Python Shell Emulator

This project is a lightweight command-line shell emulator written in Python. It implements basic command execution, tab completion, redirection features, command history, and pipeline functionality similar to those found in traditional Unix shells.

Features

• Tab Completion:
  Automatically completes both built-in commands and external executables found in your system's PATH.

• Command History:

  • Tracks commands entered during the current and previous sessions
  • Navigate through history using up/down arrow keys
  • View command history with the history command
  • Limit history display with history N (shows last N commands)
  • History persists between shell sessions in ~/.python_shell_history
  • Supports redirection of history output to files

• Built-in Commands:
  Supports a handful of common built-ins including:

  • type: Identifies whether a command is a shell built-in or an external executable.
  • cd: Changes the current working directory.
  • exit: Exits the shell.
  • pwd: Prints the current working directory.
  • echo: Displays text on the terminal.
  • export: Sets or displays environment variables.
  • history: Displays command history.
  • help: Displays information about built-in commands.

• Command Redirection:
  Basic support for redirecting input, output, and errors:

  • > or 1>: Redirect standard output to a file (write mode).
  • >> or 1>>: Redirect standard output to a file (append mode).
  • 2>: Redirect standard error to a file (write mode).
  • 2>>: Redirect standard error to a file (append mode).
  • <: Redirect standard input from a file.

• Pipes:
  Connect multiple commands together with the pipe operator (|):

  • Redirect the output of one command as input to another
  • Chain multiple commands together in a pipeline
  • Combine pipes with redirections for complex command sequences
  • Both built-in and external commands support piping

• Environment Variables:
  Full support for environment variable management:

  • Set variables using export VAR=value.
  • Use variables in commands with $VAR or ${VAR} syntax.
  • Access special variables like $$ (process ID) and $? (exit code).
  • Variables persist throughout the shell session.
  • View all variables using export without argument.

• Tilde Expansion:
  Support for using ~ as a shortcut:

  • Standalone ~ expands to the user's home directory
  • Paths starting with ~/ expand to paths relative to home
  • ~username/ expands to another user's home directory
  • Works in all contexts where paths are expected

• Help System:
  Documentation for built-in commands:

  • help lists all available built-in commands
  • help command provides detailed help on a specific command
  • Includes syntax, description, and usage information
  • Supports redirection and can be used in pipes

• External Command Execution:
  Executes external commands by searching for them in your system's PATH with proper error handling for cases like command not found or permission issues.

Requirements

• Python 3.x
• Standard Python libraries:
  • sys
  • os
  • shutil
  • subprocess
  • shlex
  • readline
  • re
  • atexit

Usage

1. Run the Shell:
   Execute the script from the terminal:

   python main.py

2. Environment Variable Examples:

   • Set a variable:

     export NAME=World

   • Use variables in commands:

     echo "Hello $NAME"

   • Check exit codes:

     ls /nonexistent
     echo $?

3. Command History Examples:

   • View all command history:

     history

   • View last 5 commands:

     history 5

   • Save history to a file:

     history > my_history.txt

   • Navigate history: Press up/down arrow keys to cycle through previous commands

4. Pipe Examples:

   • Basic pipe to filter output:

     ls | grep .py

   • Chain multiple commands:

     ls -la | grep "^d" | sort

   • Combine pipes with redirections:

     ls | grep .txt > text_files.txt

   • Use built-in commands in pipes:

     echo "Hello World" | grep "Hello"

   • Count items with pipes:

     ls | wc -l

5. Tilde Expansion Examples:

   • Navigate to home directory:

     cd ~

   • List files in home subdirectory:

     ls ~/Documents

   • Create file in home directory:

     echo "test" > ~/testfile.txt

6. Help Examples:

   • View available commands:

     help

   • Get help on specific command:

     help cd
     help export

   • Save command help to file:

     help echo > echo_help.txt

   • Search through command help:

     help | grep directory