a2 - cs201 - due in 2 weeks (feb 4).
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small shell - overview
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Write a small basic UNIX shell that parses commands, and executes
them.  Note that we expect to be able to execute only simple
commands without shell redirection or pipes.  Basically
you should be able to execute: cat, cp, ls and various
other simple commands with and without arguments.  

E.g,
% ls -l
% cat file1 file2 file3 file4  
% cp file target
% rm file1 file2

and the like.  Note that you may need to have a built-in
path; e.g,. first try ., then try /bin, then try /usr/bin.

More complex commands like
commands that assume visual terminal control commands 
(emacs, vi, and the like) may not work -- this is ok.
Nor do we expect % ls | wc to work or
ls > file.

Roughly your shell should do the following:

1. take an input string and parse it into words separated
by white space (tabs, blanks, and the like).  It should
read one line at a time (newline terminated).  Your
shell should exit at EOF (control-d).

2. execute the command using fork, execvp, wait, and exit.
I.e., the shell should fork a child, let the child call
exec, and wait for the child to terminate.  Be careful
about forking; i.e., run ps or ps -elf (or top) to make
sure you do not have vast amounts of escaped children.
This may be due to the parent exiting with the children
still running or for other reason where the parent terminates
before it can call wait.   

3. add three built-in commands: 

3.1
	% myshell string ....

Call system(3) to execute string.  This is more or less what
your shell should do without the "myshell" verb.  This is very
easy.

3.2
	% mydebug  

Reprint the last command, and show whether it was a built-in
command or not (myshell/mydebug/chdir).  This is a very primitive
history.

3.3
	% chdir somedirectory

The current working directory is an attribute of the running process.
In other words, you need to make a system call to change the shell's
directory.  

In the next assignment, we will enhance this shell so try to do
a good job.

Turn-in
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1. an ASCII report showing tests executed (use script).

2. Makefile

3. *.c and any *.h source code.

System calls
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1. system(3)
2. chdir(2)
3. fork(2), execvp(2), wait(2), exit(2)