ICSH — a small Unix shell in C

A command-line shell written in C for ICCS227 (Operating Systems), built up over seven milestones. It reads commands, runs external programs, redirects input and output, and manages background jobs with signal-based job control.

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  | Welcome to IC Shell! |  \_,` | \-'  /   )`-')
   --------------------      """) `"`    \  ((`"`
                            ___Y  ,    .'7 /|
                           (_,___/...-` (_/_/

What it does

• Runs external commands with fork + execvp, and waits on the foreground job.
• Two builtins: echo and exit [code] (the exit code is masked to 8 bits, as the assignment asks). echo $? prints the exit status of the last command.
• Repeats the previous command with !!.
• Redirects input with < file and output with > file (the output file is created or truncated, mode 0644). Redirection applies to builtins too, then the original stdin/stdout are restored.
• Runs a command in the background with a trailing & and tracks it as a job.
• Job control: jobs lists jobs by id (Running / Stopped / Done), fg %n brings a job to the foreground, bg %n resumes a stopped job in the background.
• Forwards Ctrl-C (SIGINT) and Ctrl-Z (SIGTSTP) to the foreground job only, so they don't kill the shell. A SIGCHLD handler reaps children and prints when a background job stops, resumes, or finishes.
• Ctrl-D at the prompt exits.

Build and run

You need gcc and make on a POSIX system (Linux, WSL, or macOS).

make            # builds ./icsh
./icsh          # interactive mode
./icsh test.sh  # run a script file
make clean

Commands

Command	What it does
echo <text>	Print text
echo $?	Print the last exit status
exit [code]	Leave the shell (code masked to 0–255)
!!	Run the previous command again
<cmd> &	Run in the background
jobs	List background and stopped jobs
fg %n	Move job n to the foreground
bg %n	Resume stopped job n in the background
<cmd> < in > out	Redirect input and/or output
anything else	Run it as an external program

Scripting mode

Given a file argument, the shell reads it line by line with no prompt or welcome banner. Lines that start with # or // are treated as comments. test.sh and test2.sh are small examples. test.sh is:

echo hello
echo world
!!
exit 5

How it's organized

File	Responsibility
icsh.c	The read-eval loop: prompt, builtins, redirection dispatch, forking external commands, fg/bg
parser.c	Tokenizes a line, pulls out < / > / &, and applies or restores redirection with dup2
jobs.c	The job list, kept as a linked list: add, remove, find, update status, print
signals.c	Handlers for SIGINT, SIGTSTP, and SIGCHLD
shell.h	The job struct, the status enum, and shared prototypes

Each job runs in its own process group (setpgid), and the foreground job owns the terminal through tcsetpgrp. That is what lets Ctrl-C and Ctrl-Z land on the right process instead of the shell itself.

Known limits

• A command line is capped at 255 characters (MAX_CMD_BUFFER).
• No pipes (|), no quoting, no globbing, no environment-variable expansion. One input and one output redirection per command.
• It uses POSIX job-control calls, so it builds and runs on Linux, WSL, and macOS, but not on native Windows.