Custom Shell Program

Overview

This is a custom shell program written in C that supports various features such as command execution, redirection, background execution, built-in commands, variable handling, and more. The shell provides an interactive environment for running commands similar to standard Unix shells.

Features

1. Basic Command Execution: Execute standard Unix commands with arguments.
2. Redirection:
   • Output redirection (>)
   • Append redirection (>>)
   • Error redirection (2>)
3. Background Execution: Run commands in the background using &.
4. Built-in Commands:
   • Change prompt (prompt =)
   • Print arguments (echo)
   • Change directory (cd)
   • Print the last command status (echo $?)
   • Exit the shell (quit)
   • Repeat the last command (!!)
5. Signal Handling: Custom message on Control-C.
6. Pipes: Chain multiple commands with |.
7. Variable Handling: Set and use custom variables.
8. Flow Control: Support for if/else statements.
9. User Input: Read user input and use it in commands.
10. Command History: Navigate through command history using arrow keys.

Compilation

To compile the shell program, use the following command:

make

Usage

To run the shell, execute:

./myshell

Examples

Basic Commands and Redirection

hello: ls -l
hello: ls -l > file
hello: ls -l >> file
hello: ls no_such_file 2> error.log

Built-in Commands

hello: prompt = myprompt
myprompt: echo abc xyz
myprompt: echo $?
myprompt: cd mydir
myprompt: pwd
myprompt: !!
myprompt: quit

Background Execution

hello: sleep 5 &

Using Variables

hello: $filename = "testfile.txt"
hello: echo "This is a test" > $filename
hello: cat $filename

Read Command

hello: echo Enter your name:
hello: read username
hello: echo "Hello, $username!"

Flow Control (If/Else)

hello: if grep -q "pattern" file.txt
hello: then
hello: echo "Pattern found"
hello: else
hello: echo "Pattern not found"
hello: fi

Piping Commands

hello: cat file.txt | grep "search" | sort | uniq

Notes:

• Use Ctrl + C to test the custom signal handling(eliminate child processes but not the parent).
• Navigate through command history using the up and down arrow keys.
• The if command is actuallize in one row(as in the examples above).