Minishell

A small Unix shell implemented in C as part of the 42 curriculum.

Built by omiskiny and recan.

Overview

Minishell recreates the core execution model of a Unix shell: command parsing, environment expansion, process creation, pipelines, redirections, heredocs, built-ins, and exit-status handling.

The project focuses on low-level systems programming and resource management rather than relying on an existing shell implementation.

Engineering Focus

• Lexing & parsing — converts raw command input into structured tokens and command nodes.
• Expansion — handles environment variables, $?, quote-aware expansion, and quote removal.
• Process management — uses fork(), execve(), and waitpid() to execute external commands.
• Pipelines — connects processes with Unix pipes and manages their file descriptors.
• Redirections — supports <, >, and >> through descriptor duplication with dup2().
• Heredocs — collects input until a delimiter and provides it to the command as standard input.
• Built-ins — implements echo, cd, pwd, export, unset, env, and exit.
• Signals — handles interactive shell signals without corrupting the command loop.
• Resource management — explicitly frees command structures, environment data, and execution resources.

Architecture

The implementation is split into focused modules:

input
  │
  ▼
lexer ──► parser ──► expander ──► executor
                                      │
                         ┌────────────┼────────────┐
                         ▼            ▼            ▼
                      builtin      pipeline    redirection
                                      │
                                      ▼
                                  child process
                                      │
                                   execve()

The shell keeps parsing, expansion, and execution separate so that each stage can transform the command representation before the next stage consumes it.

Supported Features

Commands

• External executables resolved through PATH
• Command pipelines (cmd1 | cmd2 | cmd3)
• Input/output redirection (<, >, >>)
• Heredocs (<<)
• Environment-variable expansion
• Exit-status expansion through $?
• Single and double quote handling

Built-ins

Built-in	Purpose
echo	Prints arguments, including repeated -n flags
cd	Changes the working directory and updates PWD / OLDPWD
pwd	Prints the current working directory
export	Creates or updates environment variables
unset	Removes environment variables
env	Prints exported environment entries with values
exit	Terminates the shell with the appropriate status

Error Handling

The shell validates common failure cases, including invalid identifiers, invalid exit arguments, missing commands, and execution failures. External commands that cannot be resolved through PATH return the conventional 127 status.

Build

Requirements

• Unix-like operating system
• C compiler
• GNU Readline development headers/library
• make

On Debian/Ubuntu:

sudo apt-get install libreadline-dev

Compile

make

Run

./minishell

Clean

make clean
make fclean

Sanitizers / Valgrind

The Makefile also provides development targets:

make sanitize
make val

Example

$ ./minishell
minishell> echo "Hello from $USER"
minishell> printf 'hello\nworld\n' | grep world
minishell> cat < input.txt > output.txt
minishell> cat << EOF
> hello from heredoc
> EOF

Project Scope

This repository targets the mandatory Minishell requirements. The bonus section of the 42 subject is not included.

Development

The repository uses a lightweight feature-branch workflow:

main
  ▲
  │ PR + CI
  │
feature/* / fix/* / refactor/* / chore/*

Changes should be isolated in a focused branch and merged through a pull request after the build checks pass.

References

• 42 Minishell subject and project documentation
• Bash Reference Manual
• Unix/Linux manual pages for fork, execve, waitpid, pipe, dup2, signals, and related system calls

License

This project was created for educational purposes as part of the 42 curriculum. No separate open-source license is currently declared.