C Shell

A Unix shell built from scratch in C. Implements process management, I/O redirection, pipelines, job control, and signal handling — all without using any shell library functions.

Features

Built-in Commands

Command	Description
hop [path...]	Change directory. Supports ~, .., - (previous dir), and multiple paths
reveal [-flags] [path]	List directory contents (ls equivalent). Flags: -a (hidden files), -l (long format)
log	Show command history (last 15 commands, persists across sessions)
log purge	Clear command history
log execute <n>	Re-run the nth most recent command
activities	List all background/stopped jobs sorted by name
fg [job_number]	Bring a background or stopped job to the foreground
bg [job_number]	Resume a stopped job in the background
ping <pid> <signal_number>	Send a signal to a process

Shell Features

• Pipelines — chain commands with | (e.g. ls | grep foo | wc -l)
• I/O Redirection — <, >, >> for input/output/append
• Background execution — run commands with &
• Sequential execution — chain commands with ;
• Signal handling — Ctrl-C (SIGINT), Ctrl-Z (SIGTSTP), Ctrl-D (logout)
• Job control — stop, resume, and track background processes
• Persistent history — command log survives shell restarts

Build & Run

cd shell
make
./shell.out

Requires GCC with C99 and POSIX support.

Architecture

shell/
├── main.c          # REPL loop, signal handler setup, startup
├── headers.h       # Shared includes and global externs
├── prompt.c/h      # Prompt rendering (user@host:path)
├── parser.c/h      # Tokenizer + recursive descent parser
├── execute.c/h     # Command dispatch and job tracking
├── commands.c/h    # Pipeline execution and I/O redirection
├── hop.c/h         # cd equivalent
├── reveal.c/h      # ls equivalent
├── log.c/h         # Command history
├── activities.c/h  # List background jobs
├── fgbg.c/h        # fg and bg commands
├── ping.c/h        # Signal sending utility
└── signals.c/h     # SIGINT, SIGTSTP, SIGCHLD handlers

The parser uses a context-free grammar to handle operator precedence:

shell_cmd → cmd_group ((&|;) cmd_group)* &?
cmd_group → atomic (| atomic)*
atomic    → word (word | < word | > word | >> word)*

Each pipeline stage runs in its own process group (setpgid), so signals from the terminal reach the correct process and not the shell itself.