MiniShell (msh)

MiniShell is a small Unix shell written in C. It reads a command, starts programs, waits for foreground work, and returns to its prompt. The project is intended to make a few important operating-system ideas visible in code: process creation, signals, pipes, exit codes, and child-process cleanup.

This is a learning shell. Its parser handles ordinary words, single and double quotes, a small set of $ variables, pipelines, and a trailing &. It does not implement every feature or every built-in found in Bash.

Build on Linux or another POSIX environment with a C compiler and make:

make
./msh

Remove the compiled program with make clean.

pwd
cd /tmp
echo "The shell PID is $$"
echo "The shell executable is $SHELL"
false
echo "The previous status was $?"
ls | wc -l
sleep 20 &
jobs

The default prompt is msh> . Set another prompt by entering an assignment with no spaces around =:

PS1=mini>

PS1 = mini is treated as a command, not a prompt assignment. The parser supports a trailing & for background commands. Use Ctrl-C to interrupt a foreground job and Ctrl-Z to stop it. fg resumes a job in the foreground; bg resumes it in the background. Both commands use the most recent job when no job number or process-group ID is supplied.

Implemented shell built-ins are cd, pwd, echo, exit, jobs, fg, and bg. Other command names are searched for in PATH and run as external programs.

The project keeps its implementation in one C file so the flow is easy to follow, with constants and shared declarations in main.h.

main() prepares the SHELL variable and prompt, then repeats these steps:

1. Reap any finished background children.
2. Print the prompt and read a line (scan_input).
3. Parse and execute that line (process_line).

An empty line is ignored. A line beginning with PS1= changes the prompt inside msh; the = must immediately follow PS1.

tokenize() turns the input into a list of words and operators. For example:

cat notes.txt | wc -l &

becomes roughly cat, notes.txt, |, wc, -l, &. Quotes keep spaces in one word. $?, $$, $SHELL, and environment variables are expanded while words are read. process_line() groups the words around each | into pipeline stages.

Some commands must change msh itself. For example, if cd ran only in a child process, that child's directory would change while the shell stayed where it was. builtin() therefore runs a standalone built-in in the shell process. For a pipeline such as pwd | wc -c, the built-in runs in a child because its output belongs in the pipe.

External commands use two system calls:

• fork() creates a child process.
• execvp() replaces that child with the requested program, searching PATH.

The parent shell uses waitpid() to wait for a foreground command and to collect its exit status. A normal exit status is the program's return value; a signal exit is represented as 128 + signal number.

run_pipeline() creates a pipe between each pair of stages. A pipe has a read end and a write end. The shell connects a stage's standard output to the next stage's standard input with dup2(), then closes the extra file descriptors. Each stage is a separate child process.

Interactive shells and their foreground jobs use different process groups. The terminal sends Ctrl-C (SIGINT) or Ctrl-Z (SIGTSTP) to the foreground process group. The shell gives the terminal to a foreground job while it runs, then takes terminal control back after the job exits or stops. Background jobs do not receive terminal input by default.

The shell ignores keyboard job-control signals so Ctrl-C does not kill msh. Children restore the default signal behavior before running a command. The SIGCHLD handler only sets a flag; reap_children() calls waitpid() later in the main loop. This keeps non-signal-safe work out of the signal handler and prevents completed background processes from remaining zombies.

Each job records its process-group ID, child PIDs, command text, and child states (running, stopped, or complete). jobs displays the table. fg gives a job the terminal and waits for it. bg sends SIGCONT and leaves the job in the background. When every process in a background job finishes, msh prints its exit status and removes the job entry.

• main.c — input loop, parser, built-ins, process launching, and job control.
• main.h — shared constants and function declarations.
• Makefile — build and clean targets.
• README.md — this guide.

This project is intentionally smaller than a production shell. It does not support redirection (>, <), command separators (;), shell scripting, globbing, command substitution, or the full Bash built-in list. Variable expansion is deliberately limited and does not perform shell word splitting.