minishell

A custom Unix shell implementation with advanced parsing and execution features, developed as part of the 42 School curriculum.

Overview

This project implements a fully functional Unix shell that replicates the behavior of bash. It features a complete lexer and parser for command-line input, supports complex command structures including pipes and logical operators, implements built-in commands, and handles advanced features like wildcards, redirections, and subshells with parentheses.

Features

Core Shell Functionality

• Interactive command prompt with readline integration
• Command history navigation and persistence
• Signal handling (Ctrl+C, Ctrl+D, Ctrl+\)
• Environment variable management and expansion
• Exit status tracking and $? variable support

Built-in Commands

• echo [-n] [args...] - Display text with optional newline suppression
• cd [path] - Change directory with HOME and OLDPWD support
• pwd - Print working directory
• export [var[=value]...] - Set environment variables
• unset [var...] - Remove environment variables
• env - Display environment variables
• exit [code] - Exit shell with optional exit code

Advanced Parsing

• Lexical Analysis: Complete tokenizer with quote handling
• Syntax Validation: Comprehensive error checking for malformed input
• AST Construction: Abstract syntax tree for command execution
• Quote Processing: Single and double quote parsing with variable expansion
• Wildcard Expansion: Glob pattern matching with * support

Execution Engine

• Pipeline Support: Multi-command pipes with proper file descriptor management
• Logical Operators: && (AND) and || (OR) command chaining
• Parenthetical Grouping: Subshell execution with () syntax
• Redirections: Input/output redirection (<, >, >>) and heredocs (<<)
• Process Management: Fork-based command execution with proper cleanup

Compilation

Prerequisites

• GCC or compatible C compiler
• Make build system
• Readline library (libreadline-dev)
• POSIX-compliant system (Linux/macOS)

Build Commands

# Compile the shell
make

# Clean object files
make clean

# Full cleanup
make fclean

# Rebuild everything
make re

Usage

Basic Commands

# Start the shell
./minishell

# Execute simple commands
minishell$ ls -la
minishell$ echo "Hello, World!"
minishell$ pwd

# Change directories
minishell$ cd /tmp
minishell$ cd        # Go to HOME

Pipes and Redirections

# Simple pipes
minishell$ ls | grep .c | wc -l

# Output redirection
minishell$ echo "test" > output.txt
minishell$ cat input.txt >> output.txt

# Input redirection
minishell$ wc -l < input.txt

# Here-documents
minishell$ cat << EOF
Hello
World
EOF

Logical Operators

# AND operator (execute second if first succeeds)
minishell$ make && ./minishell

# OR operator (execute second if first fails)
minishell$ invalid_command || echo "Command failed"

# Complex chaining
minishell$ cd /tmp && ls -la || echo "Failed to list directory"

Parenthetical Grouping

# Subshell execution
minishell$ (cd /tmp && ls) && pwd

# Complex grouping with pipes
minishell$ (echo "line1"; echo "line2") | wc -l

# Logical operators with groups
minishell$ (cd /nonexistent && ls) || echo "Directory change failed"

Variable Operations

# Set and export variables
minishell$ export NAME="John Doe"
minishell$ export PATH="/usr/local/bin:$PATH"

# Display variables
minishell$ env | grep NAME
minishell$ echo $NAME

# Unset variables
minishell$ unset NAME

# Exit status
minishell$ echo $?

Advanced Features

Wildcard Expansion

# Match all .c files
minishell$ ls *.c

# Complex patterns
minishell$ echo src/*.c include/*.h

Quote Handling

# Preserve spaces and special characters
minishell$ echo "Hello    World"
minishell$ echo 'Variable: $HOME'

# Mixed quoting
minishell$ echo "Home is $HOME, user is `whoami`"

Here-documents with Expansion

# Variable expansion in heredocs
minishell$ cat << EOF
Current user: $USER
Home directory: $HOME
EOF

# Quoted delimiter prevents expansion
minishell$ cat << 'EOF'
Literal: $USER
EOF

Implementation Details

Parsing Architecture

• Tokenization: Multi-state lexer handling quotes, operators, and words
• Validation: Comprehensive syntax checking before execution
• AST Construction: Recursive descent parser building execution trees
• Error Recovery: Detailed error messages with proper exit codes

Execution Model

• Process Management: Fork-based execution with proper signal handling
• File Descriptor Management: Safe redirection with cleanup
• Pipeline Implementation: Multi-process pipes with synchronization
• Built-in Optimization: Parent-process execution for shell built-ins

Memory Management

• Dynamic Allocation: Efficient token and AST node management
• Cleanup Strategies: Proper resource deallocation on exit
• Error Handling: Graceful recovery from allocation failures

Error Handling

# Syntax errors
minishell$ echo "unclosed quote
bash: unexpected EOF while looking for matching `"'

# Command not found
minishell$ nonexistent_command
minishell: nonexistent_command: command not found

# Permission errors
minishell$ cd /root
minishell: cd: /root: Permission denied

Signal Handling

• SIGINT (Ctrl+C): Interrupt current command, display new prompt
• SIGQUIT (Ctrl+\): Ignored in interactive mode
• SIGTERM: Graceful shutdown with cleanup
• EOF (Ctrl+D): Exit shell cleanly

Standards Compliance

• POSIX Shell: Compatible with standard shell behavior
• 42 Norm: Follows strict coding standards (Norminette compliant)
• Memory Safety: Zero memory leaks with proper resource management
• Signal Safety: Proper signal handling in multi-process environment

Authors

yyudi & nweber - 42 Heilbronn Students