MyShell - Unix Shell in C

MyShell is a feature-rich, custom Unix shell written in C as a systems programming project. It bridges the gap between a basic command interpreter and a production-ready shell by leveraging POSIX system calls, advanced tokenization, and the GNU Readline library.

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Features

• Startup banner in green ASCII art:

         __  ___      _____ __         ____
        /  |/  /_  __/ ___// /_  ___  / / /
       / /|_/ / / / /\__ \/ __ \/ _ \/ / / 
      / /  / / /_/ /___/ / / / /  __/ / /  
     /_/  /_/\__, //____/_/ /_/\___/_/_/   
            /____/                         

                  by Martin Liarte

• Interactive Command Line: Powered by readline with full support for navigation, Tab autocompletion, and persistent command history.
• Pipelines (|): Supports chaining multiple commands (up to 16 stages) to redirect the output of one command into the input of the next.
• I/O Redirection: Fully supports input redirection (<), output redirection (>), and append mode (>>).
• Quote-Aware Tokenizer: A robust state-machine parser that correctly handles arguments enclosed in single (') and double (") quotes, preserving spaces.
• Intelligent Expansion:
  • Expands environment variables (e.g., $USER, $PATH) inline, even inside double quotes.
  • Supports tilde (~) expansion to the user's $HOME directory.
• Asynchronous Execution: Supports running processes in the background using the trailing & operator, with safe zombie process cleanup (SIGCHLD handling).
• Built-in Commands:
  • cd [dir] — Changes directory (supports cd - to toggle to the previous directory).
  • pwd — Prints the current working directory.
  • echo [-n] — Prints text to stdout (with optional newline suppression).
  • export VAR=val — Sets environment variables.
  • env — Lists all current environment variables.
  • history — Shows the session's command history.
  • clear — Clears the terminal screen.
  • exit [code] — Terminate the shell cleanly.

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Cross-Platform Installation

The project includes a multi-platform Makefile that automatically detects your OS and correctly configures readline (including Homebrew paths on macOS).

1. Clone the repository

git clone https://github.com/MartinLiarte/MyShell.git
cd MyShell

2. Compile

Using make is highly recommended as it handles platform-specific linking:

make

(Optional) If you want to compile manually using GCC, you must link the readline library:

gcc -Wall -Wextra -Wpedantic -std=c11 MyShell.c -o myshell -lreadline

3. Run

./myshell

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Technical Details & Limitations

To maintain architectural simplicity, certain static thresholds are set:

• Argument Limit: Maximum of 64 arguments per command (MAX_ARGS). Excess arguments are safely truncated with a warning.
• Pipeline Limit: Maximum of 16 pipe stages per line (MAX_PIPELINE).
• Subshell Isolation Notice: Parent-altering builtins (like cd or export) executed inside a pipeline or with I/O redirections will run inside a forked subshell, meaning their environment changes will not persist in the main shell session (a warning will notify you).

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Cleaning up

make clean

Removes the compiled binary.

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Example Usage

MyShell(Desktop)> echo "Hello world" > file.txt
MyShell(Desktop)> cat file.txt | grep Hello
Hello world

MyShell(Desktop)> export PROJECT=MyShell
MyShell(Desktop)> echo "Working on $PROJECT" from ~
Working on MyShell from /Users/martin

MyShell(Desktop)> cd -
/Users/martin/PreviousFolder

MyShell(Desktop)> exit
Bye!

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Author

Martin Liarte