minishell - A Minimal POSIX Shell

A compact shell that recreates essential POSIX behaviors: command parsing, pipelines, redirections, heredocs, builtins, and signal handling. The implementation focuses on disciplined memory management to prevent cascading failures during long-running interactive sessions.

Overview

minishell interprets user input, builds an abstract representation of commands, and executes them with proper process control and I/O redirection.

Core Features

• Command execution with environment variable expansion (including special parameters like $?)
• Pipelines with correct stdin/stdout wiring
• Input/output redirections (>, >>, <)
• Heredocs with delimiter handling and signal awareness
• Builtins: echo, cd, pwd, export, unset, env, exit
• Exit status propagation across pipelines
• Signal handling for interactive use (e.g., Ctrl+C, Ctrl+\, Ctrl+D)

Memory Management & Failure Containment

The primary engineering challenge is preventing memory leaks and ensuring that any allocation failure does not cascade through the shell lifecycle. The project addresses this by:

• Owning every allocation explicitly and freeing all transient data at the end of each command loop iteration
• Centralizing cleanup paths so errors unwind resources in a single place
• Validating allocations and inputs before use; if validation fails, execution is stopped for that command but the shell remains responsive
• Separating parsing structures from execution structures to avoid partial frees or double frees

This discipline keeps the shell stable even under sustained interactive use or under intentional misuse.

Parsing and Execution Pipeline

1. Lexing: Tokenize input while respecting quotes and escape rules.
2. Parsing: Build command nodes with arguments, redirections, and pipeline links.
3. Expansion: Substitute environment variables and handle special parameters.
4. Execution: Run builtins in-process when possible; otherwise, fork/exec external commands with the correct file descriptor setup.
5. Cleanup: Release all per-command allocations before the next prompt.

Redirections and Heredocs

• Supports >, >>, and < with proper error reporting if files cannot be opened
• Heredocs capture input until the delimiter is seen; signals during heredoc are handled to avoid leaving stale temporary data

Signals

• Ctrl+C interrupts the current command and returns a fresh prompt
• Ctrl+\ is ignored in the prompt but propagated correctly to child processes
• Ctrl+D (EOF) exits the shell cleanly, releasing resources

Building & Running

make
./minishell

Learning Outcomes

• Memory ownership discipline in a long-lived interactive process
• Robust error handling that prevents small failures from escalating
• Process control, forking, and file descriptor management for pipelines
• Practical parsing: tokenization, quoting, expansion, and syntax validation

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A 42 School project showcasing shell fundamentals with a focus on memory safety and failure isolation.