},{value:"Summary",id:"summary",level:2}],p={toc:u};function c(e){let{components:t,...n}=e;return(0,r.kt)("wrapper",(0,a.Z)({},p,n,{components:t,mdxType:"MDXLayout"}),(0,r.kt)("p",null,"The shell allows you to create ",(0,r.kt)("em",{parentName:"p"},"functions")," - a set of commands that you can call at any time. In this chapter we'll see how to create functions and how function parameters and script parameters are handled. We will also look at status codes for commands and scripts and error handling."),(0,r.kt)("h2",{id:"creating-a-function--index-"},"Creating a Function "),(0,r.kt)("p",null,"A ",(0,r.kt)("em",{parentName:"p"},"function")," has the following structure:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"<function-name> {\n <function-command 1>\n <function-command 2>\n <function-command n>\n}\n")),(0,r.kt)("p",null,"First we specify the name of the function. Then between a set of opening and closing curly braces, we list the commands that should be executed when we call the function."),(0,r.kt)("p",null,"Let's take a look at a very simple function in action:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'title() {\n echo "My Script version 1.0"\n}\n')),(0,r.kt)("p",null,"This script defines a very simple function called ",(0,r.kt)("em",{parentName:"p"},"title")," that prints out a message. We call the function in the same way we would call any command in the shell, by simply writing the name of the command and hitting enter. Here's how we would call the function:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},'$ title\nMy Script version 1.0"\n')),(0,r.kt)("p",null,"Easy! Functions let you structure commands into logical blocks and can help make your scripts easier to read and manage."),(0,r.kt)("h2",{id:"variables-in-functions"},"Variables in Functions"),(0,r.kt)("p",null,"A function can read and write to any variables in the current shell session. Here's an example:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'# Set some variables.\ntitle="My Cool Script"\nversion="1.2"\nsucceeded=0\n\n# Create a function that writes a message and changes a variable.\ntitle() {\n # Note that we can read variables...\n title_message="${title} - version ${version}"\n echo "${title_message}"\n\n # ...and set them as well.\n succeeded=1\n}\n\n# Show the value of \'succeeded\' before and after the function call.\necho "Succeeded: ${succeeded}"\ntitle\necho "Succeeded: ${succeeded}"\necho "Title Message: ${title_message}"\n')),(0,r.kt)("p",null,"The output of this script will be:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"Succeeded: 0\nMy Cool Script - version 1.2\nSucceeded: 1\nTitle Message: My Cool Script - version 1.2\n")),(0,r.kt)("p",null,"This demonstrates that functions can use the variables that are available in the shell. They can also set variables. We can also create new variables in functions."),(0,r.kt)("h3",{id:"variable-scoping-index"},"Variable Scoping "),(0,r.kt)("p",null,"If you come from a programming background you might find it odd that you can create a variable in a function and use it outside of the function. This is a feature known as ",(0,r.kt)("em",{parentName:"p"},"dynamic scoping"),". Many common programming languages like Python, JavaScript, C, Java and others use an alternative mechanism called ",(0,r.kt)("em",{parentName:"p"},"lexical scoping"),"."),(0,r.kt)("p",null,"Lexical scoping is a feature that ensures that you can only use a variable from within the 'scope' that it is defined. This can reduce errors - because it means that if you define a variable in a function you don't accidentally 'overwrite' the value of another variable that is used elsewhere."),(0,r.kt)("p",null,"You can use the 'local' keyword to define a variable that is only available in the 'local' scope, i.e. the function that it is defined in. This allows you to use lexical scoping and can reduce the risk of errors. Here's an example:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'run_loop() {\n local count=0\n for i in {1..10}; do\n # Update our counter.\n count=$((count + 1))\n done\n echo "Count is: ${count}"\n}\n')),(0,r.kt)("p",null,"Let's see what happens if we run this function:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},'$ run_loop\nCount is: 10\n$ echo "Count: ${count}"\nCount:\n')),(0,r.kt)("p",null,"Notice that because we declared the ",(0,r.kt)("em",{parentName:"p"},"count")," variable using the 'local' keyword, it is only available inside the ",(0,r.kt)("em",{parentName:"p"},"run_loop")," function. If we try and access it outside of the function it is undefined."),(0,r.kt)("p",null,"In general, you should use 'local' variables inside functions. This can help to avoid problems where calling a function can have an unintended side effects:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"# Set a count variable somewhere in our script...\ncount=3\n\n# Call our 'run_loop' function.\nrun_loop\n\n# Write out the value of 'count'.\necho \"The 'count' variable is: ${count}\"\n")),(0,r.kt)("p",null,"The output of this script is:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"Count is: 10\nThe 'count' variable is: 3\n")),(0,r.kt)("p",null,"Notice that even though we used a variable named ",(0,r.kt)("em",{parentName:"p"},"count")," in the ",(0,r.kt)("em",{parentName:"p"},"run_loop")," function, we did not overwrite the value that was set outside of the function. If we were to run the same script but not use the 'local' keyword for the count variable, we would get the following output:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"Count is: 10\nThe 'count' variable is: 10\n")),(0,r.kt)("p",null,"In this case calling the function changes the 'count' variable that is outside of the function. In most cases this is not going to be what you want and will just lead to unexpected behaviour later on."),(0,r.kt)("h2",{id:"passing-parameters-to-functions"},"Passing Parameters to Functions"),(0,r.kt)("p",null,"You can pass any number of parameters to a shell function. To get the value of a parameter, we can use special built-in variables that represent each parameter. Let's take a look at an example:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'sum() {\n local value1=$1\n local value2=$2\n local result=$((value1 + value2))\n echo "The sum of ${value1} and ${value2} is ${result}"\n}\n')),(0,r.kt)("p",null,"Let's see how we can pass parameters to this function:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"$ sum 3 6\nThe sum of 3 and 6 is 9\n$ sum 10 33\nThe sum of 10 and 33 is 43\n")),(0,r.kt)("p",null,"In this script we have used the special ",(0,r.kt)("inlineCode",{parentName:"p"},"$1")," and ",(0,r.kt)("inlineCode",{parentName:"p"},"$2")," built-in variables to get the value of the first and second parameters. At the beginning of the function I have put these variables into local variables that have more descriptive names. This is purely to make the script more readable, I could also have written the function like this:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'# Create a function that calculates the sum of two numbers.\nsum() {\n echo "The sum of $1 and $2 is $(($1 + $2))"\n}\n')),(0,r.kt)("p",null,"For a short and simple function you might just use the special parameter variables directly like above. However for anything more complex than a one-line script I think that it is generally more readable to create a local variable with a more descriptive name."),(0,r.kt)("h3",{id:"parameter-variables"},"Parameter Variables"),(0,r.kt)("p",null,"There are a number of special parameter variables that the shell provides. Let's see a few in action:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'# Create a function that sums a set of numbers.\nsum() {\n local total=0\n for value in $@; do\n total=$((total + value))\n done\n\n # Write out the result.\n echo "Summed $# values for a total of: ${total}"\n}\n')),(0,r.kt)("p",null,"We can call this function with any number of parameters:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"$ sum 1 2 3 4 5\nSummed 5 values for a total of: 15\n")),(0,r.kt)("p",null,"In this script we've used two special variables. The ",(0,r.kt)("inlineCode",{parentName:"p"},"$@")," variable is expanded into a list of all of the function parameters. The ",(0,r.kt)("inlineCode",{parentName:"p"},"$#")," variable contains the number of parameters provided to the function."),(0,r.kt)("p",null,"You might recognise that these variables look quite similar to the syntax that is used to get the members of an array or the length of an array as described in ",(0,r.kt)("a",{parentName:"p",href:"/part-3-manipulating-text/variables-reading-input-and-mathematics/"},"Chapter 19 - Variables, Reading Input, and Mathematics"),". You can actually use some of the array-style operators with the special parameters variables:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"# Show the top 'n' values of a set.\nshow_top() {\n local n=$1\n local values=${@:2:n}\n echo \"Top ${n} values: ${values}\"\n}\n")),(0,r.kt)("p",null,"We can call this function with any number of parameters. The first parameter specifies how many of the subsequent parameters we will show:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"$ show_top 3 10 20 30 40 50\nTop 3 values: 10 20 30\n")),(0,r.kt)("p",null,"We have used the 'range' operator on the ",(0,r.kt)("inlineCode",{parentName:"p"},"$@")," variable to get a subset of the parameters. This script is a little odd to read because when we set the 'values' parameter we need to 'skip' past the first positional parameter, because the first positional parameter is the number of values to show."),(0,r.kt)("p",null,"The table below shows some of the common variables you can use when working with function parameters:"),(0,r.kt)("table",null,(0,r.kt)("thead",{parentName:"table"},(0,r.kt)("tr",{parentName:"thead"},(0,r.kt)("th",{parentName:"tr",align:null},"Variable"),(0,r.kt)("th",{parentName:"tr",align:null},"Description"))),(0,r.kt)("tbody",{parentName:"table"},(0,r.kt)("tr",{parentName:"tbody"},(0,r.kt)("td",{parentName:"tr",align:null},(0,r.kt)("inlineCode",{parentName:"td"},"$1")),(0,r.kt)("td",{parentName:"tr",align:null},"The first parameter")),(0,r.kt)("tr",{parentName:"tbody"},(0,r.kt)("td",{parentName:"tr",align:null},(0,r.kt)("inlineCode",{parentName:"td"},"$2")),(0,r.kt)("td",{parentName:"tr",align:null},"The second parameter")),(0,r.kt)("tr",{parentName:"tbody"},(0,r.kt)("td",{parentName:"tr",align:null},(0,r.kt)("inlineCode",{parentName:"td"},"${11}")),(0,r.kt)("td",{parentName:"tr",align:null},"The 11th parameter - if the parameter is more than one digit you must surround it with braces")),(0,r.kt)("tr",{parentName:"tbody"},(0,r.kt)("td",{parentName:"tr",align:null},(0,r.kt)("inlineCode",{parentName:"td"},"$#")),(0,r.kt)("td",{parentName:"tr",align:null},"The number of parameters")),(0,r.kt)("tr",{parentName:"tbody"},(0,r.kt)("td",{parentName:"tr",align:null},(0,r.kt)("inlineCode",{parentName:"td"},"$@")),(0,r.kt)("td",{parentName:"tr",align:null},"The full set of parameters as an array")),(0,r.kt)("tr",{parentName:"tbody"},(0,r.kt)("td",{parentName:"tr",align:null},(0,r.kt)("inlineCode",{parentName:"td"},"$*")),(0,r.kt)("td",{parentName:"tr",align:null},"The full set of parameters as a string separated by the first value in the ",(0,r.kt)("inlineCode",{parentName:"td"},"$IFS")," variable")),(0,r.kt)("tr",{parentName:"tbody"},(0,r.kt)("td",{parentName:"tr",align:null},(0,r.kt)("inlineCode",{parentName:"td"},"${@:start:count}")),(0,r.kt)("td",{parentName:"tr",align:null},"A subset of 'count' parameters starting at parameter number 'start'")))),(0,r.kt)("p",null,"The ",(0,r.kt)("inlineCode",{parentName:"p"},"$@")," and ",(0,r.kt)("inlineCode",{parentName:"p"},"@*")," parameters look quite similar. The first one is an array, just like we saw in ",(0,r.kt)("a",{parentName:"p",href:"/part-3-manipulating-text/variables-reading-input-and-mathematics/"},"Chapter 19 - Variables, Reading Input, and Mathematics"),". The second version is the parameters collected together into a single string separated by spaces (actually, separated by the first character in the ",(0,r.kt)("inlineCode",{parentName:"p"},"$IFS")," variable)."),(0,r.kt)("h3",{id:"parameter-shifting"},"Parameter Shifting"),(0,r.kt)("p",null,"We can use the ",(0,r.kt)("inlineCode",{parentName:"p"},"shift")," (",(0,r.kt)("em",{parentName:"p"},"shift positional parameters"),")"," to remove a number of parameters from the beginning of the position parameters list and 'shift' the remaining parameters to take their place."),(0,r.kt)("p",null,"This is a little hard to describe so let's see how we can use it to simplify our ",(0,r.kt)("em",{parentName:"p"},"show_top")," function:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"# Show the top 'n' values of a set.\nshow_top() {\n # Grab the number of values to show, then shift.\n local n=$1\n shift\n\n # Get the set of values to show. Notice that we start in position 1 now.\n local values=${@:1:n}\n echo \"Top ${n} values: ${values}\"\n}\n")),(0,r.kt)("p",null,"After we get the value of the first parameter, we 'shift', removing it from the list of positional parameters so that we can deal with the remaining parameters. I would avoid using 'shift' too much - if you find that you are having to write complex code to shift parameters around you might be better using a programming language rather than the shell for the task you are performing!"),(0,r.kt)("h2",{id:"return-values"},"Return Values"),(0,r.kt)("p",null,"You can return a value from a shell function in two ways. The first is to simply set the value of a variable, like so:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"is_even() {\n local number=$1\n\n # A number is even if when we divide it by 2 there is no remainder.\n # Set 'result' to 1 if the parameter is even and 0 otherwise.\n if [ $((number % 2)) -eq 0 ]; then\n result=1\n else\n result=0\n fi\n}\n")),(0,r.kt)("p",null,"A function could set any number of variables to provide output. Here's how we could use the ",(0,r.kt)("em",{parentName:"p"},"is_even")," function:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},'$ number=33\n$ is_even $number\n$ echo "Result is: $result"\nResult is: 0\n')),(0,r.kt)("p",null,"In general, this method of returning values from a function should be avoided, for the reasons we've discussed already in this chapter. It overwrites the value of a global variable and that can be confusing for the operator."),(0,r.kt)("p",null,"A more common way to return a value from a function is to write its result to ",(0,r.kt)("em",{parentName:"p"},"stdout")," - let's look at this in detail."),(0,r.kt)("h2",{id:"writing-results-to-stdout"},"Writing Results to Stdout"),(0,r.kt)("p",null,"If we write our result to ",(0,r.kt)("em",{parentName:"p"},"stdout"),", then we can capture the result of a function in a far more readable way:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"lowercase() {\n local params=\"$@\"\n # Translate all uppercase characters to lowercase characters.\n echo \"$params\" | tr '[:upper:]' '[:lower:]' \n}\n")),(0,r.kt)("p",null,"In this example we write the result of the function to ",(0,r.kt)("em",{parentName:"p"},"stdout"),". This means that we can capture the result and put it in another variable by simply executing the command in a subshell:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},'$ result=$(lowercase "Don\'t SHOUT!")\n$ echo "$result"\ndon\'t shout!\n')),(0,r.kt)("p",null,"If you have a programming background it might seem very strange that you write results in a function by writing to ",(0,r.kt)("em",{parentName:"p"},"stdout"),". Remember - the shell is a text based interface to the computer system. The majority of commands that we have seen so far that provide output write their output to the screen. This is what ",(0,r.kt)("inlineCode",{parentName:"p"},"ls")," does, what ",(0,r.kt)("inlineCode",{parentName:"p"},"find")," does, what ",(0,r.kt)("inlineCode",{parentName:"p"},"cat")," does and so on. When we ",(0,r.kt)("inlineCode",{parentName:"p"},"echo")," a result from a function, we are really just following the Unix standard of writing the results of a program to the screen."),(0,r.kt)("p",null,"This is important - if we run our function directly in a shell, we can see the result written to the screen:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"$ lowercase \"PLEASE don't SHOUT!\"\nplease don't shout!\n")),(0,r.kt)("p",null,"Remember - shell functions are designed to behave in a similar way to shell commands. They write their output to ",(0,r.kt)("em",{parentName:"p"},"stdout"),"."),(0,r.kt)("h3",{id:"dealing-with-output-in-commands"},"Dealing with Output in Commands"),(0,r.kt)("p",null,"Although it might feel a bit clunky, writing the results of a command to ",(0,r.kt)("em",{parentName:"p"},"stdout")," is a tried and tested method of returning results. However, we need to be careful. Let's take a look at an example to see why!"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'# This function creates a temporary folder for today and returns its path.\ntemp_today() {\n # Get today\'s date in the format YYYY-MM-DD.\n local today=$(date +"%Y-%m-%d")\n\n # Create a temporary directory for today and return it.\n tmpdir_today="/tmp/${today}"\n echo "Creating folder \'${tmpdir_today}\'..."\n mkdir -p "${tmpdir_today}"\n echo "${tmpdir_today}"\n}\n')),(0,r.kt)("p",null,"This function creates a temporary folder that is based on the current date. If we try and grab the result of the function and change to that folder then the script will fail:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'# Go to today\'s temporary folder.\nfolder=$(temp_today)\ncd "${folder}"\n')),(0,r.kt)("p",null,"This script fails, with the output:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"'Creating folder \\'/tmp/2021-05-28\\'...\\n/tmp/2021-05-28': No such file or directory\n")),(0,r.kt)("p",null,"What's going on here?"),(0,r.kt)("p",null,"Well in the ",(0,r.kt)("em",{parentName:"p"},"temp_today")," function we wrote a message halfway through the function, showing the name of the folder that would be created. This message is part of the output of the function. Even though in the last line we echo the path to the folder, the output of the command is ",(0,r.kt)("em",{parentName:"p"},"all")," of the text we have written."),(0,r.kt)("p",null,"It is important to remember that any command you call in a function that might write to ",(0,r.kt)("em",{parentName:"p"},"stdout")," could cause problems as it could write text to your output:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'command_exists() {\n if type "$1"; then\n echo "1"\n else\n echo "0"\n fi\n}\n')),(0,r.kt)("p",null,"What happens when we try and store the result of the function in a variable?"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},'$ result=$(command_exists "touch")\n$ echo "Result is: ${result}"\nResult is: touch is hashed (/usr/bin/touch)\n1\n')),(0,r.kt)("p",null,"This is not a well written function, we'll look at a better way to write it next. But it shows an important challenge to be aware of - when ",(0,r.kt)("inlineCode",{parentName:"p"},"type")," is used to find out whether a command exists it returns success if the command exists but also writes to ",(0,r.kt)("em",{parentName:"p"},"stdout"),"."),(0,r.kt)("p",null,"In ",(0,r.kt)("a",{parentName:"p",href:"/part-2-core-skills/thinking-in-pipelines/"},"Chapter 7 - Thinking in Pipelines")," we saw that we can send the output of a command to the 'null' device to silence its output. We can use this trick in our functions to stop commands from 'polluting' our result:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'command_exists() {\n if type "$1" >> /dev/null; then\n echo "1"\n else\n echo "0"\n fi\n}\n')),(0,r.kt)("p",null,"Now if we run this command we will not get the output from the ",(0,r.kt)("inlineCode",{parentName:"p"},"type")," command in our result - the output was redirected to the null device."),(0,r.kt)("h3",{id:"returning-status-codes"},"Returning Status Codes"),(0,r.kt)("p",null,"The ",(0,r.kt)("inlineCode",{parentName:"p"},"return")," (",(0,r.kt)("em",{parentName:"p"},"return from shell function"),")"," command causes a function to exit with a given status code."),(0,r.kt)("p",null,"This is something that often causes confusion in shell scripts. The reason is that in most programming languages, you would use a 'return' statement to return the result of a function. But in the shell, when we return, we set the ",(0,r.kt)("em",{parentName:"p"},"status code")," of the function."),(0,r.kt)("p",null,"What is a status code? We actually touched on this in ",(0,r.kt)("a",{parentName:"p",href:"/part-4-shell-scripting/mastering-conditional-logic"},"Chapter 20 - Mastering Conditional Logic"),". When a command runs, we expect it to return a ",(0,r.kt)("em",{parentName:"p"},"status code")," of 'zero' to indicate success. Any non-zero status code is used to specify an ",(0,r.kt)("em",{parentName:"p"},"error code"),"."),(0,r.kt)("p",null,"Let's see how we could re-write the ",(0,r.kt)("em",{parentName:"p"},"command_exists")," function to set a status code:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'command_exists() {\n if type "$1" >> /dev/null; then\n return 0\n else\n return 1\n fi\n}\n')),(0,r.kt)("p",null,"Now that our command sets a status code properly, we can use it in an 'if statement' like so:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'if command_exists "common"; then\n echo "The \'common\' command is installed on your system"\nelse\n echo "The \'common\' command is not installed on your system"\nfi\n')),(0,r.kt)("p",null,"Remember - only use the 'return' command to set a status code. Many shells will only allow values from 0-255 to be set, and most users will expect that a command should return zero for success and that any non-zero value is an error code. If you need to provide output for a command that is not just a status code, you should write it to ",(0,r.kt)("em",{parentName:"p"},"stdout")," or if you must, set the value of a global variable."),(0,r.kt)("p",null,"The result of the last executed command is always available in the special variable ",(0,r.kt)("inlineCode",{parentName:"p"},"$?"),". Here's how you could use it:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},'$ type "test"\ntest is a shell builtin\n$ echo "Result: $?"\nResult: 0\n')),(0,r.kt)("h2",{id:"error-handling"},"Error Handling"),(0,r.kt)("p",null,"When you run a shell script, if a command in the script fails, the script will continue to run. Like many other points in this chapter this might seem unintuitive if you come from a programming background, but this makes sense in the shell - if the shell was to terminate whenever a command fails it would be very difficult to use interactively."),(0,r.kt)("p",null,"Let's create a script called 'today' that makes a new temporary folder each day, then puts a link to that folder in our home directory:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'#!/usr/bin/env sh\n\n# Get today\'s date in the format YYYY-MM-DD.\ntoday=$(date +"%Y-%m-%d")\n\n# Create the path to today\'s temp folder and then make sure the folder exists.\ntemp_path="/tmp/${today}"\nmkdir -p "${temp_path}"\n\n# Now that we\'ve created the folder, make a symlink to it in our homedir.\nln -sf "${temp_path}" "${HOME}/today" \n\n# Write out the path we created.\necho "${temp_path}"\n')),(0,r.kt)("p",null,"Now we can run the script to create temporary folder for the current day and a link to it in our home directory:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"$ chmod +x ./today.sh\n$ ./today.sh\n/tmp/2021-05-28\n$ cd ~/today\n")),(0,r.kt)("p",null,"In this example we created a new directory in the ",(0,r.kt)("em",{parentName:"p"},"tmp")," folder and created a link to it in our home directory. But what happens if we cause one of the commands to fail?"),(0,r.kt)("p",null,"First, let's clean up the folder we created:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"$ rm -rf $(./today.sh)\n$ rm ~/today\n")),(0,r.kt)("p",null,"Now we'll create a file where we want to put our 'today' folder:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},'$ touch "/tmp/$(date +"%Y-%m-%d")"\n')),(0,r.kt)("p",null,"If we run our script now, we can see a problem:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"$ ./today.sh\nmkdir: /tmp/2021-05-28: Not a directory\n/tmp/2021-05-28\n$ cd ~/today\nbash: cd: /home/dwmkerr/today: Not a directory\n")),(0,r.kt)("p",null,"The ",(0,r.kt)("inlineCode",{parentName:"p"},"mkdir")," command failed - because there was a ",(0,r.kt)("em",{parentName:"p"},"file")," in the location where we wanted to create the folder. But the script kept on running - meaning that it created a symlink to this file. Now when we try to move to the ",(0,r.kt)("inlineCode",{parentName:"p"},"today")," folder we get another error - it is a link to a file not a folder."),(0,r.kt)("p",null,"In general in your shell scripts if a command fails you probably want the entire script to stop executing. Otherwise you can get this cascading effect as commands continue to return even after there was a failure, which can lead to all sorts of unexpected behaviour."),(0,r.kt)("p",null,"You can use the ",(0,r.kt)("inlineCode",{parentName:"p"},"set")," (",(0,r.kt)("em",{parentName:"p"},"set option"),") command to set an option in the shell. There is an option that tells the shell to exit when a command fails. Here's how we would use it:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"#!/usr/bin/env sh\n\n# Exit if any command fails.\nset -e\n\n# ...\n")),(0,r.kt)("p",null,"The 'set' command allows you to turn on and turn off shell options. The 'e' option means 'exit if any command exits with a non-zero status'."),(0,r.kt)("p",null,"Now let's clean up again:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"$ rm -rf $(./today.sh)\n$ rm ~/today\n")),(0,r.kt)("p",null,"And finally, we'll run the same script after creating the file that will cause a failure:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},'$ touch "/tmp/$(date +"%Y-%m-%d")"\n$ ./today.sh\nmkdir: /tmp/2021-05-28: Not a directory\n')),(0,r.kt)("p",null,"In this case the script stopped running as soon as there was a failure - after the ",(0,r.kt)("inlineCode",{parentName:"p"},"mkdir")," command failed."),(0,r.kt)("p",null,"One thing to be aware of is that the ",(0,r.kt)("inlineCode",{parentName:"p"},"set -e")," option only affects the ",(0,r.kt)("em",{parentName:"p"},"final")," command of a pipeline. This means that if you have a pipeline such as the below:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"grep '[:space:]*#' ~/effective-shell/scripts/common.sh | tr 'a-z' 'A-Z'\n")),(0,r.kt)("p",null,"Then the script will still run if the ",(0,r.kt)("inlineCode",{parentName:"p"},"grep")," command fails. To ensure that the shell terminates if a command in a pipeline fails we must set the ",(0,r.kt)("inlineCode",{parentName:"p"},"pipefail")," option:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"set -o pipefail\n")),(0,r.kt)("p",null,"If you set your scripts up so that they fail on errors (and this is probably something you should always do), then remember to make sure that commands that you expect ",(0,r.kt)("em",{parentName:"p"},"might")," fail are properly handled."),(0,r.kt)("p",null,"For example, if we want to delete a file in script but don't want to stop if the deletion fails for some reason, we could use an ",(0,r.kt)("inlineCode",{parentName:"p"},"if")," block to 'catch' the error and show a warning:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'if ! [ rm ~/my-file.text ]; then\n echo "warning: unable to delete file"\nfi\n')),(0,r.kt)("p",null,"Another option would be to use a conditional expression:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"rm ~/my-file.txt || true\n")),(0,r.kt)("p",null,"This expression always evaluates to 'true' so will not stop the script if an error occurs when running the ",(0,r.kt)("inlineCode",{parentName:"p"},"rm")," command."),(0,r.kt)("h2",{id:"the-function-keyword"},"The Function Keyword"),(0,r.kt)("p",null,"In some scripts you might see functions defined using the ",(0,r.kt)("inlineCode",{parentName:"p"},"function")," keyword, as below:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'function title() {\n echo "My Script version 1.0"\n}\n')),(0,r.kt)("p",null,"The 'function' keyword is not required. The keyword is available in Bash and similar shells. Using the function keyword has a minor benefit that it does not lead to an error if you have already defined an ",(0,r.kt)("em",{parentName:"p"},"alias")," with the same name as the function you are declaring. However, the drawback is that it is less standard and therefore less portable."),(0,r.kt)("p",null,"I would recommend that you do not use the 'function' keyword. Firstly, this will make your scripts more portable. Secondly, if your function is going to clash with the name of an alias that has already been defined, I would actually think that it is better that the script fails. Better to fail early and realise there is clash than to silently overwrite the alias which may cause unexpected errors later on when something else tries to call the alias and calls your function instead!"),(0,r.kt)("h2",{id:"parameters-and-status-codes-for-scripts"},"Parameters and Status Codes for Scripts"),(0,r.kt)("p",null,"Everything we have learned about parameters applies to scripts themselves. We can pass parameters to scripts and read them with the special variables such as ",(0,r.kt)("inlineCode",{parentName:"p"},"$1"),", ",(0,r.kt)("inlineCode",{parentName:"p"},"$2")," and so on."),(0,r.kt)("p",null,"The only difference is that instead of using the ",(0,r.kt)("inlineCode",{parentName:"p"},"return")," command when we want to exit a script with a status code, we use the ",(0,r.kt)("inlineCode",{parentName:"p"},"exit")," (",(0,r.kt)("em",{parentName:"p"},"exit the shell"),")"," command. The exit command exits the current shell with the provided status code."),(0,r.kt)("p",null,"Be careful when using the ",(0,r.kt)("inlineCode",{parentName:"p"},"exit")," command - if you are running a script then it is fine to use ",(0,r.kt)("inlineCode",{parentName:"p"},"exit"),", it will simply close the subshell that the script is running in. But if you type ",(0,r.kt)("inlineCode",{parentName:"p"},"exit")," in your shell that you are using interactively, it will close it."),(0,r.kt)("h2",{id:"updating-the-common-command"},"Updating the 'common' Command"),(0,r.kt)("p",null,"In the previous chapter we created the ",(0,r.kt)("inlineCode",{parentName:"p"},"common.v4.sh")," command, that shows common commands from the users shell history."),(0,r.kt)("p",null,"If you need a refresher on what is in the script, you can view it in your pager with:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"less ~/effective-shell/scripts/common.v4.sh\n")),(0,r.kt)("p",null,"The output of the command will look something like:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"1: 280 gst\n2: 144 vi\n3: 84 gc\n4: 72 ga .\n5: 62 gl\n6: 54 ls\n7: 50 gpo\n8: 48 gcm\n9: 45 make dev\n10: 44 gpr\n")),(0,r.kt)("p",null,"Let's make a couple of changes."),(0,r.kt)("p",null,"First, let's make sure we exit the script if one of the commands fails:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"# Exit if any command fails.\nset -e\n")),(0,r.kt)("p",null,"Next, we will update the script on line 7 so that we use the first parameter as the command count. If the first parameter is not set, we default to ten:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"# ...\ncommand_count=${1:-10} # The number of common commands to show\n# ...\n")),(0,r.kt)("p",null,"Here we are using the ",(0,r.kt)("inlineCode",{parentName:"p"},"$1")," variable. But we are also using ",(0,r.kt)("em",{parentName:"p"},"Shell Parameter Expansion")," as described in ",(0,r.kt)("a",{parentName:"p",href:"/part-3-manipulating-text/variables-reading-input-and-mathematics/"},"Chapter 19 - Variables, Reading Input, and Mathematics")," to provide a default value to use if the parameter is not set."),(0,r.kt)("p",null,"Next, let's change the line that writes out the count and the name of the command. At the moment, the count is shown and then the command name. Let's write a function that takes a number and line of text and writes it as a line of text with the number ",(0,r.kt)("em",{parentName:"p"},"after")," the text and in brackets:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'write_command_then_count() {\n # Get the command and count, this will be text that looks like:\n # \'43 git commit\'\n # Then write the command and the count afterwards.\n local line="$1"\n local count=$(echo "${line}" | cut -d\' \' -f1)\n local command=$(echo "${line}" | cut -d\' \' -f2-)\n echo "${command} (${count})"\n}\n')),(0,r.kt)("p",null,"We can now re-write our loop to make it a little cleaner:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},'for command in $commands\ndo\n echo "$counter: $(write_command_then_count "$command")"\n counter=$((counter + 1))\ndone\n')),(0,r.kt)("p",null,"The updated script is in the samples folder at ",(0,r.kt)("em",{parentName:"p"},"~/effective-shell/scripts/common.v5.sh"),", you can update your link to point to this version by running the ",(0,r.kt)("inlineCode",{parentName:"p"},"ln")," command:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre",className:"language-bash"},"ln -s ~/effective-shell/scripts/common.v5.sh /usr/local/bin/common\n")),(0,r.kt)("p",null,"Now when we run this command we can optionally provide the number of commands to show as a parameter. The output also is shown with the number of times the command has been called ",(0,r.kt)("em",{parentName:"p"},"after")," the command text itself:"),(0,r.kt)("pre",null,(0,r.kt)("code",{parentName:"pre"},"$ common 5\ncommon commands:\n1: gst (139)\n2: vi (74)\n3: gc (42)\n4: ga . (36)\n5: gl (31)\n")),(0,r.kt)("h2",{id:"summary"},"Summary"),(0,r.kt)("p",null,"In this chapter we looked at how to use functions to provide more structure to our shell scripts, and also how to use parameters, return values and status codes."),(0,r.kt)("p",null,"In the next and final chapter of this section, we'll look at some more advanced techniques that can be useful when writing shell scripts."))}c.isMDXComponent=!0}}]); \ No newline at end of file diff --git a/pr-preview/pr-346/assets/js/9446f79a.88b6ac45.js b/pr-preview/pr-346/assets/js/9446f79a.88b6ac45.js new file mode 100644 index 00000000..e778a05b --- /dev/null +++ b/pr-preview/pr-346/assets/js/9446f79a.88b6ac45.js @@ -0,0 +1 @@ +"use strict";(self.webpackChunkeffective_shell=self.webpackChunkeffective_shell||[]).push([[5368],{3905:(e,t,a)=>{a.d(t,{Zo:()=>m,kt:()=>p});var n=a(7294);function i(e,t,a){return t in e?Object.defineProperty(e,t,{value:a,enumerable:!0,configurable:!0,writable:!0}):e[t]=a,e}function o(e,t){var a=Object.keys(e);if(Object.getOwnPropertySymbols){var n=Object.getOwnPropertySymbols(e);t&&(n=n.filter((function(t){return Object.getOwnPropertyDescriptor(e,t).enumerable}))),a.push.apply(a,n)}return a}function r(e){for(var t=1;t<arguments.length;t++){var a=null!=arguments[t]?arguments[t]:{};t%2?o(Object(a),!0).forEach((function(t){i(e,t,a[t])})):Object.getOwnPropertyDescriptors?Object.defineProperties(e,Object.getOwnPropertyDescriptors(a)):o(Object(a)).forEach((function(t){Object.defineProperty(e,t,Object.getOwnPropertyDescriptor(a,t))}))}return e}function s(e,t){if(null==e)return{};var a,n,i=function(e,t){if(null==e)return{};var a,n,i={},o=Object.keys(e);for(n=0;n<o.length;n++)a=o[n],t.indexOf(a)>=0||(i[a]=e[a]);return i}(e,t);if(Object.getOwnPropertySymbols){var o=Object.getOwnPropertySymbols(e);for(n=0;n<o.length;n++)a=o[n],t.indexOf(a)>=0||Object.prototype.propertyIsEnumerable.call(e,a)&&(i[a]=e[a])}return i}var l=n.createContext({}),h=function(e){var t=n.useContext(l),a=t;return e&&(a="function"==typeof e?e(t):r(r({},t),e)),a},m=function(e){var t=h(e.components);return n.createElement(l.Provider,{value:t},e.children)},d="mdxType",c={inlineCode:"code",wrapper:function(e){var t=e.children;return n.createElement(n.Fragment,{},t)}},g=n.forwardRef((function(e,t){var a=e.components,i=e.mdxType,o=e.originalType,l=e.parentName,m=s(e,["components","mdxType","originalType","parentName"]),d=h(a),g=i,p=d["".concat(l,".").concat(g)]||d[g]||c[g]||o;return a?n.createElement(p,r(r({ref:t},m),{},{components:a})):n.createElement(p,r({ref:t},m))}));function p(e,t){var a=arguments,i=t&&t.mdxType;if("string"==typeof e||i){var o=a.length,r=new Array(o);r[0]=g;var s={};for(var l in t)hasOwnProperty.call(t,l)&&(s[l]=t[l]);s.originalType=e,s[d]="string"==typeof e?e:i,r[1]=s;for(var h=2;h<o;h++)r[h]=a[h];return n.createElement.apply(null,r)}return n.createElement.apply(null,a)}g.displayName="MDXCreateElement"},4859:(e,t,a)=>{a.r(t),a.d(t,{assets:()=>l,contentTitle:()=>r,default:()=>d,frontMatter:()=>o,metadata:()=>s,toc:()=>h});var n=a(7462),i=(a(7294),a(3905));const o={title:"Controlling Changes with Git",slug:"/part-5-building-your-toolkit/controlling-changes-with-git"},r=void 0,s={unversionedId:"building-your-toolkit/controlling-changes-with-git/index",id:"building-your-toolkit/controlling-changes-with-git/index",title:"Controlling Changes with Git",description:"Git is an extremely popular version control tool. You can use Git to track changes to text, code, or any type of files you might be working with. Many popular projects use Git as a tool to manage changes, allow others to contribute and collaborate, and to publish their projects.",source:"@site/docs/05-building-your-toolkit/27-controlling-changes-with-git/index.md",sourceDirName:"05-building-your-toolkit/27-controlling-changes-with-git",slug:"/part-5-building-your-toolkit/controlling-changes-with-git",permalink:"/part-5-building-your-toolkit/controlling-changes-with-git",draft:!1,editUrl:"https://github.com/dwmkerr/effective-shell/edit/main/docs/05-building-your-toolkit/27-controlling-changes-with-git/index.md",tags:[],version:"current",frontMatter:{title:"Controlling Changes with Git",slug:"/part-5-building-your-toolkit/controlling-changes-with-git"},sidebar:"sidebar",previous:{title:"Managing your Dotfiles",permalink:"/part-5-building-your-toolkit/managing-your-dotfiles"},next:{title:"Managing Remote Git Repositories and Sharing Your Dotfiles",permalink:"/part-5-building-your-toolkit/managing-rempte-git-repositories/"}},l={},h=[{value:"What is Git",id:"what-is-git",level:2},{value:"Creating a Git Repository",id:"creating-a-git-repository",level:2},{value:"Adding and Resetting Changes to the Index",id:"adding-and-resetting-changes-to-the-index",level:2},{value:"Committing Changes",id:"committing-changes",level:2},{value:"Commit Messages",id:"commit-messages",level:3},{value:"Creating Branches",id:"creating-branches",level:2},{value:"Merging",id:"merging",level:2},{value:"Merging Diverged Branches",id:"merging-diverged-branches",level:3},{value:"The Git Log",id:"the-git-log",level:3},{value:"Merge Conflicts",id:"merge-conflicts",level:3},{value:"Other Merge Strategies",id:"other-merge-strategies",level:3},{value:"Deleting and Renaming Files",id:"deleting-and-renaming-files",level:2},{value:"Restoring Your Working Tree",id:"restoring-your-working-tree",level:2},{value:"Summary",id:"summary",level:2}],m={toc:h};function d(e){let{components:t,...o}=e;return(0,i.kt)("wrapper",(0,n.Z)({},m,o,{components:t,mdxType:"MDXLayout"}),(0,i.kt)("p",null,"Git is an extremely popular version control tool. You can use Git to track changes to text, code, or any type of files you might be working with. Many popular projects use Git as a tool to manage changes, allow others to contribute and collaborate, and to publish their projects."),(0,i.kt)("p",null,"In this chapter we'll look at the most common operations for Git to allow us to quickly and easily work with Git repositories. We'll learn Git on the command line by taking our 'dotfiles' folder (a simple set of shell configuration files) and showing how we can track and manage changes."),(0,i.kt)("p",null,"Being able to use Git from the command line is essential to being an effective shell user. In this chapter we'll look at setting up a local Git repository - in the next chapter we'll see how to share it online with the popular GitHub website."),(0,i.kt)("h2",{id:"what-is-git"},"What is Git"),(0,i.kt)("p",null,"Any files or folders that we work with over time, such as our 'dotfiles' (i.e. our personal configuration files) will change. Sometimes new files get added, old files get deleted, files get changed, things get moved around and so on."),(0,i.kt)("p",null,"Git is a ",(0,i.kt)("em",{parentName:"p"},"version control system")," that allows you to track changes to files and folders. This means that you can maintain a history of all of the changes that have been made, when they were made, who made them and why. You can also maintain multiple 'branches' of your files and folders - these branches can be used as working environments where you can make changes, without affecting the current 'main' set of files."),(0,i.kt)("p",null,'Git was written by Linus Torvalds (the creator of Linux) in 2006. "Git" is slang for an annoying person - Linus joked that he always names projects after himself, first Linux and now Git. There were many version control systems around before Git, such as ',(0,i.kt)("em",{parentName:"p"},"CVS")," (Concurrent Version System) and ",(0,i.kt)("em",{parentName:"p"},"SVN")," (Subversion, an system similar to CVS but with some improvements). There were also a number of proprietary and commercial solutions."),(0,i.kt)("p",null,"In recent years Git has become without a doubt the most popular version control system globally, and many highly popular software collaboration systems such as GitHub, GitLab and BitBucket use Git as their underlying version control system, adding additional features on top."),(0,i.kt)("h2",{id:"creating-a-git-repository"},"Creating a Git Repository"),(0,i.kt)("p",null,"All of the information about a set of files or folders that you are tracking the changes for is stored in a ",(0,i.kt)("em",{parentName:"p"},"Git repository"),". We can create a Git repository by running the ",(0,i.kt)("inlineCode",{parentName:"p"},"git init")," (",(0,i.kt)("em",{parentName:"p"},"initialise Git repository"),")"," command."),(0,i.kt)("p",null,"Let's see this in action be creating a Git repository to track changes to our 'dotfiles' folder. Our 'dotfiles' folder is a folder where we keep simple shell configuration - you can read about how to create a folder like this in ",(0,i.kt)("a",{parentName:"p",href:"../../part-5-building-your-toolkit/managing-your-dotfiles"},"Chapter 27 - Managing Your Dotfiles")," or you can download the Effective Shell samples to get a copy of the 'dotfiles' folder."),(0,i.kt)("admonition",{title:"Downloading the Samples",type:"tip"},(0,i.kt)("p",{parentName:"admonition"},"Run the following command in your shell to download the samples:"),(0,i.kt)("pre",{parentName:"admonition"},(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"curl effective.sh | sh\n"))),(0,i.kt)("p",null,"If you have installed the samples, you can copy the ",(0,i.kt)("em",{parentName:"p"},"~/effective-shell/dotfiles")," folder to your home directory - this is where we will create our Git repository and start using the Git commands:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ cp -r ~/effective-shell/dotfiles ~/dotfiles\n$ cd ~/dotfiles\n")),(0,i.kt)("p",null,"We have created the ",(0,i.kt)("em",{parentName:"p"},"~/dotfiles")," folder in our home directory from the samples and moved into it."),(0,i.kt)("p",null,"Now we will initialise a Git repository with the ",(0,i.kt)("inlineCode",{parentName:"p"},"git init")," command, and choose a 'branch' name with the ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout")," command:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git init\nInitialized empty Git repository in /home/dwmkerr/dotfiles/.git/\n$ git checkout -b main\n")),(0,i.kt)("p",null,"We'll see the ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout")," command in detail soon. For now it is enough to know that we have initialised a new Git repository and chosen the name of our 'main' branch."),(0,i.kt)("admonition",{title:"Initialising a Git Repository with a Branch Name",type:"tip"},(0,i.kt)("p",{parentName:"admonition"},"If you are using Git 2.2 or later, you can initialise a repository and set the branch name with a single command:"),(0,i.kt)("pre",{parentName:"admonition"},(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"$ git init -b main\n")),(0,i.kt)("p",{parentName:"admonition"},"If you see the error message ",(0,i.kt)("em",{parentName:"p"},"error: unknown switch 'b'")," then this means that you are using a version of Git that does not have the ",(0,i.kt)("inlineCode",{parentName:"p"},"-b")," (",(0,i.kt)("em",{parentName:"p"},"initial branch name"),") flag (any version of Git lower than 2.2).")),(0,i.kt)("h2",{id:"adding-and-resetting-changes-to-the-index"},"Adding and Resetting Changes to the Index"),(0,i.kt)("p",null,"We now have an empty Git repository. We can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"git status")," (",(0,i.kt)("em",{parentName:"p"},"show the working tree status"),") command"," to show some information on the files in the ",(0,i.kt)("em",{parentName:"p"},"working tree"),". The working tree is the folder that we are using Git to track changes for, in our case ",(0,i.kt)("em",{parentName:"p"},"~/dotfiles"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ git status\nOn branch main\n\nNo commits yet\n\nUntracked files:\n (use "git add <file>..." to include in what will be committed)\n install.sh\n shell.d/\n shell.sh\n\nnothing added to commit but untracked files present (use "git add" to track)\n')),(0,i.kt)("p",null,"The first thing that ",(0,i.kt)("inlineCode",{parentName:"p"},"git status")," tells us is the name of the ",(0,i.kt)("em",{parentName:"p"},"branch")," we are on. We'll look at branches in detail shortly. The next thing we see is that there are no ",(0,i.kt)("em",{parentName:"p"},"commits")," - commits are sets of changes that we track. Finally, git is telling us that there are three files that are 'untracked'. These are the ",(0,i.kt)("em",{parentName:"p"},"install.sh")," and ",(0,i.kt)("em",{parentName:"p"},"shell.sh")," files as well as the ",(0,i.kt)("em",{parentName:"p"},"shell.d")," folder."),(0,i.kt)("p",null,"If we are going to use Git to track changes to these files, we need to add them to the repository. We can do that with the ",(0,i.kt)("em",{parentName:"p"},"git add")," (",(0,i.kt)("em",{parentName:"p"},"add file contents to index"),") command",":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git add .\n")),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"git add")," command takes a list of file paths. We have used the special ",(0,i.kt)("em",{parentName:"p"},"dot")," folder to represent the entire current directory. Let's take a look at the status again:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ git status\nOn branch main\n\nNo commits yet\n\nChanges to be committed:\n (use "git rm --cached <file>..." to unstage)\n new file: install.sh\n new file: shell.d/set_ps1.sh\n new file: shell.sh\n')),(0,i.kt)("p",null,"Git is now telling us that we have three new files which are ready to be ",(0,i.kt)("em",{parentName:"p"},"committed"),". At the moment these files are in the ",(0,i.kt)("em",{parentName:"p"},"index"),". The index, or staging area, is the set of changes that we are preparing to commit. These changes are not yet stored in the repository. When we add files to the index we are 'staging' changes. When we remove changes from the index, we are 'unstaging' changes."),(0,i.kt)("p",null,"Think of the index as a working area where we can build up a set of changes that we would later like to record in the repository. We could add more files to the index before we actually save them to the repository in a commit."),(0,i.kt)("p",null,"If we were to visualise what we've done so far, it would look like this:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram: A diagram showing how the 'git add' command tells Git to track changes to items in the working tree and adds them to the index",src:a(9126).Z,width:"1672",height:"1046"})),(0,i.kt)("p",null,"Our ",(0,i.kt)("em",{parentName:"p"},"working tree")," is the folder associated with our Git repository, this is the ",(0,i.kt)("em",{parentName:"p"},"~/dotfiles")," folder. Our ",(0,i.kt)("em",{parentName:"p"},"index")," is initially empty. When we run the ",(0,i.kt)("inlineCode",{parentName:"p"},"git add")," command, we have told Git we want to add three files to the repository. Our 'staging area' has three files in it. Our Git repository does not have any commits recorded yet."),(0,i.kt)("p",null,"What if we realised that we don't want to add one of these files to the repository? To remove a file from the index we can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"git reset")," (",(0,i.kt)("em",{parentName:"p"},"reset changes"),") command. Let's reset the ",(0,i.kt)("em",{parentName:"p"},"~/dotfiles/shell.d/set_ps1.sh")," file and check the status:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ git reset shell.d/set_ps1.sh\n$ git status\nOn branch main\n\nNo commits yet\n\nChanges to be committed:\n (use "git rm --cached <file>..." to unstage)\n new file: install.sh\n new file: shell.sh\n\nUntracked files:\n (use "git add <file>..." to include in what will be committed)\n shell.d/\n')),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"git reset")," command has removed a change from the index - telling Git that we don't want to 'stage' one of the files. Git now tells us there are two files in the index and one that is not tracked."),(0,i.kt)("p",null,"Here's how we can visualise the changes we've made:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram: The 'git reset' command removes items from the index",src:a(7878).Z,width:"2060",height:"968"})),(0,i.kt)("p",null,"You can also reset changes by using the ",(0,i.kt)("inlineCode",{parentName:"p"},"git rm --cached")," (",(0,i.kt)("em",{parentName:"p"},"remove changes from index"),") command. However, I think this is a little harder to work with as you have to remember to use the ",(0,i.kt)("inlineCode",{parentName:"p"},"--cached")," flag to tell Git that you are removing from the index and not the repository. We'll see the ",(0,i.kt)("inlineCode",{parentName:"p"},"git rm")," command a little later in the chapter."),(0,i.kt)("p",null,"Remember - at this stage we have not changed a single file! Nothing we have done has changed the content of any of the files in the working tree, and the only thing that has changed in the Git repository is the 'index' - the current set of files that we are 'staging'."),(0,i.kt)("p",null,"Now let's look at how we can commit our changes with the ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit")," command."),(0,i.kt)("h2",{id:"committing-changes"},"Committing Changes"),(0,i.kt)("p",null,"Once we are happy with the set of changes in the index, we can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit")," (",(0,i.kt)("em",{parentName:"p"},"record changes to the repository"),") command","."),(0,i.kt)("p",null,"Now run the ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit")," command:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git commit \n")),(0,i.kt)("p",null,"At this stage your shell editor will open up, with the text shown below:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"\n# Please enter the commit message for your changes. Lines staring\n# with '#' will be ignored, and an empty message aborts the commit.\n#\n# On branch main\n#\n# Initial commit\n#\n# Changes to be committed:\n# new file: install.sh\n# new file: shell.sh\n#\n# Untracked files:\n# shell.d/\n#\n")),(0,i.kt)("p",null,"The reason you shell editor opens is that the ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit")," command would like you to provide a message describing your changes. Type a short description, such as:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"add the 'install' and 'shell' scripts\n# Please enter the commit message for your changes. Lines staring\n# with '#' will be ignored, and an empty message aborts the commit.\n#\n# On branch main\n#\n# Initial commit\n#\n# Changes to be committed:\n# new file: install.sh\n# new file: shell.sh\n#\n# Untracked files:\n# shell.d/\n#\n")),(0,i.kt)("p",null,"Note that below the cursor there is some information that starts with the ",(0,i.kt)("inlineCode",{parentName:"p"},"#")," hash symbol. This is provided as a convenience - Git is telling you the status of the index. Anything that starts with a hash symbol is a comment and will not be stored in the commit message."),(0,i.kt)("p",null,"Save the file by pressing Ctrl+W and close the editor with Ctrl+X (these are the commands for the ",(0,i.kt)("inlineCode",{parentName:"p"},"nano")," editor, if you are using a different editor use whatever commands are needed to save and close)."),(0,i.kt)("p",null,"When the editor closes you'll see a confirmation below the ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit")," command:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git commit\n[main (root-commit) 01e7a10] add the 'install' and 'shell' scripts\n 2 files changed, 90 insertions(+)\n create mode 100755 install.sh\n create mode 100644 shell.sh\n")),(0,i.kt)("p",null,"This message tells us that two files have changed and 90 lines have been added. It also lists the files we have added. At this point we have created our first commit. We can visualise the process we have gone through like this:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram showing how 'git commit' commits changes to the repository",src:a(9395).Z,width:"2116",height:"1030"})),(0,i.kt)("p",null,"We 'staged' a set of changes and then 'committed' these changes. We now have a single commit in our repository. Our files are ",(0,i.kt)("em",{parentName:"p"},"still")," unchanged, but our Git repository now has a single commit in it that tracks the two files we added."),(0,i.kt)("p",null,"Let's run ",(0,i.kt)("inlineCode",{parentName:"p"},"git status")," again:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ git status\nOn branch main\nUntracked files:\n (use "git add <file>..." to include in what will be committed)\n shell.d/\n\nnothing added to commit but untracked files present (use "git add" to track)\n')),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"git status")," command tells us we're still on the 'main' branch, and that there is one file which is not tracked. Let's create a second commit by adding this file. When you run the ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit")," command below, enter a message to describe the commit:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git add .\n$ git commit\n[main d7e1bb9] add the 'shell.d' folder\n 1 file changed, 228 insertions(+)\n create mode 100644 shell.d/set_ps1.sh\n")),(0,i.kt)("p",null,"We have now created a second commit - our timeline will look like this:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram showing our second git commit",src:a(8853).Z,width:"2582",height:"1022"})),(0,i.kt)("p",null,"If we run the ",(0,i.kt)("inlineCode",{parentName:"p"},"git status")," command one last time we will see that everything in the working tree is tracked in Git:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git status\nOn branch main\nnothing to commit, working tree clean\n")),(0,i.kt)("p",null,"The concepts of the 'index', the 'working tree' and the Git repository itself can take a bit of getting used to! If you have not used Git before and this seems like a lot to take on board, don't worry, people often find Git quite hard at first. As you use it more this will all become familiar and make more sense."),(0,i.kt)("h3",{id:"commit-messages"},"Commit Messages"),(0,i.kt)("p",null,"You can use any text you like for a commit message. However, there are a couple of things that you should bear in mind:"),(0,i.kt)("ol",null,(0,i.kt)("li",{parentName:"ol"},"Try to keep the first line to 50 characters or less. This is known as the 'subject' line - keeping this short will make it easier to look through the log of changes later and see what each commit means."),(0,i.kt)("li",{parentName:"ol"},"If you want to add more detail, leave a blank line after the subject line and then include as much text as you like. Common convention is to wrap the text at 80 characters so that it will fit in a typical shell window.")),(0,i.kt)("p",null,"There are many articles available online that suggest conventions for how to write your commit messages. You can explore these as you get more familiar with Git. The only convention I would strongly recommend that you follow is to make sure that your Git message makes sense - it should describe what the change is and ideally why you have made it. It is easy to forget why changes were made after time has passed - writing good commit messages will save you a lot of time in the long run and also make it easier for others to work with your repository!"),(0,i.kt)("p",null,"Now let's take a look at how we can work on changes to our files with ",(0,i.kt)("em",{parentName:"p"},"branches"),"."),(0,i.kt)("h2",{id:"creating-branches"},"Creating Branches"),(0,i.kt)("p",null,"The commits that we have made so far have been on a 'branch' named 'main'. We can create new 'branches' and put commits on them to allow us to make a series of changes that are isolated from each other."),(0,i.kt)("p",null,"We can create branches using the ",(0,i.kt)("inlineCode",{parentName:"p"},"git branch")," (",(0,i.kt)("em",{parentName:"p"},"list, create or delete branches"),") or ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout")," (",(0,i.kt)("em",{parentName:"p"},"switch branches or restore working tree"),") command",". To show these features in action, we'll create a new branch called ",(0,i.kt)("em",{parentName:"p"},"aliases")," and add some files to it:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git checkout -b aliases\nSwitched to a new branch 'aliases'\n$ git status\nOn branch aliases\nnothing to commit, working tree clean\n")),(0,i.kt)("p",null,"We have used the ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout")," command to 'switch' to another branch. The ",(0,i.kt)("inlineCode",{parentName:"p"},"-b")," (",(0,i.kt)("em",{parentName:"p"},"new branch"),") option tells Git that we want to create a new branch. The ",(0,i.kt)("inlineCode",{parentName:"p"},"git status")," command now shows the new branch name when we run it."),(0,i.kt)("p",null,"Let's create a new file which includes an alias for the ",(0,i.kt)("inlineCode",{parentName:"p"},"git status")," command, then let's see what ",(0,i.kt)("inlineCode",{parentName:"p"},"git status")," tells us about the status of the working tree:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ echo \'alias gs="git status"\' >> ./shell.d/git_aliases.sh\n$ git status\nOn branch aliases\nUntracked files:\n (use "git add <file>..." to include in what will be committed)\n shell.d/git_aliases.sh\n\nnothing added to commit but untracked files present (use "git add" to track)\n')),(0,i.kt)("p",null,"Excellent - we have a new file and Git knows that it is not currently tracked. Let's stage this file and then commit it:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git add .\n$ git commit -m \"add alias 'gs' for 'git status'\"\n\n[aliases f61369d] add alias 'gs' for 'git status'\n 1 file changed, 1 insertion(+)\n create mode 100644 shell.d/git_aliases.sh\n")),(0,i.kt)("p",null,"In the example above I used the ",(0,i.kt)("inlineCode",{parentName:"p"},"-m")," (",(0,i.kt)("em",{parentName:"p"},"commit message"),") parameter for the ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit")," command, this means my editor will not open up as we've already provided a commit message."),(0,i.kt)("p",null,"We now have a series of commits that looks like this:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram showing how 'git checkout -b' creates a new branch",src:a(7685).Z,width:"2136",height:"560"})),(0,i.kt)("p",null,"Our new ",(0,i.kt)("em",{parentName:"p"},"~/dotfiles/shell.d/git_aliases.sh")," file has been committed to the ",(0,i.kt)("em",{parentName:"p"},"aliases")," branch."),(0,i.kt)("p",null,"We can switch back to the ",(0,i.kt)("em",{parentName:"p"},"main")," branch at any time with ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git checkout main\nSwitched to branch 'main'\n$ tree\n.\n\u251c\u2500\u2500 install.sh\n\u251c\u2500\u2500 shell.d\n\u2502\xa0\xa0 \u2514\u2500\u2500 set_ps1.sh\n\u2514\u2500\u2500 shell.sh\n")),(0,i.kt)("p",null,"When we switch back to the ",(0,i.kt)("em",{parentName:"p"},"main")," branch and look at our working tree we can see that the ",(0,i.kt)("em",{parentName:"p"},"git_aliases.sh")," file is not present. This is very cool - by passing the name of the branch we want to switch to as the parameter to the ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout")," command we can switch branches. If we are on the ",(0,i.kt)("em",{parentName:"p"},"main")," branch we don't see the ",(0,i.kt)("em",{parentName:"p"},"git_aliases.sh")," file, because the commit that added is was not on the ",(0,i.kt)("em",{parentName:"p"},"main")," branch. To go back to the ",(0,i.kt)("em",{parentName:"p"},"aliases")," branch we can just checkout again:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git checkout aliases\nSwitched to branch 'aliases'\n$ tree\n.\n\u251c\u2500\u2500 install.sh\n\u251c\u2500\u2500 shell.d\n\u2502\xa0\xa0 \u251c\u2500\u2500 git_aliases.sh\n\u2502\xa0\xa0 \u2514\u2500\u2500 set_ps1.sh\n\u2514\u2500\u2500 shell.sh\n")),(0,i.kt)("p",null,"As a nice little tip, you can always go back to the ",(0,i.kt)("em",{parentName:"p"},"last")," branch you were on by running ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout -")," just like you can use ",(0,i.kt)("inlineCode",{parentName:"p"},"cd -")," to change to the last directory you visited! The dash character is a shortcut for the last branch you were on."),(0,i.kt)("p",null,"Let's add another alias to the file and create another commit:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ echo 'alias gcm=\"git checkout main\"' >> ./shell.d/git_aliases.sh\n$ git add .\n$ git commit -m \"add alias 'gcm' for 'git checkout main'\"\n[aliases b9ae0ad] add alias 'gcm' for 'git checkout main'\n 1 file changed, 1 insertion(+)\n")),(0,i.kt)("p",null,"Our branches will now look like this:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram showing two commits on the 'aliases' branch",src:a(7745).Z,width:"2140",height:"552"})),(0,i.kt)("p",null,"You can create as many branches as you like - just remember that when you run ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout -b"),", you branch from the ",(0,i.kt)("em",{parentName:"p"},"current")," branch (and in fact, the current ",(0,i.kt)("em",{parentName:"p"},"HEAD"),", which we will see a little later)."),(0,i.kt)("p",null,"If you want to create a branch, but don't want to switch to it, you can run ",(0,i.kt)("inlineCode",{parentName:"p"},"git branch <new_branch_name>"),". This command will create a branch from your current position, but will not move to it."),(0,i.kt)("h2",{id:"merging"},"Merging"),(0,i.kt)("p",null,"You can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"git merge")," (",(0,i.kt)("em",{parentName:"p"},"join two or more branches"),") command"," to take the changes from one branch and bring them into another."),(0,i.kt)("p",null,"We can merge the changes from our ",(0,i.kt)("em",{parentName:"p"},"aliases")," branch into the ",(0,i.kt)("em",{parentName:"p"},"main")," branch of the repository by first checking out the branch we want to merge into, and then running ",(0,i.kt)("inlineCode",{parentName:"p"},"git merge"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git checkout main\n$ git merge aliases\nUpdating d7e1bb9..b9ae0ad\nFast-forward\n shell.d/git_aliases.sh | 2 ++\n 1 file changed, 2 insertions(+)\n create mode 100644 shell.d/git_aliases.sh\n")),(0,i.kt)("p",null,"When we run the ",(0,i.kt)("inlineCode",{parentName:"p"},"git merge")," command Git tells us what ",(0,i.kt)("em",{parentName:"p"},"type")," of merge it has performed. In this case we have a ",(0,i.kt)("em",{parentName:"p"},"fast forward")," ","merge, which is the most simple type of merge. When Git tries to merge the two branches, it sees that each of the commits on the ",(0,i.kt)("em",{parentName:"p"},"aliases")," branch can be applied sequentially to the ",(0,i.kt)("em",{parentName:"p"},"main")," branch:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram showing how Git prepares for a 'fast forwards' merge",src:a(8506).Z,width:"2210",height:"492"})),(0,i.kt)("p",null,"One the merge is complete, our branches look like this:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram showing a fast forward merge result",src:a(4983).Z,width:"2120",height:"364"})),(0,i.kt)("p",null,"The ",(0,i.kt)("em",{parentName:"p"},"main")," branch and ",(0,i.kt)("em",{parentName:"p"},"aliases")," branch contain the exact same set of commits."),(0,i.kt)("p",null,"Let's look at a more common merge scenario - merging branches that have diverged."),(0,i.kt)("h3",{id:"merging-diverged-branches"},"Merging Diverged Branches"),(0,i.kt)("p",null,"Let's create set of commits that show a case where we our branches have ",(0,i.kt)("em",{parentName:"p"},"diverged")," - the branches both have their own new commits:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"# Create a branch called 'more_aliases', add a file to it, then commit.\ngit checkout -b more_aliases\ntouch ./shell.d/bash_aliases.sh\ngit add .\ngit commit -m \"add a file to store 'bash' aliases\"\n\n# Create another file, add it, then commit.\ntouch ./shell.d/zsh_aliases.sh\ngit add .\ngit commit -m \"add a file to store 'zsh' aliases\"\n")),(0,i.kt)("p",null,"This snippet checks out a new branch called ",(0,i.kt)("em",{parentName:"p"},"more_aliases")," and adds two new empty files, as two separate commits. Now we'll go back to our ",(0,i.kt)("em",{parentName:"p"},"main")," branch and change a file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"# Go back to the 'main' branch, add and commit another file.\ngit checkout main\necho 'alias gm=\"git merge\"' >> ./shell.d/git_aliases.sh\ngit commit -a -m \"add the 'gm' alias for 'git merge'\"\n")),(0,i.kt)("p",null,"I have added a new alias to the ",(0,i.kt)("em",{parentName:"p"},"shell.d/git_aliases.sh")," file. By adding the ",(0,i.kt)("inlineCode",{parentName:"p"},"-a")," (",(0,i.kt)("em",{parentName:"p"},"all changes"),") flag to the ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit")," command I was able to add the changes to the index and commit them with a single command."),(0,i.kt)("p",null,"Our branches now look like this:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"A diagram showing two diverged branches",src:a(3763).Z,width:"2136",height:"548"})),(0,i.kt)("p",null,"Let's merge the ",(0,i.kt)("em",{parentName:"p"},"more_aliases")," branch into the ",(0,i.kt)("em",{parentName:"p"},"main")," branch:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git merge more_aliases\n")),(0,i.kt)("p",null,"At this point your shell editor will open up with a request for a commit message:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"Merge branch 'more_aliases'\n# Please enter a commit message to explain why this merge is needed\n# especially if it merges an updated upstream into a topic branch.\n#\n# Lines starting with '#' will be ignored, and an empty message will abort\n# the commit.\n")),(0,i.kt)("p",null,"Git is going to create a new commit on the ",(0,i.kt)("em",{parentName:"p"},"main")," branch that brings in the changes from the ",(0,i.kt)("em",{parentName:"p"},"bash_aliases")," branch. Because a new commit is going to be created, Git asks us to provide a message. The default message simply explains that this commit merges the branch named ",(0,i.kt)("em",{parentName:"p"},"more_aliases"),"."),(0,i.kt)("p",null,"You can change the message or leave it as is. Save the file when you have entered the message and the output below will be shown:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git merge more_aliases\nMerge made by the 'recursive' strategy.\n shell.d/bash_aliases.sh | 0\n shell.d/zsh_aliases.sh | 0\n 2 files changed, 0 insertions(+), 0 deletions(-)\n create mode 100644 shell.d/bash_aliases.sh\n create mode 100644 shell.d/zsh_aliases.sh\n")),(0,i.kt)("p",null,"Git now tells us that we have made a ",(0,i.kt)("em",{parentName:"p"},"recursive")," merge and our branches will look like this:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram showing the results of a recursive merge of two diverged branches",src:a(4876).Z,width:"2352",height:"502"})),(0,i.kt)("p",null,"At this stage both the ",(0,i.kt)("em",{parentName:"p"},"main")," branch and the ",(0,i.kt)("em",{parentName:"p"},"more_aliases")," branch have the full set of changes that we made to ",(0,i.kt)("em",{parentName:"p"},"both")," of the branches. Git merged the two branches together and created a new commit that joins them."),(0,i.kt)("h3",{id:"the-git-log"},"The Git Log"),(0,i.kt)("p",null,"In the diagrams we've shown we've given each commit a number. This was just to make things easier to read - Git doesn't use a number for commits. Git uses a ",(0,i.kt)("em",{parentName:"p"},"SHA"),", which is a hash. A hash is a sequence of letters and numbers that uniquely identify each commit."),(0,i.kt)("p",null,"You can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"git log")," (",(0,i.kt)("em",{parentName:"p"},"show commit logs"),") command"," to see the log of commits and their SHAs:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git log\ncommit 138b40418d5658bc64421e7bcf2680c8339f8350 (HEAD)\nMerge: a95bd90 a51ae1a\nAuthor: Dave Kerr <dwmkerr@gmail.com>\nDate: Tue Jun 15 21:00:28 2021 +0800\n\n Merge branch 'more_aliases'\n\ncommit a95bd90e3656b2e55b8708193d387c80c282a6ad\nAuthor: Dave Kerr <dwmkerr@gmail.com>\nDate: Tue Jun 15 21:00:22 2021 +0800\n\n add the 'gm' alias for 'git merge'\n\ncommit a51ae1aa42432c2f391ca782c1c20b3793c232ab (more_aliases)\nAuthor: Dave Kerr <dwmkerr@gmail.com>\nDate: Tue Jun 15 20:53:01 2021 +0800\n\n add a file to store 'zsh' aliases\n")),(0,i.kt)("p",null,"You can see that the log shows each of the commits, the branch the commit was on, the message, the date and so on."),(0,i.kt)("p",null,"If you want to see a more compact log you can use the ",(0,i.kt)("inlineCode",{parentName:"p"},"--oneline")," (",(0,i.kt)("em",{parentName:"p"},"show one line per commit"),") flag:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git log --oneline\n138b404 (HEAD) Merge branch 'more_aliases'\na95bd90 add the 'gm' alias for 'git merge'\na51ae1a (more_aliases) add a file to store 'zsh' aliases\n63ea74f add a file to store 'bash' aliases\nb9ae0ad (aliases) add alias 'gcm' for 'git checkout main'\nf61369d add alias 'gs' for 'git status'\nd7e1bb9 add the 'shell.d' folder\n01e7a10 add the 'install' and 'shell' scripts\n")),(0,i.kt)("p",null,"When you run this command yourself it will be a little easier to read as the output uses different colours for the SHAs and the branch names."),(0,i.kt)("p",null,"We can even see a 'graph' view, showing the branches we have made and when they branched off and were merged back. To do this, pass the ",(0,i.kt)("inlineCode",{parentName:"p"},"--graph")," (",(0,i.kt)("em",{parentName:"p"},"show commit graph"),") flag:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git log --oneline --graph\n* 138b404 (HEAD) Merge branch 'more_aliases'\n| \\\n| * a51ae1a (more_aliases) add a file to store 'zsh' aliases\n| * 63ea74f add a file to store 'bash' aliases\n* | a95bd90 add the 'gm' alias for 'git merge'\n|/\n* b9ae0ad (aliases) add alias 'gcm' for 'git checkout main'\n* f61369d add alias 'gs' for 'git status'\n* d7e1bb9 add the 'shell.d' folder\n* 01e7a10 add the 'install' and 'shell' scripts\n")),(0,i.kt)("p",null,"Each commit is shown with an ",(0,i.kt)("inlineCode",{parentName:"p"},"*")," asterisk symbol - we can also see when we created the ",(0,i.kt)("em",{parentName:"p"},"more_aliases")," branch and when we merged it back in."),(0,i.kt)("p",null,"The Git log is very useful to help you understand the changes that have happened in the repository."),(0,i.kt)("h3",{id:"merge-conflicts"},"Merge Conflicts"),(0,i.kt)("p",null,"One of the most important features of any version control system is the ability to manage ",(0,i.kt)("em",{parentName:"p"},"conflicts"),". Conflicts occur when a set of changes are made that cannot be merged without some kind of manual intervention to decide ",(0,i.kt)("em",{parentName:"p"},"which")," of the changes are correct."),(0,i.kt)("p",null,"Here are a few common scenarios that might lead to merge conflicts:"),(0,i.kt)("ul",null,(0,i.kt)("li",{parentName:"ul"},"In one branch a file is deleted and in another branch the file is changed - when we merge should we delete the file or keep the version with the changes?"),(0,i.kt)("li",{parentName:"ul"},"In one branch a file is edited and in another branch the ",(0,i.kt)("em",{parentName:"li"},"same")," part of the file is edited in a different way - which edit should we keep? Should we keep both?"),(0,i.kt)("li",{parentName:"ul"},"In one branch we add content to the end of a file and in another branch we add different content - which of these changes should come first?")),(0,i.kt)("p",null,"A lot of the time you can avoid conflicts by making sure that you don't keep branches for too long - if other people are merging changes into the ",(0,i.kt)("em",{parentName:"p"},"main")," branch while you are working on another branch, you are ",(0,i.kt)("em",{parentName:"p"},"drifting")," from the main branch. You should either regularly update your branch with the changes in ",(0,i.kt)("em",{parentName:"p"},"main")," or merge your changes into ",(0,i.kt)("em",{parentName:"p"},"main"),"."),(0,i.kt)("p",null,"There are many different ways for version control systems to manage conflicts. Let's see how Git does it by creating a conflict."),(0,i.kt)("p",null,"First, we will create a branch that adds a new alias to the ",(0,i.kt)("em",{parentName:"p"},"git_aliases.sh")," file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"# Create a 'glog_alias' branch and commit a file.\ngit checkout -b glog_alias\necho 'alias glog=\"git log --graph --oneline\"' >> ./shell.d/git_aliases.sh\ngit commit -a -m \"add the 'glog' alias\"\n")),(0,i.kt)("p",null,"Now we'll go back to the ",(0,i.kt)("em",{parentName:"p"},"main")," branch and add another alias:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre",className:"language-bash"},"# Go back to 'main' and commit a change to the same file as the last one.\ngit checkout main\necho 'alias glog=\"git log\"' >> ./shell.d/git_aliases.sh\ngit commit -a -m \"add the 'glog' alias\"\n")),(0,i.kt)("p",null,"We've changed the same file in two branches - if we try to merge we will get a conflict:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git merge glog_alias\n\nAuto-merging shell.d/git_aliases.sh\nCONFLICT (content): Merge conflict in shell.d/git_aliases.sh\nAutomatic merge failed; fix conflicts and then commit the result.\n")),(0,i.kt)("p",null,"When Git cannot automatically consolidate the changes into a single merge commit, it aborts the merge process. No new commits have been made - the conflicted files with their changes are in the ",(0,i.kt)("em",{parentName:"p"},"index"),". We have to now manually fix these files."),(0,i.kt)("p",null,"Let's see what the status shows:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ git status\nOn branch main\nYou have unmerged paths.\n (fix conflicts and run "git commit")\n (use "git merge --abort" to abort the merge)\n\nUnmerged paths:\n (use "git add <file>..." to mark resolution)\n both modified: shell.d/git_aliases.sh\n\nno changes added to commit (use "git add" and/or "git commit -a")\n')),(0,i.kt)("p",null,"Git is telling us that we are currently in the process of trying to fix a merge conflict. It is telling us that we need to fix the ",(0,i.kt)("em",{parentName:"p"},"shell.d/git_aliases.sh")," file, then use ",(0,i.kt)("inlineCode",{parentName:"p"},"git add")," to stage the changes and commit the result."),(0,i.kt)("p",null,"To resolve the conflict, we need to edit the file. If you open the file in an editor it will look like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'alias gs="git status"\nalias gcm="git checkout main"\nalias gm="git merge"\n<<<<<<< HEAD\nalias glog="git log"\n=======\nalias glog="git log --graph --oneline"\n>>>>>>> glog_alias\n')),(0,i.kt)("p",null,"Many modern editors will immediately recognise the sequence of symbols that Git uses to indicate conflicts and highlight them. The ",(0,i.kt)("inlineCode",{parentName:"p"},"<<<<<< HEAD")," line indicates the changes in the current branch, the ",(0,i.kt)("inlineCode",{parentName:"p"},"=======")," line is a separator, and the ",(0,i.kt)("inlineCode",{parentName:"p"},">>>>>> glog_alias")," line indicates that everything since the ",(0,i.kt)("inlineCode",{parentName:"p"},"=======")," line is the changes in the ",(0,i.kt)("em",{parentName:"p"},"glog_alias")," branch."),(0,i.kt)("p",null,"We can see why Git has not been able to merge these changes - each branch has a new line and Git doesn't know which one is correct. So rather than make an assumption (such as the most recent change should 'win'), Git is asking us to choose."),(0,i.kt)("p",null,"In your editor, update the file to look like this:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'alias gs="git status"\nalias gcm="git checkout main"\nalias gm="git merge"\nalias glog="git log --graph --oneline"\n')),(0,i.kt)("p",null,"We have chosen the version of the ",(0,i.kt)("inlineCode",{parentName:"p"},"glog")," alias that was in the ",(0,i.kt)("em",{parentName:"p"},"glog_alias")," branch. But you could choose either - or replace the content with a new line. You can make any changes you like - just be sure to remove the lines that start with ",(0,i.kt)("inlineCode",{parentName:"p"},"<<<"),", ",(0,i.kt)("inlineCode",{parentName:"p"},"===")," and ",(0,i.kt)("inlineCode",{parentName:"p"},">>>"),". We don't even have to use the changes from one of the branches - we could remove the lines or add completely new ones. We change the file to make sure that the merged result makes sense."),(0,i.kt)("p",null,"In the example above we deleted one of the ",(0,i.kt)("inlineCode",{parentName:"p"},"glog")," aliases, but we could have also just changed the name of one of them:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'alias gs="git status"\nalias gcm="git checkout main"\nalias gm="git merge"\nalias glog="git log"\nalias ggraph="git log --graph --oneline"\n')),(0,i.kt)("p",null,"We can now use ",(0,i.kt)("inlineCode",{parentName:"p"},"git add")," to mark the file as resolved, and run ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git add shell.d/git_aliases.sh\n")),(0,i.kt)("p",null,"The editor will open showing a sensible commit message:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"Merge branch 'glog_alias'\n\n# Conflicts:\n# shell.d/git_aliases.sh\n#\n# It looks like you may be committing a merge.\n# If this is not correct, please run\n# git update-ref -d MERGE_HEAD\n# and try again.\n\n\n# Please enter the commit message for your changes. Lines starting\n# with '#' will be ignored, and an empty message aborts the commit.\n#\n# On branch main\n")),(0,i.kt)("p",null,"The message is just like the earlier merge commit message, but the comments show a little more information (the files that were conflicted). Save the file and close the editor to complete the commit."),(0,i.kt)("p",null,"Our Git log will now show this new merge commit:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git log --graph --oneline\n* 2532277 (HEAD -> main) Merge branch 'glog_alias'\n|\\\n| * a8cbb15 (glog_alias) add the 'glog' alias\n* | 31548e4 add the 'glog' alias\n|/\n* 138b404 Merge branch 'more_aliases'\n|\\\n| * a51ae1a (more_aliases) add a file to store 'zsh' aliases\n| * 63ea74f add a file to store 'bash' aliases\n* | a95bd90 add the 'gm' alias for 'git merge'\n|/\n* b9ae0ad (aliases) add alias 'gcm' for 'git checkout main'\n* f61369d add alias 'gs' for 'git status'\n* d7e1bb9 add the 'shell.d' folder\n* 01e7a10 add the 'install' and 'shell' scripts\n")),(0,i.kt)("p",null,"Dealing with conflicts can be extremely complicated. We have only scratched the surface here, but there is a wealth of information online if you'd like to go deeper."),(0,i.kt)("h3",{id:"other-merge-strategies"},"Other Merge Strategies"),(0,i.kt)("p",null,"Git has a number of merge strategies that can be used to combine the changes across branches. Going into them is beyond the scope of this book. However, it is useful to know that you have a lot of functionality to control how branches are merged available to you if you want to explore further."),(0,i.kt)("p",null,"There are merge strategies that allow you to try and create a single, coherent history between two branches, rather than creating a merge commit, there are options to 'squash' all of the commits from one branch into another and more."),(0,i.kt)("p",null,'I would suggest that as you become more familiar with the basics of how Git works, you get a little deeper on this topic - searching online for "Git Merge Strategies" will bring up many articles going into detail.'),(0,i.kt)("h2",{id:"deleting-and-renaming-files"},"Deleting and Renaming Files"),(0,i.kt)("p",null,"It is quite simple to remove files from your Git Repository. You can either ask Git to remove the file for you, or just delete the file yourself and then tell Git that you have removed it."),(0,i.kt)("p",null,"Let's see both ways in action. First we'll use the ",(0,i.kt)("inlineCode",{parentName:"p"},"git rm")," (",(0,i.kt)("em",{parentName:"p"},"remove files from the working tree and index"),") command",":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git rm install.sh\nrm 'install.sh'\n")),(0,i.kt)("p",null,"The ",(0,i.kt)("inlineCode",{parentName:"p"},"git rm")," command removes the file from your working tree and stages the deletion. We can see that the file is staged for deletion if we run ",(0,i.kt)("inlineCode",{parentName:"p"},"git status"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ git status\nOn branch main\nChanges to be committed:\n (use "git restore --staged <file>..." to unstage)\n deleted: install.sh\n')),(0,i.kt)("p",null,"The deletion is only in the ",(0,i.kt)("em",{parentName:"p"},"index")," at this stage - we need to run ",(0,i.kt)("inlineCode",{parentName:"p"},"git commit")," to commit the deletion."),(0,i.kt)("p",null,"If we want to restore the file, we can use ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout")," to get the file back from Git:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git checkout HEAD install.sh\n")),(0,i.kt)("p",null,"We use ",(0,i.kt)("inlineCode",{parentName:"p"},"HEAD")," to tell Git what commit we want to checkout the file from - head means the current commit we are on (which is the most recent commit on ",(0,i.kt)("em",{parentName:"p"},"main"),")."),(0,i.kt)("p",null,"Another common way to delete a file is to simply remove it from the file-system:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ rm install.sh\n$ git status\nOn branch main\nChanges not staged for commit:\n (use "git add/rm <file>..." to update what will be committed)\n (use "git restore <file>..." to discard changes in working directory)\n deleted: install.sh\n')),(0,i.kt)("p",null,"At this point Git knows the file is missing, but the deletion is not yet ",(0,i.kt)("em",{parentName:"p"},"staged")," - because we haven't explicitly told Git that we want to remove this file as part of the commit we are creating. To confirm that we want to remove the file we can use:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git rm install.sh\n")),(0,i.kt)("p",null,"We could also run ",(0,i.kt)("inlineCode",{parentName:"p"},"git add install.sh")," - think of this like saying 'stage my change to ",(0,i.kt)("em",{parentName:"p"},"index.sh"),"' - we are ",(0,i.kt)("em",{parentName:"p"},"adding")," a change to the ",(0,i.kt)("em",{parentName:"p"},"index"),". I often just run ",(0,i.kt)("inlineCode",{parentName:"p"},"git add .")," to add all of my changes (including deletions) to the stage. At this point we can make any other changes we like, stage them, and then commit."),(0,i.kt)("p",null,"Let's restore the file:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git reset .\nUnstaged changes after reset:\nD install.sh\n$ git checkout .\nUpdated 1 path from the index\n")),(0,i.kt)("p",null,"This is another way to restore the file - first we reset all of the changes to the index with ",(0,i.kt)("inlineCode",{parentName:"p"},"git reset ."),", then we checkout all of the files in the current commit with ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout ."),". But be careful with this method - it will also reset any other changes you have made. I prefer the ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout HEAD <file_path>")," method as it is more explicit and only restores one file."),(0,i.kt)("p",null,"What about if we rename a file? Let's rename the install file and see what the status is:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ mv install.sh install_dotfiles.sh\nChanges not staged for commit:\n (use "git add/rm <file>..." to update what will be committed)\n (use "git restore <file>..." to discard changes in working directory)\n deleted: install.sh\n\nUntracked files:\n (use "git add <file>..." to include in what will be committed)\n install_dotfiles.sh\n\nno changes added to commit (use "git add" and/or "git commit -a")\n')),(0,i.kt)("p",null,"We've renamed the file, but when we run ",(0,i.kt)("inlineCode",{parentName:"p"},"git status")," it is telling us that the file is missing, and there is a new untracked file. But Git is smart enough to know when we move a file - if we run ",(0,i.kt)("inlineCode",{parentName:"p"},"git add .")," to add all changes in the working tree to the index, Git will recognise that we have not deleted and added a file, but instead renamed it:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ git add .\n$ git status\nOn branch main\nChanges to be committed:\n (use "git restore --staged <file>..." to unstage)\n renamed: install.sh -> install_dotfiles.sh\n')),(0,i.kt)("p",null,"Let's restore the file by renaming it back to what it was and adding these changes:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ mv install_dotfiles.sh install.sh\n$ git add .\n$ git status\nOn branch main\nnothing to commit, working tree clean\n")),(0,i.kt)("p",null,"We can also use the ",(0,i.kt)("inlineCode",{parentName:"p"},"git mv")," (",(0,i.kt)("em",{parentName:"p"},"move or rename a file"),") command"," to move or rename a file and stage the changes in one go:"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},'$ git mv install.sh install_dotfiles.sh\nChanges to be committed:\n (use "git restore --staged <file>..." to unstage)\n renamed: install.sh -> install_dotfiles.sh\n')),(0,i.kt)("p",null,"We can also restore the changes by using ",(0,i.kt)("inlineCode",{parentName:"p"},"git mv"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git mv install_dotfiles.sh install.sh\n$ git status\nOn branch main\nnothing to commit, working tree clean\n")),(0,i.kt)("p",null,"These commands work equally well with folders, or lists of files and folders."),(0,i.kt)("h2",{id:"restoring-your-working-tree"},"Restoring Your Working Tree"),(0,i.kt)("p",null,"We've already seen how the ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout")," command can be used to switch branches. We can also use the ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout")," command to restore our working tree to a certain point in our commit history. This is extremely useful if you want to see how your files looked at an earlier point in time, or restore your files to a previous state."),(0,i.kt)("p",null,"Let's take a quick look at our Git log so far."),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git log --graph --oneline\n* 2532277 (HEAD -> main) Merge branch 'glog_alias'\n|\\\n| * a8cbb15 (glog_alias) add the 'glog' alias\n* | 31548e4 add the 'glog' alias\n|/\n* 138b404 Merge branch 'more_aliases'\n|\\\n| * a51ae1a (more_aliases) add a file to store 'zsh' aliases\n| * 63ea74f add a file to store 'bash' aliases\n* | a95bd90 add the 'gm' alias for 'git merge'\n|/\n* b9ae0ad (aliases) add alias 'gcm' for 'git checkout main'\n* f61369d add alias 'gs' for 'git status'\n* d7e1bb9 add the 'shell.d' folder\n* 01e7a10 add the 'install' and 'shell' scripts\n")),(0,i.kt)("p",null,"The Git log is showing the first seven digits of the SHA for each commit. We can checkout any commit by providing its SHA. We don't need to provide the entire SHA (which is good, because they are normally quite long), we only need to provide enough letters to uniquely identify the commit, so normally four or five digits is plenty."),(0,i.kt)("p",null,"Notice that the most recent commit is marked with the text ",(0,i.kt)("inlineCode",{parentName:"p"},"HEAD"),". The ",(0,i.kt)("em",{parentName:"p"},"HEAD")," is where Git is currently 'pointing' to - we will see how to move the head shortly (the term ",(0,i.kt)("em",{parentName:"p"},"HEAD")," also refers to the most recent commit of a branch)."),(0,i.kt)("p",null,"At the moment we can visualise our Git log, with the commits, branches and HEAD like so:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram showing our Git log with HEAD indicated",src:a(5524).Z,width:"2992",height:"854"})),(0,i.kt)("p",null,"We can restore the working tree to the state of any of the commits by running ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout <commit_sha>"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git checkout f61369d\nNote: switching to 'f61369d'.\n\nYou are in 'detached HEAD' state. You can look around, make experimental\nchanges and commit them, and you can discard any commits you make in this\nstate without impacting any branches by switching back to a branch.\n")),(0,i.kt)("p",null,"This snippet moved our HEAD to the third commit in our repository. If you look at the files in your working tree you'll see that they are in exact state that they were when we made our third commit. Visually, we have moved the HEAD as shown below:"),(0,i.kt)("p",null,(0,i.kt)("img",{alt:"Diagram showing a detached HEAD",src:a(7107).Z,width:"2486",height:"742"})),(0,i.kt)("p",null,"The warning that says we are in a 'detached HEAD' state is telling us that our current position in the history is not at the 'tip' of a branch - this means we cannot create a new commit without first starting a new branch."),(0,i.kt)("p",null,"If we want to move back to the 'tip' of our main branch (or any other branch), we can use ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout <branch_name>"),":"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git checkout main\n")),(0,i.kt)("p",null,"This moves us back to the 'tip' of the main branch."),(0,i.kt)("p",null,"Git has a very convenient syntax we can use to move backwards. We can specify a branch name, a SHA or 'HEAD', use a ",(0,i.kt)("inlineCode",{parentName:"p"},"~")," tilde character, and then provide the number of commits we want to move 'backwards':"),(0,i.kt)("pre",null,(0,i.kt)("code",{parentName:"pre"},"$ git checkout HEAD~1\n")),(0,i.kt)("p",null,"This command moves the 'head' backwards one commit. This is useful if you realise you have made a mistake with your commit or recent commits and want to go backwards."),(0,i.kt)("p",null,"Remember - whenever you run ",(0,i.kt)("inlineCode",{parentName:"p"},"git checkout <branch_name>")," Git moves the HEAD to the tip of the branch by default."),(0,i.kt)("h2",{id:"summary"},"Summary"),(0,i.kt)("p",null,"In this chapter we looked at some of the core concepts behind Git - the repository, the working tree and the index. We saw how to stage and unstage changes, commit changes, create branches, merge branches, deal with conflicts and remove and rename files."),(0,i.kt)("p",null,"In the next chapter we will look at how to work with 'remote' repositories - this will allow us to push our changes up to a location on the internet and share these changes with other users, or to download these changes to other machines we might work on."),(0,i.kt)("p",null,"We've introduced a lot of commands in this chapter - you can use this table as a quick reference:"),(0,i.kt)("table",null,(0,i.kt)("thead",{parentName:"table"},(0,i.kt)("tr",{parentName:"thead"},(0,i.kt)("th",{parentName:"tr",align:null},"Command"),(0,i.kt)("th",{parentName:"tr",align:null},"Description"))),(0,i.kt)("tbody",{parentName:"table"},(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git init")),(0,i.kt)("td",{parentName:"tr",align:null},"Initialise a new Git Repository.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git status")),(0,i.kt)("td",{parentName:"tr",align:null},"Show the status of the working tree and index.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git add <files>")),(0,i.kt)("td",{parentName:"tr",align:null},"Stage ",(0,i.kt)("inlineCode",{parentName:"td"},"files")," - you can use patterns and wildcards.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git reset <files>")),(0,i.kt)("td",{parentName:"tr",align:null},"Unstage ",(0,i.kt)("inlineCode",{parentName:"td"},"files")," - you can use patterns and wildcards.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git rm --cached <files>")),(0,i.kt)("td",{parentName:"tr",align:null},"Unstage ",(0,i.kt)("inlineCode",{parentName:"td"},"files")," - you can use patterns and wildcards.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git commit")),(0,i.kt)("td",{parentName:"tr",align:null},"Create a commit from the current index - the shell editor will open for the commit message.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git commit -m 'message'")),(0,i.kt)("td",{parentName:"tr",align:null},"Create a commit with message ",(0,i.kt)("inlineCode",{parentName:"td"},"message"),".")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git commit -a")),(0,i.kt)("td",{parentName:"tr",align:null},"Stage and commit all changes in the working tree.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git checkout <branch>")),(0,i.kt)("td",{parentName:"tr",align:null},"Checkout a branch called ",(0,i.kt)("inlineCode",{parentName:"td"},"branch"),".")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git checkout -b branch")),(0,i.kt)("td",{parentName:"tr",align:null},"Create and checkout a new branch called ",(0,i.kt)("inlineCode",{parentName:"td"},"branch"),".")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git branch <name>")),(0,i.kt)("td",{parentName:"tr",align:null},"Create a branch called ",(0,i.kt)("inlineCode",{parentName:"td"},"name")," but do not check it out.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git branch -m <new_name>")),(0,i.kt)("td",{parentName:"tr",align:null},"Change the current branch name to ",(0,i.kt)("inlineCode",{parentName:"td"},"new_name"),".")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git merge <branch>")),(0,i.kt)("td",{parentName:"tr",align:null},"Merge the branch named ",(0,i.kt)("inlineCode",{parentName:"td"},"branch")," into the current branch.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git log")),(0,i.kt)("td",{parentName:"tr",align:null},"Show the log of commits.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git log --oneline --branch")),(0,i.kt)("td",{parentName:"tr",align:null},"Show the log of commits, one line per commit, with the branch graph.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git rm <files>")),(0,i.kt)("td",{parentName:"tr",align:null},"Stage the removal of ",(0,i.kt)("inlineCode",{parentName:"td"},"files")," from the repostiry - you can use patterns and wildcards.")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git mv <source> <destination>")),(0,i.kt)("td",{parentName:"tr",align:null},"Stage the movement of ",(0,i.kt)("inlineCode",{parentName:"td"},"source")," to ",(0,i.kt)("inlineCode",{parentName:"td"},"destination"),".")),(0,i.kt)("tr",{parentName:"tbody"},(0,i.kt)("td",{parentName:"tr",align:null},(0,i.kt)("inlineCode",{parentName:"td"},"git checkout 8342bec")),(0,i.kt)("td",{parentName:"tr",align:null},"Checkout a commit with SHA 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Hack On! See those system calls!
I mentioned earlier on that if you make a call like
fopen, the Kernel is going to provide access to a file. It's quite easy to see this in action. Check the code below:#include <stdio.h>
void main()
{
void* handle = fopen("/tmp/some-file");
fwrite(handle, "some text");
fclose(handle);
}If you compile this program, then run XXX you will see the actual calls made to the Kernel. It can be very useful to use this technique to see what is going on with programs under the hood, particularly when diagnosing issues.
Hack On! Ahah! So that's TTY?
Consider a command like:
docker -itConsider a command like:
ssh user@remote.com my-script
no ttyOnce you understand the concept of shells and terminals in a bit more detail, more obscure messages like this start to make sense.
In the first instance, we are telling Docker we are interactive - i.e. we are going to use an interface to send commands. The second parameter,
-tsays use a TTY - which is short for teletype terminal, old fashioned lingo for the screen.TODO TTY picture
TODO
Summary; technical
For technical readers, there's quite a lot of terms which get thrown about almost interchangeably; shell, command-line, terminal, tty, command-prompt, CLI and so on. But each of these have a very specific meaning. It's important to understand exactly what each of these terms really means, where they came from, and how these different types of system or concept relate to each other.
Summary; non-tech
A computer in a nutshell.
-- Could link to other layers of abstractions - such as sandboxes in webpages? +-- Could link to other layers of abstraction, such as containers?