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Bash Scripting

Some useful links

Scripting vs Programming

Scripting languages are those which do not require an explicit compilation step. ex: Javascript,Lua.
Programming Languages on the other hand require an explicit compilation step. ex: C,C++.
Infact whether a language is programming or scripting, depends on the environment,meaning
to say, we can actually write a 'C interpreter' or a 'Javascript compiler'.
https://stackoverflow.com/questions/17253545/scripting-language-vs-programming-language

What is Shebang?

You probably have seen the starting line of many shell scripts, as the following

#!/bin/bash

This is called shebang It actually defines which program actually runs the current script.In fact we can use shebang on any file be it Python, Javascript..
https://unix.stackexchange.com/questions/87560/does-the-shebang-determine-the-shell-which-runs-the-script
The which command in terminal helps locate where in the linux the command is actually stored.

which bash
/bin/bash
which python
/usr/bin/python

What is $PATH?

$PATH is an environment variable, which lists colon seperated list of directories. 

echo $PATH
/home/mohit123/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:
/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/home/mohit123/bin

Whenever you type a command on the shell, the command is searched in these directoris, in the order
of their occurrance. If the command is not found here, an error is displayed.
https://askubuntu.com/questions/551990/what-does-path-mean
Adding a command to one of these directories will make your command accessible from anywhere.
https://unix.stackexchange.com/questions/3809/how-can-i-make-a-program-executable-from-everywhere

Floating point numbers in Bash

How to use floating point in bash.(Bash does not support floating point arithmetic inherently) https://stackoverflow.com/questions/12722095/how-do-i-use-floating-point-division-in-bash

Storing in a variable

var1=5                                  #store a constant
var2=$var1                              #store a variable
var3=$((var1+var2))                     #store the result of an arithmetic expression   
var4=$(($var1+$var2))                   #store the result of an arithmetic expression
var5=$( ls | wc -l )                    #substituting a command/or return value from a function
var6=$( func_name $param1 $param2 )     #storing the return value  from a function

It is important that there is no space between the varialble the '=' sign and the rhs.

Single quotes vs Double quotes!!

var1=5
echo 'the value of var1 is $var1'
echo "the value of var1 is $var1"

The first one will print 

the value of var1 is $var1

The second one will print 

the value of var1 is 5

I/O

  1. Simple Read
  2. Read with prompt (p)
  3. Read with silence (s)
  4. echo

Conditionals

#/bin/bash
echo $1
echo $2
if [ $1 -gt $2 ]
then
  echo $1 greater than $2
elif [ $2 -gt $1 ]
then
  echo $2 is greater than $1
else
  echo equal
fi

Notice that $1 and $2 are the command line args. How to run??

bash example.sh 4 5
4
5
5 is greater than 4

Don't forget the fi in the end. 

    -lt : less than
    -ge : greater than equal to
    -le : less than equal to
    -ne : not equal to
    -eq : interprets both sides as integer and compares them
     == : interprets both sides as string and compares them

== vs eq

https://unix.stackexchange.com/questions/16109/bash-double-equals-vs-eq The following program will output yes since -eq does a integer comparison and

if[[ 01 -eq 1 ]] 
then
echo yes
else
echo no

The following program will output no since == does a string comparison and "01" is not equal to "1"

if[[ 01 == 1 ]] 
then
echo yes
else
echo no

Directory existence

How to detect whether a directory exists or not.
https://stackoverflow.com/questions/59838/check-if-a-directory-exists-in-a-shell-script?answertab=votes#tab-top

File exists

filevar=$1 [ -e filevar ]

filevar="a b c d"
for name in $filevar
do
  echo name
done

Output: 
a
b
c
d

This will first split the filevar into a, b,c, and d and hence can be a problem at times,
when we want to treat a b c d as a single filename(which is not recommended ) we can modify the code
by adding a double quotes around $filevar in the for loop.

filevar="a b c d"
for name in "$filevar"
do
  echo name
done

Output:
a b c d

Read more https://unix.stackexchange.com/questions/131766/why-does-my-shell-script-choke-on-whitespace-or-other-special-characters

&& and || follows the same short-ciruit trick as in other languages.
i.e.

[ <cond1> ] && [ <cond2> ]

If [ <cond1> ] is not satisfied, [ <cond2> ] is not executed.
Similarly [ <cond1> ] || [ <cond2> ] If [ <cond1> ] is satisfied, [ <cond2> ] is not executed.

Hence for creating a file we can do: [ -f "$filevar" ] || [ touch "$filevar" ] if [ -f "$filevar" ] is satisfied, it means the file is present, so no need to execute the second block

Additional

Linux Users and groups
Only two individual can change the permission of a file or directory

  1. The owner(generally the creator of the file)
  2. The root user https://www.linode.com/docs/tools-reference/linux-users-and-groups