BuildShellUsingJava

A feature-rich custom shell implementation in Java that provides Unix-like shell functionality with support for built-in commands, pipelines, command history, and more.

Features ✨

Built-in Commands

• exit - Exit the shell
• echo [text] - Print text to standard output
• pwd - Print current working directory
• cd [directory] - Change directory (supports ~, relative and absolute paths)
• type [command] - Check if a command is a builtin or show its executable path
• cat [files...] - Display contents of one or more files
• history [n] - Display command history

Advanced Features

• 🔗 Pipeline Support - Chain commands using | operator
• 📝 Persistent History - Automatic history save/load using HISTFILE environment variable
• ⬆️⬇️ Arrow Key Navigation - Browse command history (Unix/Linux/Mac)
• 🎯 Quote Handling - Support for single quotes, double quotes, and backslash escaping
• 🚀 External Command Execution - Run any program available in your system's PATH
• 💾 History Management - Read, write, and append history to custom files

Prerequisites 📋

• Java Development Kit (JDK) 8 or higher
• Java compiler (javac)

Installation & Setup 🛠️

1. Clone the repository

   git clone https://github.com/smritii73/BuildShellUsingJava.git
   cd BuildShellUsingJava

2. Compile the code

   javac Main.java

3. Run the shell

   java Main

Usage Examples 💡

Basic Commands

$ pwd
/home/user/projects

$ echo Hello World
Hello World

$ cd ~
$ pwd
/home/user

Pipelines

Chain multiple commands together:

$ echo "Hello World" | cat
Hello World

$ pwd | cat
/current/directory

Command History

# View all history
$ history

# View last 5 commands
$ history 5

# Save history to file
$ history -w commands.txt

# Append new commands to file
$ history -a session_log.txt

# Load history from file
$ history -r previous_commands.txt

File Operations

$ cat file1.txt
Contents of file1

$ cat file1.txt file2.txt file3.txt
Contents of file1
Contents of file2
Contents of file3

Quote Handling

$ echo 'single quotes preserve everything'
single quotes preserve everything

$ echo "double quotes allow \"escapes\""
double quotes allow "escapes"

$ echo backslash\ works
backslash works

History Management 📚

Persistent History with HISTFILE

Set the HISTFILE environment variable before starting the shell:

Unix/Linux/Mac:

export HISTFILE=~/.myshell_history
java Main

Windows (PowerShell):

$env:HISTFILE="$HOME\.myshell_history"
java Main

The shell will automatically:

• Load history from HISTFILE on startup
• Save history to HISTFILE on exit

History Commands

Command	Description
history	Display all commands with line numbers
history N	Display last N commands
history -r file	Read/load history from file
history -w file	Write all history to file (overwrite)
history -a file	Append new session commands to file

Keyboard Shortcuts ⌨️

Shortcut	Action
↑ (Up Arrow)	Previous command in history
↓ (Down Arrow)	Next command in history
Backspace	Delete character
Ctrl+C	Cancel current line
Ctrl+D	Exit shell (when line is empty)
Enter	Execute command

Note: Arrow key navigation works on Unix/Linux/Mac systems. On Windows, it falls back to simple line reading.

Technical Details 🔧

Architecture

• Built-in Commands: Implemented directly in Java
• External Commands: Executed via ProcessBuilder
• Pipeline Execution: Mixed threading model supporting both built-ins and external commands
• Input Parsing: Custom parser handling quotes and escapes

Quote and Escape Rules

• Single quotes ('): Preserve all characters literally
• Double quotes ("): Allow escape sequences for $, `, ", \, and newline
• Backslash (\): Escape the next character

Pipeline Implementation

• Supports multiple commands in a single pipeline
• Uses PipedInputStream and PipedOutputStream for inter-command communication
• Threads handle concurrent execution of pipeline stages
• Both built-in and external commands can be chained

Platform Compatibility 🖥️

Feature	Windows	Unix/Linux/Mac
Basic commands	✅	✅
Pipelines	✅	✅
Arrow key history	❌	✅
External commands	✅	✅
History file	✅	✅

Limitations ⚠️

This shell does not support:

• Input/output redirection (>, <, >>)
• Background jobs (&)
• Environment variable expansion ($VAR)
• Command substitution ($(command))
• Wildcards/globbing (*.txt)
• Logical operators (;, &&, ||)

Contributing 🤝

Contributions are welcome! Feel free to:

• Report bugs
• Suggest new features
• Submit pull requests

License 📄

This project is open source and available under the MIT License.

Author 👨‍💻

Smriti - @smritii73 Vaishnavi - @vaishnavidhule Jaykit - @Jaykit1907 Nikshit - @143Nikshit

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