MYSHELL - USER MANUAL - JOHN MURRAY, OCTOBER 2019

This is a simple implementation of a shell, or command-line interpreter.
It has almost as much functionality as any shell you might have used that is 
builtin to your computer system (bash, tcsh, etc.). 

COMMANDS:
The functionality is based on executing and manipulating programs, files, and commands.
The commands can be categorized into builtin and external commands.
Builtin commands are coded directly in the shell program, whereas external commands include 
any command that is on the system running the shell, including commands contained in shells
builtin to the system.
Builtin commands are simply functions within the program, namely:
    cd <directory> - Change the current default directory to <directory>. If the <directory> 
    argument is not present, report the current directory. 

    clr - clear the screen

    dir <directory> - List the contents of directory <directory>

    environ - List all the environment strings

    echo <comment> - Display <comment> to stdout followed by a new line 

    help - Display the user manual 

    pause - Pause operation of the shell until 'Enter' is pressed

    quit - Quit the shell

    path - display the path from which the shell has been executed

Any external command that is called will create a copy of the current process that is running, 
and execute the command specified, then the original process will return to the prompt.
Some external commands include ls, wc, man, gcc, rm, mkdir, nice, etc...
(Note that many commands require arguments to do anything of substance)

OPERATORS:
There are several special operators featured in this shell that offer powerful functionality:
    Redirection - '<', '>', and '>>' - redirect input or output of command to specified file 
    use: 
        <program> < <file>
        <program> > <file>
        <program> >> <file>

    Input Redirection - '<' - Redirects stdin of program given to file specified. Essentially
    reads from that file rather than reading from the keyboard

    Output Redirection - '>' or '>>' - Displays output of program given to file specified rather 
    than the screen. 
    Single '>' means that if the given file already exists, clear its contents and write to the 
    beginning of the file, or create and write to file with that name if it does not exist.
    Double '>>' means that if the given file already exists, write to the end of the file, or create
    and write to file with that name if it does not exist.

    Pipes - '|' - redirect output of one program to input of other program
    use: <program1> <optionalargs> | <program2> <optargs>

    Parallel - '&' - Run multiple commands in parallel (at the same time)
    use: <program> <optionalargs> & <program> & <command> <optargs>
    Simple use this operator if you want to execute multiple commands at once

    Background Execution - '&' - Run program in background
    use: <command> <args> &
    If the '&' character is at the end of the command line, the shell executes the command specified,
    and does not wait for it to finish, but immediately returns to user prompt

BATCHFILE:
Instead of running the shell normally and typing commands into the prompt, you can include a batchfile 
as an argument when you invoke the shell. You can do this by typing commands, one per line, into a .txt
file. Say the file is called batch.txt. 
When you open the shell, type the command:
        myshell batch.txt

NOTE: for more information on any external command, type "man <command>" into the shell to open the 
manual page for that command