Mini Shell (C)

A small but capable POSIX-style shell

Overview

This group project implements a simple shell in C that runs on openlab with GCC 11. It supports built-in commands, executes local binaries in the foreground or background, manages jobs with signals, and handles input and output redirection.


Features

  • Built-ins: cd, pwd, jobs, bg, fg, kill, quit. Built-ins run in the shell process. jobs lists running and stopped background tasks. fg and bg accept either %JID or PID.
  • Foreground execution: spawn with fork, replace with execv or execvp, and waitpid for completion. Ctrl-C sends SIGINT to the foreground child only.
  • Background execution: append & to run jobs without blocking the prompt. Children are reaped by a SIGCHLD handler.
  • Job control: track pid, status, and command line in a job table. Ctrl-Z stops the foreground job with SIGTSTP, fg and bg send SIGCONT. kill terminates by id or pid.
  • I/O redirection: <, >, and >> are parsed and applied via open and dup2 with the required modes. One input and one output redirect at most, as specified.
  • Parsing: whitespace-tokenized argv with background and redirection handling. Both absolute and PATH-based execution are supported through execv and execvp.


Architecture

  • Parser: parseline tokenizes the command line, detects &, and resolves optional <, >, >> into file descriptors stored for later dup2.
  • Built-ins: buildin_command implements control-flow commands directly. jobs, fg, bg, and kill read or update the job table.

  • Signal layer:

    • SIGINT handler targets the foreground child.
    • SIGTSTP stops the foreground child and updates status to Stopped.
    • SIGCHLD reaps finished background jobs without blocking.
  • Evaluator: eval wires parsing, built-ins, forking, redirection, and waiting. For background jobs, it prints the pid and returns the prompt immediately.


How to Build and Run

# Build the shell
gcc -o shell hw2.c

# Optional helpers from the assignment packet
gcc -o counter counter.c      # prints a line every second
gcc -o add add.c              # prints n+2 for input n

Examples

prompt > pwd
/home/uci-user
prompt > cd ..
prompt > counter
Counter: 0
Counter: 1
^C
prompt > counter &
12345 counter &
prompt > jobs
[1] (12345) Running counter &
prompt > kill %1
prompt > jobs

Redirection

# number.txt contains: 40
prompt > add < number.txt > added_number.txt
prompt > cat added_number.txt
42

The above follows the assignment’s redirection contract.


Notes on Design Choices

  • The shell accepts both /bin/ls style absolute commands and ls resolved via PATH. This is achieved by trying execv first, then execvp.
  • Foreground and background states are tracked in a fixed-size job array with pid, status text, and the original command line, which makes jobs, fg, and bg straightforward.