unix-shell

A POSIX-compliant Unix shell written from scratch in C — pipes, I/O redirection, job control, and signal handling, all built on raw fork/exec/wait primitives.

Features

Feature	Details
Pipelines	cmd1 | cmd2 | cmd3 — stdout of each stage wired to stdin of the next
I/O Redirection	> overwrite, >> append, < input from file
Background Jobs	cmd & — tracked by job ID, listed with jobs
Job Control	fg %N, bg %N — move jobs between foreground and background
Signal Handling	SIGINT (Ctrl-C), SIGTSTP (Ctrl-Z), SIGCHLD (child reaping)
Built-ins	cd, exit, jobs, fg, bg
Process Groups	Each pipeline gets its own pgid for correct signal delivery

Build & Run

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

Usage

# Multi-stage pipeline
ls -la | grep ".c" | wc -l

# I/O redirection
sort < input.txt > sorted.txt
cat log.txt >> archive.txt

# Background job + control
sleep 60 &           # [1] 4821
jobs                 # [1]+ Running   sleep 60
fg %1                # bring to foreground
# Ctrl-Z             # suspend it
bg %1                # resume in background

# Nested pipes
cat file.txt | tr 'a-z' 'A-Z' | rev

How It Works

Command lifecycle

Input string
    │
    ▼
Tokenizer          quoted strings, whitespace, metachar splitting
    │
    ▼
Parser             builds pipeline: [ cmd1 | cmd2 | cmd3 ]
    │
    ▼
Executor
  ├── for each stage: fork() → child calls execvp()
  ├── pipe(2) connects adjacent stages
  ├── dup2(2) wires stdin/stdout to pipe ends
  ├── parent calls setpgid() to assign pipeline to a process group
  └── tcsetpgrp() hands terminal control to foreground pgid
    │
    ▼
Reaper             waitpid(WNOHANG) in SIGCHLD handler, updates job table

Signal handling

Signal	Source	Shell behavior
SIGINT	Ctrl-C	Forwarded to foreground process group; shell ignores it
SIGTSTP	Ctrl-Z	Stops foreground process group; shell marks job STOPPED
SIGCHLD	Child exit/stop	waitpid loop reaps zombies, updates job state
SIGTTOU	Background write to tty	Ignored by shell to prevent self-stops

Process groups and terminal control

Every pipeline runs in its own process group. tcsetpgrp() grants the terminal to the foreground pgid, so SIGINT/SIGTSTP only reach the running job — not the shell itself. When the job finishes or is suspended, tcsetpgrp() returns control to the shell's pgid.

Source Structure

unix-shell/
├── shell.c          # REPL loop, prompt, readline
├── tokenizer.c/h    # Lexer: splits input into tokens, handles quoting
├── parser.c/h       # Builds pipeline structs from token stream
├── executor.c/h     # fork/exec, pipe wiring, pgid management
├── jobs.c/h         # Job table: add, update, print, reap
├── builtins.c/h     # cd, exit, jobs, fg, bg
├── signals.c/h      # Signal handler setup and dispatch
└── Makefile

Implementation Notes

• waitpid(-1, WNOHANG, WUNTRACED) in the SIGCHLD handler reaps all finished children without blocking
• dup2 is called in the child after fork but before execvp to redirect file descriptors before the process image is replaced
• The tokenizer handles single/double quotes and backslash escapes before the parser sees any input
• Built-ins are checked before execvp because they need to run inside the shell process (e.g. cd must change the shell's own cwd)

Tech Stack

C · POSIX APIs (fork, execvp, pipe, dup2, waitpid, tcsetpgrp, sigaction) · GNU Make