minishell

A simplified Unix shell written in C, built from scratch as an OS/Linux learning project. It implements a REPL, a lexer/parser, $VAR/$? expansion, pipes, redirections (<, >, >>, <<), signal handling, and a set of builtins — the same core ideas real shells like bash are built on.

This is a learning project, not a production shell. Scope was deliberately capped (see knowledge.md) once the core OS concepts — processes, file descriptors, and signals — were solidly covered.

Features

• REPL with readline (arrow-key history, line editing)
• Lexer that tokenizes input, correctly handling 'single' and "double" quotes
• Parser that builds a pipeline of commands, each with its own argument list and redirections
• Expansion of $VAR and $?, respecting quoting rules (single quotes suppress expansion, double quotes don't)
• Execution via fork + execve + $PATH resolution
• Builtins (run in-process, not forked): cd, pwd, echo, export, unset, env, exit
• Pipes — any number of commands chained with |
• Redirections — <, >, >>, and heredocs (<<)
• Signal handling — Ctrl-C and Ctrl-\ behave like in bash (the shell survives; a running child is what gets interrupted/killed)
• Exit status propagation — $? and process exit codes match bash's conventions (e.g. 128 + signal when a command is killed)
• Error handling — command not found, bad redirection targets, failed fork/pipe, and redirection-only lines (> file.txt with no command) are all handled without crashing

Build

make

Produces a minishell binary. Requires libreadline-dev (or your distro's equivalent readline development package).

Other targets:

make clean   # remove object files
make fclean  # remove object files + binary
make re      # fclean + all

Run

./minishell

You'll get a minishell$ prompt. Type commands as you would in bash:

minishell$ echo hello | wc -c
6
minishell$ echo "user is $USER, last status was $?"
minishell$ cat << EOF
heredoc line with $HOME expanded
EOF
minishell$ ls > out.txt
minishell$ export NAME=world
minishell$ exit

Exit with exit, or Ctrl-D (EOF), just like a real shell.

Project structure

minishell/
├── Makefile
└── src/
    └── main.c

Everything currently lives in one file by design (see knowledge.md for why) — it was kept single-file deliberately while the project scope stayed at this size, to avoid header/linkage indirection with no payoff yet.

What's intentionally out of scope

These were identified as high effort during planning, and skipped in favor of hardening what's already built:

• && / || logical operators
• Wildcards (*)
• Subshells ((...))
• A fully custom envp array (currently uses the process environment directly via getenv/setenv/unsetenv)

See knowledge.md for the full reasoning, the build-up of the project step by step, and what each step was meant to teach.