daemonSH

A custom Unix shell in C built on top of the starter for SUTD 50.005's PA1 assignment, then taken well beyond the base requirements as a personal, post-course rebuild.

What it does

• Real shell loop. The starter shell ran exactly one command and exited (execv replaces the process image with no fork/loop around it). This implements the actual fork → exec → wait → re-prompt cycle, plus graceful handling of blank input and Ctrl+D.

• Seven builtins, run in-process (never forked, since things like cd need to persist in the shell's own state): cd, help, exit, usage, env, setenv, unsetenv.

• .daemonshellrc startup config. On launch, the shell reads .daemonshellrc from its working directory if present. Two directive types: PATH=... extends where external commands are looked up, and any other line is run as a startup command before the first prompt.

• PATH is opt-in, not inherited. By default the shell only knows its own bundled bin/ programs plus the seven builtins — nothing from the host system's PATH is visible unless .daemonshellrc explicitly adds it. Bundled bin/ always takes priority even when more directories are added, so a same-named system binary (e.g. /usr/bin/ld, the real linker) can never shadow this shell's own ld.

• Seven bundled system programs under bin/, built alongside the shell:

  • find, ld, ldr — from the original assignment scope.
  • sys — a small neofetch-style system info dump (OS, kernel, CPU, memory, disk, uptime), shown above.
  • dspawn — spawns a proper Unix daemon via the full double-fork, setsid, signal-handling, fd-redirection sequence, not just a backgrounded process.
  • dcheck — counts how many dspawn daemons are currently alive.
  • backup — zips whatever $BACKUP_DIR points to (a file or a directory) into a timestamped archive under archive/.

Building and running

make            # builds ./daemonshell and everything under bin/
./daemonshell

Testing

make test       # unit + integration
make unit       # Unity-based tests for pure helpers (perms, .daemonshellrc parsing)
make integration # black-box bash scripts that drive ./daemonshell as a subprocess

11 unit tests, 7 integration tests, all passing as of this writing.

Layout

• source/ Shell loop, builtins, .daemonshellrc loader
• libs/ Pure, unit-testable helpers
• system_programs/ The seven bin/ programs
• includes/ Matching headers
• tests/
• unit/ Unity tests for source/libs/
• integration/ Bash scripts exercising the built shell end-to-end
• files/ Fixture data the integration tests run against

Credit

Forked from natalieagus/cse-pa1-starter-template, the starter scaffold for SUTD 50.005's PA1. Everything beyond the original one-shot shell and the three bundled programs (find/ld/ldr) is my own solo rebuild, done after the course had concluded.