Description
Background
One of the fundamental functionalities of Linux (and Linux-like OSs) is the ability to pipe
commands. When the | character is used in a shell between two commands, the shell takes
the output from the first command and “pipes” it to the second command as the input. You
can try it yourself! If you run the command ls -la / | grep tmp on systems1, you
should be able to see only the tmp directory output from ls -la. This is because you gave
grep, a common search tool, the output of ls -la directly. There are many more
applications of piping but that is for you to discover as necessary in your
adventures/projects/jobs/etc etc etc.
The programming part
For this assignment you are to write a simple program that will act as a shell. The program
should:
Display a command prompt and read in a command line from the user (the prompt must
be CS361 >, otherwise it cannot be detected by the test script)
Your shell must support basic piping like the unix shell.
1. $ command Run command, with stdin and stdout connected to their usual files. When
command finishes, the parent should output pid:%d status:%d\n (with the proper
relevant values inserted into the format string), and then print the prompt again and
wait for more input.
CS 361 Summer 2020 Home Schedule Homeworks
6/15/2020 CS 361 Summer 2020 – HW3
https://sites.google.com/uic.edu/cs361summer2020/homeworks/hw3?authuser=1 2/3
2. $ command1 | command2 Run command1 as in #1, but redirects the output of
command1 as input to command2. Note that there should be two instances of pid:%d
status:%d\n output.
3. $ command1 ; command2 Run command1 as in #1, wait for it to finish, and then run
command 2 as in #1.
Also…
Your shell should handle the following signals:
SIGINT – Generated by Ctrl-C. This signal allows a user to terminate a running
program. Your shell should not exit when user presses Ctrl-C or the process receives
SIGINT but simply report that the SIGINT signal has been received by the shell. If the
process receives SIGINT, you must print the string “caught sigint” on a line by itself,
and then show the prompt again.
SIGTSTP – Generated by Ctrl-Z. This signal allows a user to terminate a running
program. Your shell should not exit when user presses Ctrl-Z or the process receives
SIGTSTP but simply report that the SIGTSTP signal has been received by the shell. If
your shell receives SIGTSTP, you must print the string “caught sigtstp” on a line by
itself, and then show the prompt again.
The shell does NOT need to support background processes, running more than one child
at a time, or handling multiple chained pipes. Essentially, if anything not matching the
use cases/requirements above is in question, you probably do not have to do it.
Due to the breadth of this assignment, it is difficult to clarify every case in writing. Please
ask in office hours or on Piazza if you have questions or need clarification.
Tips for primary components
Parse the command line into arguments, creating an array of character pointers, where
array[0] points to the actual command and rest of the array elements point to the
arguments to the command (Similar to main()’s argv[])
Fork off a child and have the child load the requested program by passing the argument
vector created in step 2 to exec() family of system calls. The parent should report the PID
of the child before proceeding to the next step. (report means you are required to return
strings contain “pid” and “status”, e.g. pid:11111 status:0)
Wait for the child to complete executing and report why it ended (exited or uncaught
signal) and its exit value if available.
Repeat the first step forever until the user enters the command exit
Your personal repository
CS 361 Summer 2020 Home Schedule Homeworks
6/15/2020 CS 361 Summer 2020 – HW3
https://sites.google.com/uic.edu/cs361summer2020/homeworks/hw3?authuser=1 3/3
p p y
Note that there is no skeleton code for the homework, you can find the repository for hw3
here (this is an empty repository, start by creating a file named “hw3.c” and add it to your
github repository). Going to lab sections is highly advisable, as the TAs have been and will
be explaining basics of getting started with a shell. Both the lab section code, as well as code
found in the book or book slides, are “fair game” from which to begin your coding. Previous
solutions, or other students’ work, are off limits and any cheating will be prosecuted to the
fullest extent of university rules. There are solutions on the Internet. If we find you using them,
we will give you an F in the class. Don’t look at them, don’t use them. It is not worth it.
Requirements
See above, but here is a summary:
1. Display a command prompt (CS361 >) and read in a command line from the user
2. Handle $ command (run command, with stdin and stdout connected to their usual files,
must additionally output pid:%d status:%d\n, then continue)
3. Handle $ command1 | command2 (run command1 as in #1, but redirects the output of
command1 as input to command2, then continue)
4. Handle $ command1 ; command2 (run command1 as in #1, wait for it to finish, and then
run command 2 as in #1, then continue)
5. Handle SIGINT – Generated by Ctrl-C (print “caught sigint” and then continue)
6. Handle SIGTSTP – Generated by Ctrl-Z (print “caught sigtstp” and then continue)
It is highly recommended you develop and test your code on systems1 or a similar Linux
environment. This is simply to ensure standardization of comparisons between our and
your tests whilst you are developing your code.
Turn-in instructions and Grading
You need to submit hw3.c file to Gradescope. When you turn in the assignment, the TA
should be able to compile your program by running gcc -Wall -Werror -o hw3 hw3.c
in the hw3 directory (case sensitive). You are free to include any files e.g. hw3.h, it just has
to compile using the above command.
Grading will be done automatically using Gradescope. Submitting to GitHub is not sufficient
– your code must be submitted to Gradescope. If you have issues with the autograder,
please contact us via Piazza ASAP. Technical issues within 36 hours of the deadline will
not be an excuse for submitting the assignment improperly or late.
CS 361 Summer 2020 Home Schedule Homeworks
