CS 450 Homework 2

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Due Wednesday, March 22 at 10:00 AM. There is an automatic grace period of 24 hours associated with this deadline. This is the only consideration that will be given for minor emergencies.

You may work in groups of up to three students. Please have just one group member submit the assignment (see instructions below).

As a reminder, the course collaboration policy will be strictly enforced on this assignment.

Project Description

For this project, you are to write a simple shell-like program in C that reads user commands and executes them using a combination of "fork", "exec", and "pipe".

Basics

• Your shell should allow the user to enter lines of input until the user hits Ctrl-D. You can print a prompt at the beginning of each line if you want, but it's not mandatory.
• You can assume that the user will enter valid Unix commands that are NOT Bash builtins.
• You can assume that the user will enter 1024 or fewer characters per line (counting the newline) and 100 or fewer "words" per line (where a "word" is a contiguous block of text delimited by whitespace).
• You can assume that the user will put whitespace around the redirection operators <, >, and |.
• You are required to implement error handling for open(), close(), pipe(), fork(), and exec (see the provided pipedemo.c for examples)

Sample Commands

Your shell should be able to handle commands of the following types. I recommend developing your code in this order, but it's not required:

• Simple commands with no redirection. Example:

  ps -u yourusername

  Your shell should fork a new process to exec each of these commands.
• Commands with one pipe. Example:

  who | wc -l

  Your shell should fork twice, once to create a process to run "who" and the second time to run "wc" (with option "-l"). You should use a pipe for the two children to communicate. That is, the process that runs "who" will read from stdin and write to your pipe, and the process that runs "wc" will read from that pipe and write to its standard output.
• Commands with two pipes. Example:

  ls /usr/bin | head -10 | tail -5

  This prints the 6th through the 10th commands from the list of commands in /usr/bin. Your shell will need to create three processes and link them together using two pipes. The first child process runs "ls" and writes its output to a pipe. The second child process runs "head" (with argument "-10"), with its input coming from the first process's pipe, and its output going to a second pipe. The third child process, which runs "tail", will read from that second pipe and write its output to stdout.
• Commands with stdin and/or stdout redirected. Examples:

  wc -l < inputfile

  who > outputfile

  wc -l < inputfile > outputfile

• Commands with an arbitrary number of pipes, where the first command's stdin and the last command's stdout may be redirected. Example:

  tr "A-Z" "a-z" < wcExample.cpp | tr -cs "a-z" '\012' | sort | uniq -c | sort -nr | head -1

  This series of commands finds the most common word in "wcExample.cpp" and prints it along with the number of times that it has been repeated. Note that you will have to strip the quotes from the arguments that are quoted before passing them to exec.

Starter Code

To get started, copy the files in ~srivoire/cs450/pickup/hw2 to your directory on the CS department server (blue.cs.sonoma.edu). You should have the following files, which you can use as a basis for your code:

• constants.h -- Defines the maximum number of characters per line and the maximum number of words per line your program is responsible for
• parsetools.h/.c -- Defines a function that can split a C-string into an array of "words" using whitespace as a delimiter
• main.c -- Skeleton code for your shell. Currently loops until the user hits Ctrl-D. For each line of input, it splits the line into words and prints out one word per line
• pipedemo.c -- The file we went over in class. Feel free to steal any or all of this code.
• makefile -- Creates two executables: pipedemo.x and pipes.x. Your final executable should be named pipes.x.

Grading Criteria

Design and style will be assessed on the following criteria:

• Your algorithm -- is it efficient and clear, or convoluted with a lot of wasted work?
• Your decomposition -- have you divided the work among well-defined functions with clear interfaces?
• Your variable names and documentation -- are they helpful? Please note that this does NOT mean putting a comment on every line!

Submission

In addition to your source files, your submission should include a makefile and a file named groupmembers.txt listing your group members (unless you worked by yourself).

Run "make clean" before submitting. Then, back up to your parent directory and run

tar czvf hw2.tgz yourdirectory

Submit your tarfile on Moodle.