shell-c

A Unix shell written in C.

Features

• Pipelines: arbitrary-length cmd1 | cmd2 | cmd3
• I/O redirection: <, >, >>, 2>, 2>>, 2>&1
• Command substitution: $(cmd) runs in a subshell and splices stdout inline; works mid-token (/usr/$(uname -m)/lib) and with pipelines inside ($(cat f | wc -l))
• Expansion: $VAR, $?, ~, ~user
• Job control: jobs, fg [%N], bg [%N]; Ctrl+Z stops a foreground job, fg restores terminal ownership and resumes with SIGCONT
• Background execution: trailing &
• Signal handling: Ctrl+C and Ctrl+Z reach the foreground pipeline, not the shell; SIGCHLD is blocked around fork loops to eliminate handler races
• Script execution: ./shell-c script.sh runs non-interactively; prompts and history are suppressed automatically
• Persistent history: saved to ~/.shell-c_history on exit, restored on startup, capped at 100 entries
• Built-ins: cd, exit, export, history, jobs, fg, bg
• Trace mode: --trace logs every fork, exec, and pipe decision to stderr

Build

make
make test   # run the regression suite (48 tests)

Requires a C11 compiler and POSIX.1-2008.

Usage

./shell-c                    # interactive
./shell-c script.sh          # run a script file
./shell-c --trace            # interactive with execution tracing
./shell-c --trace script.sh  # trace a script

Examples

# Multi-stage pipeline
cat /etc/passwd | grep root | cut -d: -f1 > out.txt

# Stderr redirection
gcc bad.c 2> errors.txt       # stderr to file
make 2>&1 | grep error        # merge stderr into stdout then pipe
make > build.log 2>&1         # stdout and stderr to the same file

# Job control
sleep 60 &    # [1] 12345 — runs in background
jobs          # [1] running   sleep 60 &
fg %1         # bring to foreground
              # Ctrl+Z to stop it
bg %1         # resume in background

# Command substitution
echo "today is $(date)"
echo /usr/$(uname -m)/lib
echo "$(cat /etc/hosts | wc -l) lines"

# Variable and tilde expansion
ls /nonexistent
echo $?        # 1
echo ~         # /Users/you
echo $HOME     # /Users/you

Built-ins

Command	Description
cd [dir]	Change directory; defaults to $HOME; cd - returns to the previous directory
exit [code]	Exit with optional status code
export VAR=VALUE	Set an environment variable
history	Print command history
jobs	List active background and stopped jobs
fg [%N]	Bring job N to the foreground
bg [%N]	Resume stopped job N in the background

Implementation notes

Concern	Approach
Pipeline wiring	pipe(2) + dup2(2) per child before execvp; all unused ends closed in parent and child
Stderr redirect	stdout is redirected before stderr so > file 2>&1 correctly points both fds at the file
Command substitution	subshell stdout captured via pipe; parent drains with a doubling read loop; stdin set to /dev/null to suppress terminal handoff inside the subshell; SIGCHLD blocked around fork and waitpid to prevent the reaping handler stealing the exit status
Depth-aware tokenizer	next_arg() and split_pipes() track $( nesting depth so spaces and `
Process isolation	setpgid(2) puts every pipeline in its own process group; both parent and child call it to close the TOCTOU race
Terminal handoff	tcsetpgrp(3) transfers the terminal to the foreground pipeline; reclaimed by the shell on return
Job table safety	SIGCHLD blocked via sigprocmask around fork loops and job_add/job_remove; handler writes only volatile int fields, no stdio
Stopped job detection	SA_NOCLDSTOP omitted so SIGCHLD fires on stop; waitpid uses WUNTRACED in the foreground wait path
Background reaping	waitpid(-1, WNOHANG | WUNTRACED) in SIGCHLD handler updates job status; main loop notifies before each prompt
Persistent history	loaded in history_init; rewritten on exit capped at 100 lines; gated on isatty(fileno(src)) so script runs don't pollute the file
Script mode	shell_loop takes FILE *src; isatty(fileno(src)) controls prompts, history, and job notifications with no extra flags