The Pipe Shell
By Elijah Montgomery

The P shell

This program implements a very simple UNIX shell in C using Flex and Bison. Originally, this was created for CS485G Systems Programming with Professor Griffioen at University of Kentucky. I have since modified it slightly and plan to do more work on it in the future. Also, I believe it serves as a good example of how one can implement a simple shell or other programs in C with Flex and Bison. 

Included files are:
 - psh.y: Input parser implemented with GNU bison. Note that this also includes the functions to execute the parsed input
 - psh.l: Input lexical analyzer implemented with flex
 - psh.h: Pipe shell header file. Defines the node type used by the bison parser as well as a few other things needed by the shell
 - varlist.c: Symbol table implemented using a singly-linked list in C
 - varlist.h: Header file for symbol table. Defines node type used by symbol table and a few other things needed by the symbol table.
 - Makefile: Makefile to build the entire project
 - README: information about the program

To compile:
 - Type "make" from the root directory of the project

To delete compilation files:
 - Type "make clean" from the root directory of the project

Features:
 - Includes all builtin commands listed in the project spec
 - Uses path to execute command, so ls can be executed with "ls" or "/bin/ls" 
 - Checks errno's value after system calls to create more informative error messages

Limitations:
 - Cannot currently execute builtin commands using a variable as the command name. For example:
	setvar $mycmd echocmd
	$mycmd on
   would attempt to execute echocmd as a system command
 - Prompt cannot be a dynamic prompt like many shells support. It must be a static string
   and is only changed when the user explicitly changes it.
 - Cannot use complex variable substitution like having $myvar.txt substitute in the value of myvar