This project has been created as part of the 42 curriculum by ydylan-k, sming-zh.

Minishell

As beautiful as a shell

Description

Minishell is a 42 project aimed at creating a minimal yet fully functional shell based on Bash. The primary goal of this project is to understand core system architecture by building it from scratch, specifically focusing on Lexical Analysis, Process Management, Inter-Process Communication (IPC), Environment Management, Signal Handling.

Key Features Implemented

• libreadline-dev is used to await and read for user input. It's prefixed with :3.
• Mandatory Built-ins:
  • echo: Prints text into standard output followed by a line break. Prepend with -n flag to disable line break.

     :3 echo hello
     hello
     :3 echo -n hello
     hello:3 echo hello
     hello
     :3

  • cd: Changes current working directory to another
  • pwd: Shows current working directory
  • export: Shows or creates an environment variable with format <key>=<value>

     :3 export six=seven
     :3 echo $six
     seven
     :3 export
     declare -x six="seven"
     :3

  • unset: Deletes an environment variable
  • env: Shows all shell variables
  • exit: Exits current shell with optional exit code
• Quote Interpretation:
• ' (Single Quotes): Prevents the shell from interpreting meta-characters (literal string).
• " (Double Quotes): Prevents the shell from interpreting meta-characters, except for $ (allows variable expansion).
• Redirections:
  • <: Redirects terminal input from a specified file.
  • >: Redirects terminal output to a specified file (truncates).
  • << (Heredoc): Reads multi-line input from the terminal until a specified delimiter is seen.
  • >>: Redirects terminal output to a specified file in append mode.
• Pipelines (|): Connects the output of one command directly into the input of the next using precise file descriptor management.
• Environment Variables: Expands variables (e.g., $USER) and handles $?, which dynamically expands to the exit status of the most recently executed foreground pipeline.
• Signal Handling: Custom behavior for Ctrl+C (SIGINT), Ctrl+\ (SIGQUIT), and Ctrl+D (EOF) depending on whether the shell is in an interactive prompt, reading a heredoc, or executing a blocking command.
• Wildcard Expansion (*): Scans your directory to replace * patterns with real filenames. It ignores hidden files, won't expand if inside quotes, and keeps the pattern if no files match.

Instructions

Compilation

Minishell requires libreadline-dev which can be downloaded from your operating system's package manager.

• Run make all to compile the program.
• make clean to remove the object files.
• make fclean to remove both the binary and the object files.
• make re to make fclean and make all again.

Usage

./minishell
:3 echo hello
hello
:3 ls -a
.  ..  minishell
:3 exit
exit

Resources

Lexer

• Is space a meta-character
• Token Recognition

Parser

• Precedence climbing
• Shunting yard algorithm
• Shell grammar for recursive descent parser
• Parsing with recursive descent
• Shell Operation
• Abstract Syntax Tree

Heredoc

• Pipe buffer limit
• Heredoc deep dive
• O_TMPFILE
• GNU Readline Library
• Readline usage tutorial
• Readline: getting a new prompt on SIGINT

Environment Variable

• Understanding environment variables

Execution

• dup2
• f_DUPFD
• Open pipe limit
• Why shell calls fork()

Signal

• Dangers of global and static variables
• Readline signal handling
• Double prompt issue
• Readline to original state when receiving SIGINT

Wildcard

• Matching Wildcard

Others

• Subshell
• Visual Studio Code sigaction not defined
  • Some solutions
• Git Gud
• getcwd() Manpage
• history Manpage
• add_history()
• Readline dependency issue

The usage of AI in this project was used for all parts of this project. Like brainstorming and understanding data structure, execution pipeline, pipes, redirection, file descriptor handling, refactoring code, documentations, wildcard solving, and writing documentations.

rentry version