Mini Shell

A Unix shell written in C from scratch. Built as part of learning systems programming — processes, file descriptors, and how the OS actually works underneath every terminal you've ever used.

What it does

• Runs any command on your system (ls, pwd, echo, git, anything)
• Forks a child process per command, execs the binary, waits for it to finish
• Tokenizes raw input into an argument vector
• Handles blank input cleanly
• Reports errors when a command isn't found or can't be executed

What's coming

• Built-in commands: cd, exit
• I/O redirection: echo hello > file.txt, cat < file.txt
• Pipes: ls | grep .c
• Signal handling: Ctrl+C kills the running command, not the shell

How to build

git clone https://github.com/yourusername/mini-shell
cd mini-shell
mkdir build &&cd build
cmake ..
make
./mini_shell

Or with gcc directly:

gcc -std=c99 -Wall -Wextra -o xsh main.c
./xsh

How it works

Every command goes through the same path:

read input
↓
tokenize into argv[]
↓
fork() — creates child process
↓
child: execvp(argv[0], argv) — child becomes the command
parent: waitpid() — shell waits, then reprints prompt

The shell is the parent process. It never becomes the command — it forks a child, the child transforms into the command via execvp, runs, and exits. The parent collects the exit status and loops.

What I learned building this

• The difference between a terminal and a shell
• How fork() clones a process and copy-on-write defers the actual memory duplication
• Why execvp never returns on success — it replaces the process entirely
• Why cd and exit must be built-ins, not forked children
• How file descriptors are inherited across fork() — the mechanism that makes pipes possible
• What a zombie process is and why waitpid() prevents it

Environment

• Language: C99
• Platform: macOS (Apple Silicon)
• Built with CLion + CMake
• Memory checked with AddressSanitizer

Part of

Building toward EduOS — a privacy-first, AI-native Linux-based OS for African schools.