Advanced Operating Systems P1

                        Advanced Operating Systems
                                 


Write a C/C++ program that implements a portion your own shell.  It should
print a prompt which is the concatenation of your login id and the char
string "_sh> " (note the blank at the end). So, if I were to implement
the project, the prompt I would print would be "rbutler_sh> " without
the quotes.  However, the shell should not print a prompt if its stdin
is not a tty or if it is given a single command-line arg which is a
file to accept commands from instead.  It is valid to assume that no
command-line will be more that 64 bytes.

For ease of parsing, you may assume that all command-line values are
separated by at least one char of white-space and that a single command
is on a single line.  A # anywhere on the line marks the beginning of
a comment and thus the end of the line.

The shell should provide these builtin commands:
   exit (or end-of-file, e.g. cntl-D on tty)
   set var val  (shell variable - not env)
   unset var    (shell variable - not env)
   prt word $var  (use shell's $var if same name in both shell and env)
   envset VAR VAL
   envunset VAR
   envprt (print the entire environment)
   witch (like which :)  (looks thru AOSPATH for a cmd - see below)
   pwd
   cd  (no args -> no change) (one arg may be relative or absolute)
Builtin commands are executed in the shell's process.  Variables can
be used on any command-line, but no variable value can contain a $
that might cause recursive variable interpolation.  Shell variables
are to be used before env vars if both exist unless otherwise stated.
A valid value on a command-line might be:
    cd $HOME/$nextdir    ## assuming both vars have been defined
where both HOME and nextdir must be interpolated.
Valid vars consist of alphas, numerics, and underscores, beginning with
alpha or underscore.

The shell should provide these environment variables at start-up:
    AOSPATH (default is /bin:/usr/bin)
    AOSCWD  (current working directory) (cd should cause it to be updated)
And, those two vars should be the ONLY two at start-up.
So, I should be able to immediately execute:
    prt $AOSPATH $AOSCWD

For this project, you do NOT have to execute any commands other than those
built into the shell.


Submitting the project for grading:

The project should compile and link as p1.

You should turnin a tar file containing all of the required files.

To build the project, I will cd to my directory containing your files
and simply type:

    rm -rf p1 *.o
    make