A Mock Shell: crash

Built a minimal Unix-style shell called crash that supports job control, background/foreground execution, signal handling, and job tracking.

Summary

Implemented a command-line shell that:

• Parses input lines into commands
• Supports foreground and background execution (&)
• Manages multiple jobs concurrently
• Handles user signals like Ctrl+C, Ctrl+Z, and Ctrl+\
• Implements job control commands like jobs, fg, bg, and nuke

This project emphasizes signal safety, concurrency, and process control — core concepts in systems programming.

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Specification

Supported Commands

Command	Description
quit	Exit the shell
jobs	List current background/suspended jobs
nuke [PID/%JOBID ...]	Kill one or more jobs or PIDs with SIGKILL
fg [PID/%JOBID]	Resume a job in the foreground
bg [PID/%JOBID ...]	Resume job(s) in the background
program args	Run a program in the foreground
program args &	Run a program in the background

• Job IDs use %N format (e.g. %3)
• PIDs are direct numeric values
• Commands can be separated by ; or &

Job Execution

• Foreground jobs block the shell until finished or suspended.
• Background jobs allow continued shell use immediately.
• Each job receives a unique job ID starting from 1.
• Shell supports up to 32 concurrent jobs.

Signals

Signal	Action
Ctrl+C (SIGINT)	Sends signal to foreground process
Ctrl+Z (SIGTSTP)	Suspends foreground process
Ctrl+\ (SIGQUIT)	Quits the shell if no foreground job, else sends to foreground process
Ctrl+D (EOF)	Exits shell if idle

Output Formatting

Messages look like:

[1] (12345)  running    sleep
[1] (12345)  suspended  sleep
[1] (12345)  continued  sleep
[1] (12345)  finished   sleep
[1] (12345)  killed     sleep
[1] (12345)  killed (core dumped)  sleep

• Commands are displayed without arguments
• Output are atomic (no interleaved prints)

⚠ Error Messages

All error messages go to standard error and follow this format:

ERROR: fg needs exactly one argument
ERROR: no job 23
ERROR: cannot run foo

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Notes

• This shell does not support features like pipes (|), redirection (>, <), or quoting.
• Everything was implemented in C, in a single file: crash.c
• No external libraries used beyond standard libc