42 Minishell 101

Welcome to Minishell 101! This is a beginner-friendly guide and README template for the 42 School minishell project. The goal of this project is to create a simple UNIX shell—a program that takes commands from the user and executes them, much like bash.

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Overview

Minishell is a journey into process creation, file descriptors, signals, and parsing. By the end of this project, you'll have a deep understanding of how your terminal interprets what you type and how it asks the operating system to do the work.

Core Features

• Prompt: Displays a prompt when waiting for a new command.
• History: Keeps a working history of commands (using readline).
• Execution: Finds and executes executables using the PATH variable or relative/absolute paths.
• Built-ins: Recreates 7 essential built-in functions (see below).
• Pipes (|): Connects the output of one command to the input of the next.
• Redirections:
  • > (Redirect output)
  • < (Redirect input)
  • >> (Redirect output in append mode)
  • << (Here-document)
• Environment Variables: Expands variables like $USER or $? (exit status).
• Signals: Handles ctrl-C, ctrl-D, and ctrl-\ just like bash.

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The 7 Built-in Commands

Your shell must natively implement the following commands without calling external binaries:

1. echo (with -n option)
2. cd (with relative or absolute paths)
3. pwd (print working directory)
4. export (export environment variables)
5. unset (unset environment variables)
6. env (print the environment)
7. exit (exit the shell, handling exit codes)

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Architecture 101 (How it works)

A well-structured minishell usually follows these four steps:

1. Lexer (Tokenization): Breaks the user's input string into small, manageable chunks (tokens) like WORD, PIPE, REDIRECT_IN, etc.
2. Parser: Organizes those tokens into a structure (like an Abstract Syntax Tree or a linked list of commands) that the computer can easily understand.
3. Expander: Loops through the parsed arguments and replaces any $VARIABLES with their actual values from the environment.
4. Executor: Iterates through the command structures. It sets up pipes and redirections (using dup2), forks processes (using fork), and runs the commands (using execve or built-in functions).

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Installation & Usage

Compilation

To compile the project, simply run make in the root directory.

make

Running the Shell

Start the shell by running the executable:

./minishell

Cleaning Up

• make clean: Removes the object files.
• make fclean: Removes object files and the executable.
• make re: Recompiles the whole project.

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Resources & Tips

• Waitpid & Fork: Understand how parent processes wait for child processes.
• File Descriptors: Grasp how dup2() is used to wire stdout of one process into the stdin of another.
• Memory Leaks: valgrind is your best friend. Make sure you free your abstract syntax tree or command lists after every prompt loop!
• Bash is your standard: Whenever you are unsure how minishell should behave, type the same command into bash and mimic its behavior.

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Created for the 42 Cursus (yel-mota).