*****WELCOME TO SEAN'S SHELL PROGRAM!*****

INTRODUCTION

This is my second project for the Temple University course:
CIS3207 - Introduction to Systems Programming and Operating Systems

A shell is a user interface for access to an operating system's services.
This progam will allow the user to run my version of a shell within a 
linux shell itself. While running Sean's Shell, the user will have access
to both the internal commands built into the Shell as well as external commands
from the linux environment its running in.

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GETTING STARTED

Once you have downloaded the nessesary files:
    shell.c
    shell.h
    makefile

Open the directory of the files within a linux shell.
Execute the makefile to build the program:
    $:~/SeansShellPath> makefile

The makefile will add a new executable file for Sean's shell in the current directory.
Execute this "shell" file to run Sean's Shell!
    $:~/SeansShellPath> shell

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INTERNAL COMMANDS

Sean's Shell features built in commands which let the user make use of their operating system:

Change directory command: $cd
    This internal command changes the current working directory to the
    directory specified as the second arguement of the command line input.
    If no arguements follow the cd command, the directory shall change to
    the HOME directory. *MUST use "./" to specify relative path.*

Clear screen command: $clr
    This internal command clears the screen of the shell.

Print directory command: $dir
    Lists contents of the directory provided by the second arguement.
    If there are no arguements passed with the command, the contents of
    the current working directory are printed.
    NOTE: The full path name of the arguement must be provided!

        good: $:~/SeansShellPath> dir /home/TU/tuh42070/CIS3207/shell
        bad:  $:~/SeansShellPath> dir /shell

Environ command: $environ
    This internal command lists all current environment variables.

Echo command: $echo
    This file will display the command line input following the echo command.

Help command: $help
    Prints out the user manual for the shell. Command specific commands can also
    be displayed by entering the desired command as the first arguement:

        $:~/SeansShellPath>help dir

Pause command: $pause
    Pause operation of the shell until 'Enter' is pressed.

Quit command: $quit
    Exits out of Sean's Shell and ends its process.


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EXTERNAL COMMAND SUPPORT

Sean's Shell also supports commands that are provided by the Linux operating system. 
You can learn more about Linux commands here: http://man7.org/linux/man-pages/index.html

Sean's Shell also offers built in opperators for external commands:

Running a background process:
    A process can be executed in the background of the shell. To do so,    
    an '&' character must be at the end of a command line:

        $:~/SeansShell>exefile &

Input/Output Redirection:
A command's input or output file descriptors can be specified in
the command line. This allows the user to change where the entered command
process reads and writes from. i/o redirection can be done
using the following characters:

    '<'  - Redirects the input for the cmd process from the preceding arguement.

    '>'  - Redirects the output of the cmd process to the proceeding arguement.
            If the output file exists, the output will TRUNCATE the existing file.
            If the output file does not exist, a new file will be created.

    '>>' - Redirects the output of the cmd process to the proceeding arguement.
            If the outfile exists, the output will be APPENDED to the file.
            If the output file does not exist, a new file will be created.

    $:~/SeansShell>cat < inputfile.txt > outputfile.txt

Piping:
Piping allows the user to execute multiple external commands on a single command line
while interconnecting the file descriptors between the commands. The output of the first
command will be connected through a "pipe" into the second command as input.nPiping can be 
used by entering the '|' character following the first command, then the second command after:

    $:~/SeansShell>exe1 inputForExe2.txt | exe2