Minishell — POSIX-Compliant Command Execution Engine

A deterministic command execution runtime in C replicating core POSIX.1-2017 shell behaviors. Engineered with an Abstract Syntax Tree (AST) parser, asynchronous signal traps, multi-stage inter-process communication pipelines, and zero-leak memory invariants.

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Architecture & Systems Design

The engine executes an iterative read-eval-print loop (REPL) backed by a 4-tier processing pipeline:

flowchart TD
    A["Raw Command Line (POSIX readline)"] --> B["Lexer / Tokenizer (FSM Quote Handling)"]
    B --> C["AST Parser (Grammar & Operator Precedence)"]
    C --> D["Expander (Environment Variables & Expansions)"]
    D --> E["Executor (Process Trees & File Redirections)"]
    
    subgraph Execution Subsystem
        E --> F["Built-in Dispatch (cd, echo, env, exit, export, pwd, unset)"]
        E --> G["Fork / Execve (PATH Binary Resolution)"]
        E --> H["IPC Pipeline Multiplexing (pipe / dup2)"]
    end
    
    subgraph Signal & Lifecycle Supervisor
        S["Signal Trapping (SIGINT / SIGQUIT)"] -.-> E
        W["Process Reaping (waitpid / Status Decoding)"] -.-> E
    end

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1. Lexical Analysis & Token Stream

• State-machine tokenization handling single (') and double (") quote states.
• Exact delimiter tokenization for metacharacters: |, <, >, <<, >>.
• Semantic expansion of environment variables ($VAR, $?) while preserving quoted literals.

2. Abstract Syntax Tree (AST) Parser

• Converts linear token streams into an executable binary tree representation.
• Enforces strict operator precedence: Command sequences and pipes (|) partition nodes, with input/output redirections bound directly to command execution contexts.
• Immediate syntax error detection with standard POSIX exit status code 2.

3. Process Lifecycle & Execution Engine

• Process Trees: Subprocess creation via fork(), executing binaries with execve().
• IPC Pipelines: Multiplexes standard input/output streams using pipe() and atomic file descriptor duplication via dup2().
• Heredoc Management: Captures multi-line delimiter inputs without polluting user terminal state.
• Asynchronous Signal Safety: Context-sensitive trapping for SIGINT (Ctrl+C) and SIGQUIT (Ctrl+) during both interactive prompts and child process execution.

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Technical Specifications

Component	Technical Implementation	Conformance Target
Language & Standard	Pure C (ISO/IEC 9899:1999)	-Wall -Wextra -Werror
Parser Architecture	Abstract Syntax Tree (AST) Recursive Descent	Structured Precedence
Process Orchestration	fork, execve, waitpid, pipe, dup2	POSIX.1-2017 Process Model
Built-in Commands	echo, cd, pwd, export, unset, env, exit	IEEE Std 1003.1
Memory Policy	Deterministic allocation / zero reachable heap bytes	Valgrind Memcheck Verified
Signal Handling	Safe state transitions via sigaction / signal	Re-entrant async-safe

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Build & Verification

Compilation

# Build production executable
make

# Clean compilation object files
make clean

# Purge binaries and libraries
make fclean

# Rebuild from scratch
make re

Memory & Stability Audits

Every pull request and push is continuously audited under Valgrind Memcheck with custom suppression configurations (readline.supp):

# Test complex pipeline execution under Valgrind
echo "ls | wc -l" | valgrind --suppressions=readline.supp --leak-check=full ./minishell

# Test redirection and file descriptor isolation
printf "echo hello > test.txt\ncat test.txt\nrm -f test.txt\n" | valgrind --suppressions=readline.supp --leak-check=full ./minishell

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Author & Engineering Standards

Rubén D. Higuita — Systems & Embedded Software Engineer
Madrid, Spain • LinkedIn • GitHub • Portfolio

Engineering Invariant:
Zero memory leaks, deterministic process reaping, and absolute compliance with POSIX runtime standards.