/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 
	 *                                                             *
	 *    MyShell - basic C shell that emulates the unix bash      *
	 *    Author - Danju Visvanthan                                *
	 *    SID - 312095252                                          *
	 *    Written for COMP3520 Assignment 1 2015 Semester 1.       *
	 *                                                             *
	 *                                                             *
	 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 NAME
	myshell - basic implementation mirroring the functionality of the UNIX bash.

 SYNOPSIS
	myshell [file]
	myshell

	ERRORS: if the batchfile does not exist, an appropriate error message is displayed, and the
			shell is invoked as it would be without a batchfile.

 DESCRIPTION
	A basic shell that is developed to closely mirror the fundamental commands of the UNIX
	bash. It supports the basic commands (cd, echo, environ, help, quit, clr, dir) and
	externally executes any other commands. It also has support for I/O redirection.

 COMMANDS
	cd <directory>: Changes the directory to the specified argument. Does not necessarily 
					have to be an absolute path (although this is supported), and
					support for shortcuts such as "..", "." exist, but "~" is not supported.
					entering cd with no arguments prints the current working directory.
					The PWD environment variable is changed to reflect the new directory.
					
				ERRORS: If the location does not exist, the command is ignored and an appropriate
				messageis outputted.

				ARGS: The path to change to. If none specified, prints the current working directory.

	dir <n>:		Lists the contents of the current directory if no argument is specified,
					otherwise lists the contents of directory n, if it exists. supports
					multiple directories e.g. dir . .. ../.. will print the contents of the
					current, parent, and the parent's parent directories respectively. 

				ERRORS: issues error message and aborts if directory specified does not exist.

				ARGS: directory to list files for, if no arguments specified; lists current working
					directory.

	echo <n>:		Prints to stdout, if no argument is specified, a blank line is displayed.

				ERRORS: None

				ARGS: tokens to print out, can have multiple arguments. If no argument is specified,
					  a blank line is instead displayed.


	clr:			clears all output displayed on the screen.

	environ: An array of string pointers of every environment variable

	help: Displays the readme.

	quit: Exits the shell.

 I/O REDIRECTION
		Input redirection [<]: replaces the STDIN stream with any file F (all input required for a program
							   is replaced by each line in F).

			ERRORS: If the file does not exist, an error message is displayed, and the command is ignored.
					If no file is specified, an error message is displayed, and the command is ignored.

			USAGE: programname arg1 < filename

		Output Redirection [>]: replaces the STDOUT stream with any File F (all output generated from a program
								is instead inserted into f.). If F does not exist, it is created. If F exists,
								it is replaced in its entirety by the output from STDOUT.

			ERRORS: If no file is specified, an error message is displayed, and the command is ignored.

			USAGE: programname arg1 > filename

		Appended Output redirection [>>]: See output redirection. If the file exists, it is instead appended to.

			ERRORS: If no file is specified, an error message is displayed. The command proceeds
					as it would if no redirection was specified.

			USAGE: programname arg1 >> filename

 BACKGROUND EXECUTION [&]

	Allows a program to execute in the background, allowing the shell to execute another program(s) in parallel with the
	initial program.
	This is achieved by continuing the parent process when a child process' program has been flagged as in the background,
	not waiting for the process to return.

	ERRORS: if no programname is specified, a syntax error is issued.
			if the program errors or doesn't exist, the process will
				first begin, then exit on error.
	USAGE: programname arg1 &