42-minishell

This repository contains one of my projects of the core curriculum at 42 Heilbronn, done together with tjensen42.

The project minishell is about creating a simple, bash-like shell. As with all C projects at 42 Heilbronn, the code has to be written according to the 42 Norm (maximum 25 lines per function, no for loops, a certain set of allowed functions, ...).

Description

The task in this project is to write a small shell with several features. The requirements include:

• a prompt
• history of previous input
• launch executables (based on PATH variable, using relative or absolute path)
• single (') and double (") quotes to prevent the interpretation of meta-characters
• redirections:
  • input (<)
  • output (>)
  • here document (<<)
  • output append (>>)
• pipes (|) to connect commands
• expand environment variables ($ followed by a sequence of characters)
• last exit status ($?)
• signal handling (ctrl+C, ctrl+D, ctrl+\)
• builtins:
  • echo with option -n
  • cd with relative or absolute path, - for OLDPWD, no argument for HOME
  • pwd without options
  • export without options
  • unset without options
  • env without options
  • exit with specific exit status
• wildcards (*) in the current working directory (bonus)
• AND (&&) and OR (||) groups of commands with parentheses for priorities (bonus)

The behavior of bash is to be considered as a general reference.

The use of GNU's readline library is allowed for handling terminal interaction (input reading, history).

Approach

We decided to split the handling of user input into four basic parts:

• lexer: split input into basic tokens, implemented as list elements
• parser: interpret tokens and rearrange them into sublists
• expander: expand environment variables and wildcards
• executer: execute commands

In order to be able to handle groups of commands for the bonus part, we had the parser and the executer work recursively. For this purpose, we also decided to implement a system of subshells.

As a general rule, we tried to reproduce the bahavior of bash as closely as possible, checking the bash manual a lot. To facilitate the comparison between our minishell and the original bash, we also built a small tester, which compares the output and the exit status of both shells (see Testing).

For debugging purposes, we decided to implement a printing function, which shows the different parsing steps with some syntax highlighting (see Debug Mode).

How to Use

The current version of minishell is developed and tested on macOS, but it should work on all UNIX/LINUX based systems as well.

Requirements: GCC / CLANG Compiler, GNU Make, GNU Readline Library

Clone this repository:

git clone https://github.com/hepple42/42-minishell.git

Change to the cloned directory and build the project:

cd 42-minishell && make

Run the minishell:

./minishell

Dependencies

Install Readline with Brew

How to install readline using brew.

brew install readline

brew link --force readline

echo 'export C_INCLUDE_PATH="/usr/local/opt/readline/include:$C_INCLUDE_PATH"' >> ~/.zshrc

echo 'export LIBRARY_PATH="/usr/local/opt/readline/lib:$LIBRARY_PATH"' >> ~/.zshrc

source ~/.zshrc

Install Readline on 42 Macs

How to install readline at 42HN.

Install brew if it is not already installed:

rm -rf $HOME/.brew && git clone --depth=1 https://github.com/Homebrew/brew $HOME/.brew && echo 'export PATH=$HOME/.brew/bin:$PATH' >> $HOME/.zshrc && source $HOME/.zshrc && brew update

Install readline library:

brew install readline

brew link --force readline

echo 'export C_INCLUDE_PATH="$HOME/.brew/include:$C_INCLUDE_PATH"' >> ~/.zshrc

echo 'export LIBRARY_PATH="$HOME/.brew/lib:$LIBRARY_PATH"' >> ~/.zshrc

source ~/.zshrc

Testing

To test our minishell, a small tester is included in this repository. It compares most of the shell's features with the current bash installation. The output (stdout) and the exit-code are compared directly. In test cases which produce an error, the tester only makes sure that both shells print some error output (stderr).

Run tester:

make test

Debug Mode

To visualize the different steps of the lexing and parsing process, we implemented a debug mode. It can be activated by exporting an environment variable.

Enter the minishell and export the DEBUG environment variable:

export DEBUG=printer

Change Prompt

The prompt displayed in the command line of our minishell can be customized by exporting an environment variable.

Enter the minishell and export the PS1 environment variable:

export PS1='[prompt]$ '