Problem statement

Implement a simple modular UNIX-like shell using Python

* Requirement:
    We need a fully programmable user-land ecosystem purely on Python for deployment on IoT boards like RaspberryPi and BeagleBone. Ideally, a Linux system need to be integrated with just the kernel and the userland ecosystem that consists only of Python. This would provide a very lean, flexible and a dynamic programmer-friendly user-land environment.

    In order to achieve this goal we need to build a rudimentary UNIX-like shell that allows extensibility using custom python modules to implement various features as shell commands.

    This mini-project should implement the rudimentary skeleton shell that can be extended easily with new commands and functionality with minimal effort by a developer. The shell should support user-friendly command-line interface and also have a command history support that could be recalled across invocations (similar to the ~/.bash_history feature available on bash shell).

    The shell should be packaged such that it should be easily installable on any target platform.

    Create a shell that implements the following commands:

* Directory related commands:
    ls (with support for -R or --recursive switch for listing contents of directory recursively)
    mkdir
    pwd
    rmdir
    cd

* File related commands:
    cat
    cp
    rm (with support for -r switch to recursively delete folder/files)
    mv
    grep
    head
    tail
    sizeof (print size of a file in bytes)
    find

* Miscellaneous commands:
    date (built-in command)
    whoami (built-in command)
    hostname (built-in command)
    timeit (built-in command - print time taken by a command in seconds)
    exit (built-in command)
    history (built-in command)
    
Implement the shell using modular/scalable approach.

The shell should compromise of the following components for scalability:

The core event loop - code that accepts user input, tokenize and evaluate commands

Dispatch routines - the actual functions that implement the command. All miscellaneous commands must be implemented as a built-in command that involves adding new functions for any new future commands to be implemented without changing the core event loop and argument parser.

Plug-in commands - All directory/file related commands must be implemented as a separate module that must be dynamically imported matching the command name. Thus, to add a new command into the shell, create a python module with the same name as the command in a specific folder. This module should have run() function that would contain the command implementation.

Cross-cutting functionality - code that implements recursive traversal of files, parsing command-line switches and arguments passed to commands.

All commands must be implemented purely within Python using the standard python library, without invoking external programs.

The commands must mimic the behaviour of GNU coreutils commands (Linux shell). But not all command-line switches need to be implemented for this project. Just implement the command-line switches mentioned in the above commands list requirements.

The shell must be highly extensible and efficient. The interface for adding new custom commands must be simple.

Usage example when the shell is executed should be as described below (support all of the following features):

    PyShell> ls
    a.txt b.txt pyshell.py testfile.txt

    PyShell> ls /
    bin usr sbin lib dev etc opt root home tmp

    PyShell> sdfsdf
    `sdfsdf` is an invalid command

    PyShell> mkdir testfolder

    PyShell> ls
    a.txt b.txt pyshell.py testfile.txt testfolder

    PyShell> mkdir testfolder
    mkdir: `testfolder` already exists.

    PyShell> cp a.txt testfolder

    PyShell> ls testfolder
    a.txt

    PyShell> rmdir testfolder
    rmdir: `testfolder` is not empty.

    PyShell> pwd
    /home/pythonista/python_exercises

    PyShell> cd testfolder

    PyShell> pwd
    /home/pythonista/python_exercises/testfolder

    PyShell> cd ..

    PyShell> rm -r testfolder

    PyShell> ls
    a.txt b.txt pyshell.py testfile.txt

    PyShell> sizeof a.txt
    47 bytes

    PyShell> timeit cp a.txt c.txt
    `cp a.txt c.txt` took 0.23125 seconds.


    PyShell> head -3 a.txt
    this is first line in a.txt
    this is second line in a.txt
    this is third line in a.txt

    PyShell> tail -1 a.txt
    this is the last line in a.txt

    PyShell> grep -n seven a.txt
    a.txt:7:this is seventh line in a.txt

    PyShell> find / words
    /usr/share/dict/words

    PyShell> find / passwd
    /etc/passwd

    PyShell> ls werewr
    * Unhandled `FileNotFoundError` exception: [Errno 2] No such file or directory: 'werewr'

    PyShell> exit
    PyShell exited.