Minishell Parser

Overview

The parser component of the minishell project is responsible for processing and validating user input commands. It consists of several stages that transform raw input into a structured format that can be executed by the shell.

Components

1. Early Error Checking

Located in 01early_err.c and related utility files, this stage performs initial validation of the input:

• Spaces Check: Ensures the input isn't just whitespace
• Quotes Check: Validates matching quotes (single and double)
• Pipes Check: Ensures proper pipe syntax
• Special Characters: Validates allowed special characters
• Redirections: Checks proper syntax for input/output redirections

2. Parser

The main parser (02parser.c) orchestrates three main operations:

• Removes consecutive quotes
• Tokenizes the input
• Expands environment variables and special parameters

3. Tokenizer

Implemented in 03tokenizer.c and its utility files, this component breaks down the command into meaningful tokens:

Token Types:

• WORD: Regular command words and arguments
• PIPE: Command pipe operator (|)
• SEP: Separators (spaces)
• IN_RED: Input redirection (<)
• OUT_RED: Output redirection (>)
• HERE_DOC: Here document (<<)
• APP_RED: Append redirection (>>)

Token Initialization Functions:

• init_in_quotes: Handles quoted strings
• init_pipe_or_sep: Processes pipes and separators
• init_single_redirection: Handles < and > operators
• init_double_redirection: Handles << and >> operators
• init_word: Processes regular words

4. Expander

Located in 04expander.c and related files, this component handles variable expansion:

Features:

• Environment variable expansion ($VAR)
• Exit status expansion ($?)
• PID expansion ($$)
• Quote handling (prevents expansion in single quotes)

Implementation Details

Error Handling

• Proper memory allocation checks
• Syntax validation
• Quote matching
• Redirection syntax verification

Memory Management

• Dynamic allocation for tokens
• Proper freeing of resources
• Prevention of memory leaks

Quote Handling

• Support for both single (') and double (") quotes
• Preservation of quoted content
• Special handling of nested quotes

Usage Example

# Valid command examples
echo "Hello $USER"    # Expands environment variable
echo 'Hello $USER'    # Preserves literal string
ls -l | grep "file"   # Pipe operation
echo test > outfile   # Output redirection

# Invalid commands (will be caught by error checking)
echo "unclosed quote  # Missing closing quote
|                     # Pipe without command
ls >>                 # Redirection without target

Technical Notes

1. Quote Removal:

   • Consecutive quotes of the same type are removed
   • Quotes inside other quotes are preserved
   • Example: echo hi""bye becomes echo hibye

2. Environment Variables:

   • Expanded in double quotes
   • Preserved literally in single quotes
   • $? expands to last exit status
   • $$ displays as a single $

3. Redirections:

   • Supports standard input/output
   • Handles append mode
   • Implements here documents
   • Validates file names and descriptors

Error Messages

The parser provides clear error messages for various scenarios:

• minishell: unsupported open quotes
• minishell: token error: '|'
• minishell: unsupported special char
• minishell: token error: '<' or '>'

Data Structures

Token Structure

typedef struct s_token {
    char            *string;    // Token content
    enum e_type     type;      // Token type
    enum e_quotes   quote_type; // Quote type if applicable
    struct s_token  *next;     // Next token in list
} t_token;

Limitations

• Does not support certain special characters (;, &, , *, etc.)
• Limited to basic redirection operations
• No support for advanced shell features (&&, ||, etc.)