Smallsh: A Simple Shell in C

Introduction

Smallsh is a lightweight shell written in C that offers a command line interface similar to more complex shells, such as bash. It is designed as an educational project to demonstrate key concepts related to Unix process APIs, signal handling, and I/O redirection.

Features

• Interactive input prompt
• Tokenizes command line input
• Parameter expansion including:
  • '$$' for shell's process ID
  • '$?' for exit status of the last foreground command
  • '$!' for the process ID of the most recent background process
  • '${parameter}' for environment variables
• Built-in commands:
  • exit
  • cd
• Execution of non-built-in commands
• I/O redirection operators: <, >, >>
• Background command execution using &
• Custom signal handling for SIGINT and SIGTSTP

How it Works

• Input: Manages background processes and reads a line of input from stdin or a script file.
• Word Splitting: Breaks the input line into individual words.
• Expansion: Performs variable and parameter expansion.
• Parsing: Converts words into syntactic tokens.
• Execution: Determines if the command is built-in and executes it. If not, it spawns a child process to run the command.
• Waiting: Handles the synchronization between the parent smallsh process and any child processes it spawns.

Signal Handling

In interactive mode:

• SIGTSTP is ignored.
• SIGINT is ignored except when reading input.

Usage

To compile the program, run make. This will create an executable program named smallsh. To run the program in interactive mode, use ./smallsh in the same directory as the executable. To run the program in non-interactive mode, use ./smallsh < [script_name]

Learning Outcomes

By exploring and working with smallsh, you'll get hands-on experience with:

• The Unix process API
• Writing programs using Unix process API
• Understanding the concept of signals and their applications
• Writing programs utilizing the Unix signal handling API
• Understanding and implementing I/O redirection

Final Thoughts

While smallsh does not aim to replace fully-fledged shells, it provides a simplified platform to understand and explore key Unix programming concepts. Feel free to dive into the source code and provide feedback or modifications!