vshl - A Custom Shell Implementation

A lightweight POSIX shell written in C with modern features like command history, tab completion, job control, and alias support.

Features

Core Functionality

• Command Execution: Execute system commands with full argument support
• Pipelines: Chain commands with | operator
• Redirection: Support for >, >>, <, and 2> (stdout, append, stdin, stderr)
• Logic Operators: Sequential (;), conditional (&&, ||) command execution
• Background Jobs: Run commands in background with & and manage with jobs, fg

Interactive Features

• Command History: Navigate previous commands with arrow keys (stored in ~/.vshl_history)
• Tab Completion: Auto-complete commands and filenames
• Aliases: Create command shortcuts with alias and unalias
• Startup Config: Auto-loads ~/.vshlrc on launch for persistent settings

Built-in Commands

• cd [dir] - Change directory (supports ~ expansion)
• export VAR=value - Set environment variables
• unset VAR - Remove environment variables
• alias name="command" - Create command alias
• unalias name - Remove alias
• jobs - List background jobs
• fg [job_id] - Bring job to foreground
• exit [code] - Exit shell

Advanced Features

• Globbing: Wildcard expansion with * (match all) and ? (match one)
• Environment Expansion: $VAR, ${VAR}, and ~ expansion
• Signal Handling: Ctrl-C interrupts current job, Ctrl-Z suspends to background
• Quote Handling: Single and double quotes, backslash escaping

Installation

Prerequisites

• GCC compiler
• GNU Make
• libedit library (for macOS/BSD) or libreadline (for Linux)

Build

make

Run

./vshl

Configuration

On first launch, vshl creates ~/.vshlrc with default settings. Customize it with:

# Environment variables
export PATH="$PATH:$HOME/bin"
export EDITOR=vim

# Aliases
alias ll="ls -la"
alias gs="git status"

Security

The codebase has been hardened against common vulnerabilities:

• Buffer overflow protection with bounds checking
• NULL pointer validation for all allocations
• Error handling for system calls (fork, dup2, kill)
• Safe signal handling with sig_atomic_t
• Input validation for environment variables and job IDs

Limitations

• Globbing in Quotes: Wildcards expand even inside quotes (ls "*.c" behaves like ls *.c)
• No Noclobber: Output redirection (>) always overwrites files
• Limited Expansion: No command substitution, arithmetic expansion, or special parameters ($?, $$, etc.)
• Tilde Expansion: Only ~ (not ~user) is supported

Project Structure

include/          Header files
  ├── builtins.h
  ├── executor.h
  ├── globbing.h
  ├── jobs.h
  ├── parser.h
  ├── shell.h
  ├── signals.h
  └── utils.h
src/              Source files
  ├── builtins.c
  ├── executors.c
  ├── globbing.c
  ├── jobs.c
  ├── parsers.c
  ├── shell.c
  ├── signals.c
  ├── utils.c
  ├── main.c
  ├── alias.c
  └── line_editing.c