WitsShell

A custom Unix shell implementation written in C that provides basic shell functionality including command execution, built-in commands, path management, I/O redirection, and parallel command execution.

Features

Built-in Commands

• cd <directory> - Change current working directory
• path [directories...] - Set executable search paths
• exit - Exit the shell (must be called without arguments)

Advanced Features

• I/O Redirection - Redirect command output to files using >
• Parallel Commands - Execute multiple commands concurrently using & separator
• Environment Variables - Support for environment variable expansion using $VARIABLE
• Quoted Strings - Handle arguments with spaces using double quotes
• Batch Mode - Execute commands from script files
• Interactive Mode - Standard shell prompt interface

Compilation

gcc -o witsshell witsshell.c

Usage

Interactive Mode

Run the shell interactively:

./witsshell

You'll see the witsshell> prompt where you can enter commands.

Batch Mode

Execute commands from a file:

./witsshell script.txt

Command Examples

Basic Commands

witsshell> ls -la
witsshell> cat file.txt
witsshell> pwd

Built-in Commands

# Change directory
witsshell> cd /home/user

# Set executable paths
witsshell> path /bin /usr/bin /usr/local/bin

# Clear all paths
witsshell> path

# Exit shell
witsshell> exit

I/O Redirection

# Redirect output to file
witsshell> ls -la > output.txt
witsshell> echo "Hello World" > greeting.txt

Environment Variables

# Use environment variables
witsshell> echo $HOME
witsshell> ls $HOME

Quoted Arguments

# Handle arguments with spaces
witsshell> echo "This is a quoted string"
witsshell> mkdir "My Documents"

Parallel Commands

# Execute commands in parallel
witsshell> sleep 5 & echo "Hello" & ls

Technical Details

Path Management

• Default path is set to /bin/ on startup
• Use the path command to modify executable search paths
• Commands are searched in the order paths were specified
• If no paths are set, external commands cannot be executed

Error Handling

• All errors output the standard message: "An error has occurred\n"
• Errors are written to stderr
• Invalid commands, file operations, and syntax errors are handled gracefully

Process Management

• External commands are executed in child processes using fork() and execv()
• Parent process waits for child completion unless running in parallel
• Proper cleanup of zombie processes in parallel execution

Memory Management

• Dynamic memory allocation for paths and command arguments
• Proper cleanup of allocated memory on exit
• Handles memory allocation failures gracefully

Implementation Notes

Redirection Rules

• Only supports output redirection (>)
• Redirects both stdout and stderr to the specified file
• File is created with permissions 0644
• Multiple > symbols in a command result in an error
• Redirection must have a valid filename

Parallel Execution

• Commands separated by & run concurrently
• Each command runs in its own child process
• Shell waits for all parallel commands to complete before accepting new input

Environment Variable Expansion

• Variables prefixed with $ are expanded to their values
• Undefined variables are treated as empty strings
• Expansion occurs during command tokenization

Error Conditions

The shell will output "An error has occurred\n" for:

• Invalid file operations
• Command not found
• Memory allocation failures
• Invalid syntax (malformed redirection, etc.)
• Built-in command errors (invalid directory for cd, etc.)
• Exit command called with arguments

Limitations

• No support for input redirection (<)
• No piping between commands
• No background process management beyond parallel execution
• No command history or tab completion
• No support for complex quoting or escaping beyond basic double quotes
• No globbing or wildcard expansion

File Structure

witsshell.c          # Main source file containing all implementation
README.md            # This documentation file

License

This project was created as an educational exercise. Feel free to use and modify as needed.

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Created as part of a systems programming exercise to understand Unix shell internals and process management.