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You have just had the wonderful insight that the interface to the operating
system could be nicer than what is currently available with punch card sets.
New technology -- called "terminals" and "terminal emulators" -- could be used
to write a program (you decide to call it a "command interpreter", or maybe just
a "shell" because that is shorter) that
interacts in a friendly fashion so as to allow you to simply type in commands
and have the operating system jump to carry out those commands.
You decide to write the shell in two steps. The first version of the command
interpreter (V1) will let you explore the basic ideas and make sure that
you've got the hang of this "programming UNIX" stuff. Later, you will work on a fancier
version.
Your goals for the first version of the command interpreter are as follows:
- To get better acquainted with UNIX.
- To get a feel for system tool/application level programming.
- To gain experience with UNIX process creation and management.
The program requirements for your shell
You decide that your shell will exhibit the following behavior:
- To exit the shell, you can type either <CTRL-D> at the beginning of a line
(that's how UNIX simulates end of file from keyboard input) or "exit".
- Run standard UNIX commands just like the regular shell (bash, sh, csh, etc.)
Syntax: command [option ]*
Test command: $ ls -sF file1 file2
Man pages: fork, execvp, wait and waitpid.
NOTE: you must use fork and execvp, as well as your choice of wait or waitpid.
Using execvp requires that you build an argument vector à la argv for the new process.
Be very careful to use fork( ) properly. The correct use will
be explained in class. If you mess up on this, you will quite possibly tie up/lock
up your account until a snickering (or after the fourth time, grumpy) sysadmin
cleans up the mess. Do take the warnings seriously and follow instructions carefully.
- Change the current working directory.
Syntax: cd path_to_new_directory
Test command: $ cd /
Man pages: chdir.
Groups
You decide to work on this in groups of exactly 1 people :-). Seriously, this
is a great time to get going on brushing up you 'C' skills, reading "man" pages,
and generally getting in shape for the semester. Group programs will come later.
Suggested reading
Sample code
Due date and deliverables
- Due midnight, Monday Feb 10, via the electronic submission system.
Submit under "P1 - Shell V1".
- Submit the directory with your source code and executable. Please call the
primary source code file "shell1.c" and the executable (your command interpreter)
"a.out" to make it easier for the grader to identify them.
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