unix-shell

A POSIX-compliant Unix shell implemented from scratch in C. Supports the core features you'd expect from a real shell — pipes, I/O redirection, background jobs, signal handling, and a set of built-in commands.

Written as a deep dive into Unix process management, file descriptors, and the exec family of syscalls.

Features

• Command execution — fork/exec with PATH resolution
• Pipes — arbitrary pipeline chains (cmd1 | cmd2 | cmd3)
• I/O redirection — >, >>, <, 2>, 2>&1
• Background jobs — cmd & with job table tracking
• Job control — jobs, fg, bg, Ctrl-Z (SIGTSTP)
• Signal handling — SIGINT, SIGQUIT, SIGCHLD handled correctly
• Built-ins — cd, pwd, exit, export, unset, jobs, fg, bg, history
• Environment variables — $VAR expansion, export, unset
• Command history — up/down arrow navigation (via readline)
• Quoting — single quotes, double quotes, backslash escaping

Building

git clone https://github.com/Harsh7115/unix-shell
cd unix-shell
make
./hsh

Requires: GCC, GNU Make, readline (sudo apt install libreadline-dev on Ubuntu)

Usage

# Basic command
$ ls -la

# Pipeline
$ cat /etc/passwd | grep root | cut -d: -f1

# Redirection
$ gcc main.c -o prog 2> build.log
$ ./prog < input.txt > output.txt

# Background job
$ sleep 60 &
[1] 12345

# Job control
$ jobs
[1]+ Running    sleep 60 &
$ fg 1

Architecture

unix-shell/
├── src/
│   ├── main.c          # REPL loop, readline integration
│   ├── lexer.c         # Tokeniser (handles quoting, escaping)
│   ├── parser.c        # Builds command tree from tokens
│   ├── executor.c      # fork/exec, pipes, redirections
│   ├── builtins.c      # cd, export, jobs, fg, bg, history …
│   ├── jobs.c          # Job table, SIGCHLD handler
│   ├── signals.c       # Signal setup for interactive shell
│   └── env.c           # Environment variable management
├── include/
│   ├── shell.h         # Shared types and prototypes
│   └── jobs.h
├── tests/
│   └── run_tests.sh    # Integration tests
├── Makefile
└── README.md

Implementation Notes

• Uses waitpid(WNOHANG) in the SIGCHLD handler to reap background children without blocking
• Pipes are built left-to-right: each stage creates a pipe(2) pair, the left child writes to pipefd[1] and the right reads from pipefd[0]
• The shell forks a separate process group for each pipeline so Ctrl-C only kills the foreground job, not the shell itself
• Readline callbacks set the terminal back to canonical mode before exec so child processes see a normal tty

Running Tests

make test

License

MIT