cshell - Minimal Unix Shell

A minimal Unix shell implementation in C with support for pipes, redirections, built-in commands, and command history.

Features

• External Commands: Execute any external command by forking child processes

• Built-in Commands:

  • cd [dir] - Change directory (default: home directory)
  • pwd - Print working directory
  • help - Display help message with all features
  • history - Show command history (last 256 commands)
  • exit - Exit the shell

• Piping: Chain multiple commands with |

  • Example: ls | grep .c | wc -l

• Input/Output Redirection:

  • < - Redirect input from file
  • > - Redirect output to file (truncate)
  • >> - Append output to file
  • Example: echo "hello" > file.txt

• Signal Handling: Ctrl+C does not kill the shell, only the current command

• Command History: All commands are stored and accessible via the history command

Build Instructions

Prerequisites

• GCC compiler
• Standard C library (libc)
• POSIX-compliant operating system (Linux, macOS, BSD)

Building

cd /path/to/cshell
make

This will compile all source files and create the cshell executable.

Running

./cshell

Or use the convenience target:

make run

Cleaning

Remove compiled files:

make clean

Usage Examples

cshell> pwd
/Users/himanshujaiswal/Documents/cshell

cshell> cd ~
cshell> ls

cshell> cat file.txt | grep "search" | wc -l

cshell> echo "test" > output.txt

cshell> cat < input.txt

cshell> history

cshell> help

cshell> exit

Project Structure

cshell/
├── main.c          - Main event loop, signal handling, history management
├── parser.c        - Input parsing, tokenization, command structure building
├── parser.h        - Parser interface and structures
├── executor.c      - Command execution, piping, fork/exec handling
├── executor.h      - Executor interface
├── builtins.c      - Built-in command implementations
├── builtins.h      - Built-in command interface
├── Makefile        - Build configuration
├── README.md       - This file
└── CHANGELOG.md    - Version history

Implementation Details

Parser

• Tokenizes input by whitespace
• Handles pipes (|) to identify command boundaries
• Handles redirections (<, >, >>) with associated filenames
• Returns a pipeline structure with individual commands

Executor

• Handles single commands, built-in or external
• Creates pipes between commands in a pipeline
• Forks child processes for external commands using fork() and execvp()
• Handles input/output redirections via file descriptors and dup2()
• Waits for all child processes to complete

Built-ins

• cd: Uses chdir() to change directories
• pwd: Uses getcwd() to get current directory
• help: Displays usage information
• history: Shows all stored commands
• exit: Terminates the shell

Main Loop

• Initializes signal handler for SIGINT
• Maintains command history (max 256 entries)
• Reads user input and calls parser/executor
• Continues on signal interruption

Limitations

• Piping only works with external commands (built-ins cannot be piped)
• No background process support (no & operator)
• No job control (no fg, bg, jobs commands)
• No environment variable expansion
• No quoting or escape sequences
• No aliases or command substitution
• Limited to 64 arguments per command
• Limited to 128 tokens per input line

Technical Notes

• Uses only POSIX APIs and standard C library
• No external dependencies
• Handles process termination and cleanup properly
• Safe memory management with malloc/free
• Robust error handling and reporting

License

Public domain. Free to use and modify.