POSIX Mini Shell

A small interactive Unix-like shell written in C, with foreground and background execution, a handful of built-in commands, SIGINT handling, and command history. It's a systems-programming exercise: fork/exec, signal handling, and manual memory management, not a drop-in replacement for bash.

Features

• Interactive prompt showing the current working directory
• Foreground and background execution (&)
• Built-ins: cd, pwd, exit, help, history
• History recall: !! re-runs the last command, !n runs command number n
• SIGINT (Ctrl+C) is caught so the shell doesn't die when you interrupt it
• Zombie process cleanup after each command
• Automated tests via GoogleTest

Tech stack

• C (shell) and C++ (test suite)
• CMake, with GoogleTest pulled in via FetchContent
• Linux / POSIX

Project structure

.
├── CMakeLists.txt
├── include
│   ├── internal_commands.h
│   └── msgs.h
├── src
│   ├── internal_commands.c
│   └── main.c
└── gtest
    └── shell_test.cpp

Building

cmake -S . -B build
cmake --build build

This builds two targets: build/shell, the shell itself, and build/shell_test, the test binary.

Running the shell

./build/shell

Running tests

The test suite uses GoogleTest, fetched automatically by CMake. After building, run it through ctest:

cd build
ctest --output-on-failure

Or run the test binary directly for more verbose GoogleTest output:

./build/shell_test

AddressSanitizer

To build with AddressSanitizer and LeakSanitizer enabled, configure with ENABLE_ASAN:

cmake -S . -B build-asan -DCMAKE_BUILD_TYPE=Debug -DENABLE_ASAN=ON
cmake --build build-asan
cd build-asan && ctest --output-on-failure

This is also run as a separate job in CI (see below), so any memory errors in the built-ins or history handling get caught before merge.

Continuous integration

GitHub Actions builds the project and runs the test suite on every push and pull request to main, in two jobs: a plain build/test, and a second build/test under AddressSanitizer. The workflow is in .github/workflows/ci.yml.

Built-in commands

Command	Description
pwd	Print the current working directory
cd <dir>	Change directory
cd or cd ~	Change to the home directory
cd -	Switch back to the previous directory
history	Show the last 10 commands
!!	Re-run the last command
!n	Run command number n from history
help	Show help for a command
exit	Exit the shell

Background execution

Append & to run a command in the background:

sleep 10 &

History example

/home/user$ history
9	history
8	pwd
7	ls
6	sleep 5 &
5	cd /usr
4	pwd
3	ls -l
2	echo Hello
1	whoami
0	pwd
/home/user$

Error handling

Fork failures, exec failures, bad cd targets, and invalid history references are reported to stderr rather than crashing the shell. SIGINT during a foreground command interrupts it without killing the shell process.

Limitations

This is a minimal shell, not a POSIX-compliant one. Known gaps:

• No pipes (|) or I/O redirection (<, >, >>)
• No &&, ||, or ; to chain multiple commands on one line
• No quoting or escaping — arguments are split on whitespace with strtok_r, so quoted strings and spaces in arguments aren't handled
• No environment variable expansion ($HOME, $PATH, etc.) or globbing (*.txt)
• No job control — background jobs aren't tracked, so there's no jobs, fg, or bg
• History holds the last 10 commands only, and commands are limited to 64 whitespace-separated tokens
• & is only recognized as its own token at the end of a command, not appended directly to a word (e.g. sleep 10& won't background it)

If you need any of these, you're probably looking for bash or zsh — this project is meant for learning process control and signal handling, not daily use.