42SH

A POSIX-compliant shell implementation in C

---

Table of Contents

• Overview
• Features
• Project Structure
  • Project Index
• Getting Started
  • Prerequisites
  • Installation
  • Usage
  • Testing
• Roadmap
• Contributing
• Acknowledgments

---

Overview

42sh is a Unix shell implementation written in C, developed as part of the EPITA curriculum. It aims to replicate the behavior of a POSIX-compliant shell (sh/bash), supporting a full pipeline from lexing and parsing to AST construction and execution. This project was realized by students within a one-month timeframe as part of their school curriculum, and as such, it may not be perfect or fully complete.

The shell is built around a classic compiler-like architecture: source input is tokenized by the lexer, parsed into an AST by the parser, and then evaluated by the execution engine. It supports interactive use as well as script execution.

---

Features

• Lexer — tokenizes raw shell input including keywords, operators, quotes, and redirections
• Parser — builds an Abstract Syntax Tree (AST) from the token stream following POSIX grammar
• AST execution engine — walks and evaluates the AST node by node
• Control flow — if/elif/else/fi, while, until, for loops with proper break/continue
• Pipelines — | operator with correct fd wiring between processes
• Redirections — >, >>, <, <>, >&, 2>&1, fd duplication and closing
• Logical operators — && and || with short-circuit evaluation
• Negation — ! prefix operator
• Subshells — ( ) with isolated environment
• Brace groups — { } command blocks
• Variables — assignment, expansion ($VAR, ${VAR}), special variables ($?, $$, $#, $@, $*, $0–$9)
• Quoting — single quotes (literal), double quotes (with expansion), backslash escaping
• Command substitution — `cmd` and $(cmd)
• Functions — definition and call, local positional parameters, return
• Built-in commands — echo, cd, exit, export, unset, true, false, . (dot/source), break, continue
• Comments — # comment support
• Pretty printer — AST pretty-printing for debugging
• Test suite — comprehensive integration and unit tests

---

Project Structure

└── 69sh/
    ├── 69sh/
    │   ├── configure.ac          # Autotools configuration
    │   ├── Makefile.am           # Top-level Makefile template
    │   ├── author_setup.sh       # Author/environment setup script
    │   ├── pretty_print.sh       # Pretty-print helper script
    │   └── src/
    │       ├── 42sh.c            # Entry point (main)
    │       ├── lexer/            # Tokenizer
    │       ├── parser/           # Grammar rules → AST
    │       ├── ast/              # AST node constructors + pretty printer
    │       ├── exec/             # AST execution engine
    │       ├── vars/             # Variable storage and expansion
    │       ├── funcs/            # Shell function registry
    │       └── io/               # I/O helpers
    └── tests/
        ├── testsuite.sh          # Main test runner
        ├── lib/runner.sh         # Test runner library
        ├── basics/               # Integration tests (step1, step2, step3)
        └── unitary/              # Unit tests (lexer, builtins, options)

Project Index

42SH/

src/

Module	Description
42sh.c	Entry point — handles argument parsing, REPL loop, and script mode
lexer/	Transforms raw input into a stream of tokens (words, keywords, operators, redirections, quotes)
parser/	Recursive-descent parser implementing the POSIX shell grammar; produces an AST
ast/	AST node types and constructors for every shell construct (cmd, if, for, while, until, pipe, redir, block, subshell, func, operator, negation, list)
ast/pretty_printer/	Walks the AST and prints a human-readable representation for debugging
exec/	Evaluates the AST: forks processes, sets up pipes, redirections, and executes builtins or external commands
vars/	Shell variable storage (get/set/unset), expansion of $VAR, ${VAR}, and special variables
funcs/	Registry for user-defined shell functions (store and look up by name)
io/	Low-level I/O utilities used across the shell

tests/

Directory	Description
basics/step1/	Builtins (echo), simple commands, comments, if/elif/else, lists
basics/step2/	Double quotes, variables, for, while/until, pipes, redirections, operators, negation
basics/step3/	Blocks, subshells, functions, command substitution, break/continue, cd, exit, export/unset, dot builtin
unitary/	Unit tests for the lexer, option handling, and specific builtins

---

Getting Started

Prerequisites

• C compiler: gcc or clang
• Autotools: autoconf, automake, libtool
• make

On Debian/Ubuntu:

sudo apt-get install build-essential autoconf automake libtool

Installation

1. Clone the repository:

   git clone https://github.com/69sh-epita/69sh
   cd 69sh/69sh

2. Bootstrap the build system:

   autoreconf -i

3. Configure:

   ./configure

4. Build:

   make

The compiled binary will be available as src/42sh.

Usage

Interactive mode:

./src/42sh

Script mode:

./src/42sh script.sh

Command string:

./src/42sh -c "echo hello world"

Testing

The project includes a shell-based test suite. From the 69sh/ directory:

cd tests
./testsuite.sh

To run only a specific category (e.g. step2 variables):

./testsuite.sh basics/step2/variables

Unit tests are located in tests/unitary/ and follow a .t format compatible with the runner in tests/lib/runner.sh.

---

Roadmap

• Lexer — tokenization of all POSIX shell constructs
• Parser — recursive-descent grammar, AST construction
• Execution engine — commands, pipes, redirections, control flow
• Variables & expansion — assignment, $VAR, special variables
• Built-ins — echo, cd, exit, export, unset, break, continue, .
• Functions — definition, call, local args, return
• Subshells & blocks — ( ) and { }
• Command substitution — `cmd` and $(cmd)
• Here-documents — <<EOF
• Arithmetic expansion — $(( ))
• Job control — &, fg, bg, jobs
• Signal handling — trap

---

Acknowledgments

• EPITA — for the project specification and curriculum framework
• The POSIX shell standard — IEEE Std 1003.1
• Contributors of the 69sh-epita organization : Anatole Debus, Etân Delabie, Martin Vitte, Aymeric Leclerre

---