C-Shell

This project is a lightweight, custom shell implementation written in C. It demonstrates the core mechanics of a command-line interface, including process management, environment variable handling, and command dispatching via a lookup table.

Made this while exploring how fork() and exec() work, and this is done with 0 AI. (except this readme)

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Features

• Built-in Commands: Native support for common operations (cd, pwd, echo, type, exit).
• System Integration: Automatically searches the system PATH to execute external binaries.
• Process Management: Uses fork, execvp, and wait to handle external program execution safely.
• Lookup Table Architecture: Uses a function-pointer-based dispatch system, making it trivial to add new built-in commands.

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Supported Commands

Built-ins

• echo [args]: Prints text to the standard output.
• pwd: Displays the current absolute working directory.
• cd [path]: Changes the current directory. Supports ~ for the home directory.
• type [command]: Identifies if a command is a shell built-in or an external binary (providing the file path).
• exit: Terminate the shell session.

External Commands

Any command not found in the built-in lookup table (e.g., ls, grep, cat, gcc) is searched for in the directories listed in your environment's PATH. If found, the shell spawns a child process to execute it.

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How It Works

The shell operates on a standard REPL (Read-Eval-Print Loop) cycle:

1. Read: Captures user input via fgets and strips the newline.
2. Eval:

• Tokenizes the string into an argument array.
• Checks the LookupTable for a matching internal function.
• If no match is found, it forks a new process to execute the command as an external program.

3. Print: Output is directed to stdout.
4. Loop: Returns to the prompt until exit is called.

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Technical Details

Dependencies

• Standard C Library (stdio, stdlib, string)
• POSIX API (unistd, sys/wait)

Memory Management

The shell dynamically allocates memory for command strings to handle varying input lengths and ensures all allocated memory is freed before the next prompt, preventing leaks during long-running sessions.

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Getting Started

Compilation

Use any standard C compiler (like gcc or clang):

gcc -o cshell main.c

Running the Shell

./cshell

Once inside, you will see the $ prompt. You can then run commands like:

$ echo hello world
$ pwd
$ type ls
$ cd /tmp

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Future Roadmap

• Support for I/O redirection (> and <).
• Command piping (|).
• Background process execution (&).
• Support for quoted strings (e.g., echo "hello world" as a single argument).