--- Name --- 

MyShell
Made public 04/10/2019

--- Author --- 

Sean Fradl
Dublin City University
As part of CA216

--- Overview ---

MyShell is a python 3 based UNIX command interpreter usable as a script command processor. 

--- Usage ---

-- Commands -- 

cd [directory]          - changes directory, if none supplied then prints out the current working directory. 
echo [options] strings  - simply prints the additional arguments passed. 
clr                     - clears the screen. 
dir [directory]         - displays the contents of the directory passed as an argument. If none are passed displays contents of current working directory.
environ                 - lists all the currently set evironment variables. 
pause                   - pauses the execution of the shell until the enter key has been pressed.
quit                    - exits the shell.
pwd                     - prints the current working directory to the terminal. 
help                    - displays this readme within the terminal using more.

-- Script Files -- 

The shell supports scripts that can be passed when starting the shell,
Example:./myshell.py script.mysh
The above would run the script line by line as if it was manually inputed into the shell.

-- i/o redirection --

Before a command is executed, its input and output may be redirected and specified using a special notation interpreted by the shell.
The shell supports standard shell i/o redirection as follows: 

> - appends output to file.
>> - creates file/overwrites output to file.
<< and < - both read input from file to command. 

Usage: command < input >> ouput. 

--- Environment Variables --- 

The following variables are set within the shell to provide additional support for the inbuilt commands.

Home - string containing the path of the home directory for the user that is currently operating the shell.
PWD - string containing the path of the current working directory the user is operating within.
SHELL - string containing the executable path to the shell. 


--- Reserved Keywords ---

The shell reserves the following keywords for inbuilt commands:
cd, echo, clr, dir, environ, pause, exit, environ, help, pwd.

These commands have a higher priority and binaries attempted to run with the same name will be unable to do so.

--- Running Binaries --- 

By default, the shell will take in the OS path environment variable if possible. This will allow the shell to run programs typically found in /usr/bin. 
For example, instead of typing the full path to python3 '/usr/bin/python3 test.py', users can simply type 'python3 test.py' in the shell to run a python3 program.