Mini-Shell 42

Mini-Shell is a project completed as part of the 42 School curriculum. The goal is to recreate a minimal UNIX shell that behaves similarly to Bash, implementing parsing, execution, pipelines, redirections, and built-in commands, while managing environment variables and handling signals.

"The objective of this project is to create a simple shell, reproducing the behavior of Bash as closely as possible, with a focus on understanding how a shell works internally."

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Grade	Evaluation Information
	3 peers 1 hour

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Table of Contents

• Features
• Installation
• Usage
• Builtins Implemented
• Parsing & Execution
• Signals & Error Handling
• Bonus / Extra Features
• Known Issues
• Authors

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Features

• Prompt display in interactive mode
• Command line parsing with:
  • Quotes (single and double)
  • Escapes
  • Environment variable expansion
• Execution of simple commands and pipelines (|)
• Support for redirections (>, >>, <, <<)
• Built-in command support (see below)
• Handling of environment variables (export, $VAR, etc.)
• Error handling (syntax, execution, etc.)
• Signal handling for SIGINT (Ctrl+C), SIGQUIT (Ctrl+), heredoc signals
• Exit status management
• No memory leaks (tested with Valgrind)

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Installation

Requirements

• Unix-based system (Linux/macOS)
• GCC compiler (or compatible)
• make
• readline library (libreadline-dev on Ubuntu)

Build

git clone https://github.com/DanielFonsecaa/Mini-Shell-42.git
cd Mini-Shell-42
make

The executable will be created as ./minishell.

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Usage

Start the shell by running:

./minishell

You will see a prompt similar to:

minishell$ 

Type commands as you would in Bash:

• Execute binaries:

  minishell$ ls -la /tmp
  

• Use pipes:

  minishell$ ls | grep src
  

• Use redirections:

  minishell$ echo hello > file.txt
  minishell$ cat < file.txt
  

• Use environment variables:

  minishell$ export MYVAR=42
  minishell$ echo $MYVAR
  

Exit with exit, Ctrl+D, or by typing exit 42 (with the desired code).

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Key components:

• Parsing: Lexical analysis, tokenization, syntax parsing, expansions
• Execution: Built-in/execve, pipelines, redirections
• Env: Environment variables management
• Signals: Signal handlers for interactive mode
• Utils: Helper functions

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Builtins Implemented

The following built-in shell commands are implemented:

Built-in	Description
echo	Print arguments to stdout
cd	Change current directory
pwd	Print working directory
export	Set environment variables
unset	Remove environment variables
env	Display environment variables
exit	Exit the shell

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Parsing & Execution

Lexer:

• Scans and tokenizes user input into meaningful tokens, correctly handling:
  • Single and double quotes
  • Escape characters (backslashes)
  • Special shell symbols (|, >, <, etc.)
  • Whitespace and delimiters

Parser:

• Constructs an internal representation (such as an Abstract Syntax Tree) from the tokenized input, with support for:
  • Command grouping (sequences and compound commands)
  • Pipelines (|) connecting commands
  • Input/output redirections (>, >>, <, <<)
  • Environment variable expansion (e.g., $HOME, $?)
  • Comprehensive syntax error detection and reporting

Executor:

• Executes parsed commands by:
  • Running built-in commands internally without spawning new processes
  • Creating child processes with fork and executing external programs with execve
  • Establishing pipes and setting up file descriptors for redirections
  • Managing file creation (e.g., truncation, appending, heredoc files) and verifying file access permissions
  • Cleaning up resources and restoring shell state after execution

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Signals & Error Handling

• SIGINT (Ctrl+C): Interrupts current command or clears prompt
• SIGQUIT (Ctrl+): Ignored in interactive mode, handled in child
• Heredoc: Handles SIGINT to safely interrupt input
• Syntax errors: Informative messages on invalid input
• Exit codes: Consistent with Bash where possible

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Bonus / Extra Features

• Support for multiple consecutive pipes and redirections
• Heredoc (<<) with variable expansion
• Proper file descriptor management
• Memory leak checks (Valgrind clean)
• History navigation (if readline is available)

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Known Issues

• Wildcard expansion (*) not implemented
• Wildcard (&& || \) not implemented
• Some advanced Bash features (subshells, process substitution, etc.) are out of scope
• May behave differently from Bash in edge cases (export without arguments, etc.)

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Authors

• DanielFonsecaa
• NukyYuki