Making Own Shell(CLI) in C

Overview

This project involves creating a simple shell, which operates by creating child processes to execute commands entered by the user. The shell functions similarly to the bash shell but includes several simplifications to make it more manageable.

Features

Prompt and Input

• Interactive Prompt: The shell displays a prompt (shell>) and waits for user input.
• Command Input: Users can enter commands with the full path to the executable (e.g., /bin/ls) along with command-line options.

Command Execution

• Non-built-in Commands: When a user enters a command, the shell creates a new process to run the specified program.
• Foreground Execution: Commands without an ampersand (&) run in the foreground, and the shell waits for them to complete.
• Background Execution: Commands ending with an ampersand (&) run in the background, allowing the shell to prompt for new commands immediately. The shell also outputs the PID of the background process.

Process Management

• Graceful Error Handling: The shell handles cases where the specified executable cannot be invoked.
• Background Process Completion: The shell checks for completed background processes and outputs their PIDs, ensuring zombie processes are reaped using waitpid with WNOHANG.
• Moving Background Commands to Foreground: The fg command moves the most recent background process to the foreground. If this process has already finished, the next most recent background process is moved to the foreground.

Getting Started

1. Clone your repository on your Raspberry Pi (RPi).
2. Run your shell interactively by executing the compiled program.

Main Function

The main function in main.c includes:

1. Displaying the prompt (shell>)
2. Reading user input into a buffer
3. Tokenizing the input command, where tokens are delimited by whitespace

Sample Commands

Here are some example commands to test the shell:

shell> /usr/bin/ls
shell> /usr/bin/ls -a
shell> /usr/bin/date
shell> /usr/bin/sleep 5
shell> /usr/bin/sleep 10 &
Process 5328 started
shell> /usr/bin/sleep 5 &
Process 5329 started
shell> /usr/bin/ps
shell> /usr/bin/sleep 15
shell> /usr/bin/ps
Process 5329 finished
Process 5328 finished
shell> /usr/bin/sleep 7 &
Process 5333 started
shell> fg
shell> /usr/bin/echo Hello
Hello

Notes

• Foreground Execution: The shell waits for commands like /usr/bin/sleep 5 to complete before displaying the prompt again.
• Background Execution: Commands like /usr/bin/sleep 10 & run in the background, and the prompt appears immediately.
• Process Listing: Use /usr/bin/ps to list processes and verify background processes.
• Foreground Command: The fg command moves the most recent background process to the foreground.