CMPSC 311 Fall 2019, Project 3
Shell Lab : Writing Your Own Unix Shell
Assigned: Wednesday Oct. 25th, Due: Friday, Nov. 8th, 11PM
Yanling Wang ([email protected]) is the lead person for this assignment.
Introduction
The purpose of this assignment is to become more familiar with the concepts of process control and sig-
nalling. You’ll do this by writing a simple Unix shell program that supports job control.
Hand Out Instructions
Start by copying the file shlab-handout.tar from Canvas to the protected directory (the lab directory)
in which you plan to do your work. Then do the following:
• Type the command tar xvf shlab-handout.tar to expand the tarfile.
• Type the command make to compile and link some test routines.
• Type your name and PSU ID in the header comment at the top of tsh.c.
Looking at the tsh.c (tiny shell) file, you will see that it contains a functional skeleton of a simple Unix
shell. To help you get started, we have already implemented the less interesting functions. Your assignment
is to complete the remaining empty functions listed below. As a sanity check for you, we’ve listed the
approximate number of lines of code for each of these functions in our reference solution (which includes
lots of comments).
• eval: Main routine that parses and interprets the command line. [70 lines]
• builtin cmd: Recognizes and interprets the built-in commands: quit, fg, bg, and jobs. [25
lines]
• do bgfg: Implements the bg and fg built-in commands. [50 lines]
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• waitfg: Waits for a foreground job to complete. [20 lines]
• sigchld handler: Catches SIGCHILD signals. 80 lines]
• sigint handler: Catches SIGINT (ctrl-c) signals. [15 lines]
• sigtstp handler: Catches SIGTSTP (ctrl-z) signals. [15 lines]
Each time you modify your tsh.c file, type make to recompile it. To run your shell, type tsh to the
command line:
unix> ./tsh
tsh> [type commands to your shell here]
General Overview of Unix Shells
A shell is an interactive command-line interpreter that runs programs on behalf of the user. A shell repeat-
edly prints a prompt, waits for a command line on stdin, and then carries out some action, as directed by
the contents of the command line.
The command line is a sequence of ASCII text words delimited by whitespace. The first word in the
command line is either the name of a built-in command or the pathname of an executable file. The remaining
words are command-line arguments. If the first word is a built-in command, the shell immediately executes
the command in the current process. Otherwise, the word is assumed to be the pathname of an executable
program. In this case, the shell forks a child process, then loads and runs the program in the context of the
child. The child processes created as a result of interpreting a single command line are known collectively
as a job. In general, a job can consist of multiple child processes connected by Unix pipes.
If the command line ends with an ampersand ”&”, then the job runs in the background, which means that
the shell does not wait for the job to terminate before printing the prompt and awaiting the next command
line. Otherwise, the job runs in the foreground, which means that the shell waits for the job to terminate
before awaiting the next command line. Thus, at any point in time, at most one job can be running in the
foreground. However, an arbitrary number of jobs can run in the background.
For example, typing the command line
tsh> jobs
causes the shell to execute the built-in jobs command. Typing the command line
tsh> /bin/ls -l -d
runs the ls program in the foreground. By convention, the shell ensures that when the program begins
executing its main routine
int main(int argc, char *argv[])
the argc and argv arguments have the following values:
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• argc == 3,
• argv[0] == ‘‘/bin/ls’’,
• argv[1]== ‘‘-l’’,
• argv[2]== ‘‘-d’’.
Alternatively, typing the command line
tsh> /bin/ls -l -d &
runs the ls program in the background.
Unix shells support the notion of job control, which allows users to move jobs back and forth between back-
ground and foreground, and to change the process state (running, stopped, or terminated) of the processes
in a job. Typing ctrl-c causes a SIGINT signal to be delivered to each process in the foreground job. The
default action for SIGINT is to terminate the process. Similarly, typing ctrl-z causes a SIGTSTP signal
to be delivered to each process in the foreground job. The default action for SIGTSTP is to place a process
in the stopped state, where it remains until it is awakened by the receipt of a SIGCONT signal. Unix shells
also provide various built-in commands that support job control. For example:
• jobs: List the running and stopped background jobs.
• bg : Change a stopped background job to a running background job.
• fg : Change a stopped or running background job to a running in the foreground.
• kill : Terminate a job.
The tsh Specification
Your tsh shell should have the following features:
• The prompt should be the string “tsh> ”.
• The command line typed by the user should consist of a name and zero or more arguments, all sepa-
rated by one or more spaces. If name is a built-in command, then tsh should handle it immediately
and wait for the next command line. Otherwise, tsh should assume that name is the path of an
executable file, which it loads and runs in the context of an initial child process (In this context, the
term job refers to this initial child process).
• tsh need not support pipes (|) or I/O redirection (< and >).
• Typing ctrl-c (ctrl-z) should cause a SIGINT (SIGTSTP) signal to be sent to the current fore-
ground job, as well as any descendents of that job (e.g., any child processes that it forked). If there is
no foreground job, then the signal should have no effect.
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• If the command line ends with an ampersand &, then tsh should run the job in the background.
Otherwise, it should run the job in the foreground.
• Each job can be identified by either a process ID (PID) or a job ID (JID), which is a positive integer
assigned by tsh. JIDs should be denoted on the command line by the prefix ’%’. For example, “%5”
denotes JID 5, and “5” denotes PID 5. (We have provided you with all of the routines you need for
manipulating the job list.)
• tsh should support the following built-in commands:
– The quit command terminates the shell.
– The jobs command lists all background jobs.
– The bg command restarts by sending it a SIGCONT signal, and then runs it in
the background. The argument can be either a PID or a JID.
– The fg command restarts by sending it a SIGCONT signal, and then runs it in
the foreground. The argument can be either a PID or a JID.
• tsh should reap all of its zombie children. If any job terminates because it receives a signal that
it didn’t catch, then tsh should recognize this event and print a message with the job’s PID and a
description of the offending signal.
Checking Your Work
We have provided some tools to help you check your work.
Reference solution. The Linux executable tshref is the reference solution for the shell. Run this program
to resolve any questions you have about how your shell should behave. Your shell should emit output that is
exactly identical to the reference solution (except for PIDs, of course, which change from run to run).
Shell driver. The sdriver.pl program executes a shell as a child process, sends it commands and signals
as directed by a trace file, and captures and displays the output from the shell.
Use the -h argument to find out the usage of sdriver.pl:
unix> ./sdriver.pl -h
Usage: sdriver.pl [-hv] -t -s -a
Options:
-h Print this message
-v Be more verbose
-t Trace file
-s Shell program to test
-a Shell arguments
-g Generate output for autograder
We have also provided 16 trace files (trace{01-16}.txt) that you will use in conjunction with the shell
driver to test the correctness of your shell. The lower-numbered trace files do very simple tests, and the
higher-numbered tests do more complicated tests.
You can run the shell driver on your shell using trace file trace01.txt (for instance) by typing:
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unix> ./sdriver.pl -t trace01.txt -s ./tsh -a "-p"
(the -a "-p" argument tells your shell not to emit a prompt), or
unix> make test01
Similarly, to compare your result with the reference shell, you can run the trace driver on the reference shell
by typing:
unix> ./sdriver.pl -t trace01.txt -s ./tshref -a "-p"
or
unix> make rtest01
For your reference, tshref.out gives the output of the reference solution on all races. This might be
more convenient for you than manually running the shell driver on all trace files.
The neat thing about the trace files is that they generate the same output you would have gotten had you run
your shell interactively (except for an initial comment that identifies the trace). For example:
unix> make test15
./sdriver.pl -t trace15.txt -s ./tsh -a "-p"
#
# trace15.txt - Putting it all together
#
tsh> ./bogus
./bogus: Command not found.
tsh> ./myspin 10
Job (9721) terminated by signal 2
tsh> ./myspin 3 &
[1] (9723) ./myspin 3 &
tsh> ./myspin 4 &
[2] (9725) ./myspin 4 &
tsh> jobs
[1] (9723) Running ./myspin 3 &
[2] (9725) Running ./myspin 4 &
tsh> fg %1
Job [1] (9723) stopped by signal 20
tsh> jobs
[1] (9723) Stopped ./myspin 3 &
[2] (9725) Running ./myspin 4 &
tsh> bg %3
%3: No such job
tsh> bg %1
[1] (9723) ./myspin 3 &
tsh> jobs
[1] (9723) Running ./myspin 3 &
[2] (9725) Running ./myspin 4 &
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tsh> fg %1
tsh> quit
unix>
grading script The grade-shlab.pl program tests your program through all 16 trace files and compare
them against the ref solution. It gives out a preliminary correctness score based on the comparison. This
script will take a while to run and sometimes will stall on a trace if you have not implemented features
needed for what that trace is checking.
You may run this command as:
unix>./grade-shlab.pl -f tsh.c
Hints
• Read every word of Chapter 8 (Exceptional Control Flow) in your textbook.
• Use the trace files to guide the development of your shell. Starting with trace01.txt, make
sure that your shell produces the identical output as the reference shell. Then move on to trace file
trace02.txt, and so on.
• The waitpid, kill, fork, execve, setpgid, and sigprocmask functions will come in very
handy. The WUNTRACED and WNOHANG options to waitpid will also be useful.
• When you implement your signal handlers, be sure to send SIGINT and SIGTSTP signals to the en-
tire foreground process group, using ”-pid” instead of ”pid” in the argument to the kill function.
The sdriver.pl program tests for this error.
• One of the tricky parts of the assignment is deciding on the allocation of work between the waitfg
and sigchld handler functions. We recommend the following approach:
– In waitfg, use a busy loop around the sleep function.
– In sigchld handler, use exactly one call to waitpid.
While other solutions are possible, such as calling waitpid in both waitfg and sigchld handler,
these can be very confusing. It is simpler to do all reaping in the handler.
• In eval, the parent must use sigprocmask to block SIGCHLD signals before it forks the child,
and then unblock these signals, again using sigprocmask after it adds the child to the job list by
calling addjob. Since children inherit the blocked vectors of their parents, the child must be sure
to then unblock SIGCHLD signals before it execs the new program.
The parent needs to block the SIGCHLD signals in this way in order to avoid the race condition where
the child is reaped by sigchld handler (and thus removed from the job list) before the parent
calls addjob.
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• Programs such as more, less, vi, and emacs do strange things with the terminal settings. Don’t
run these programs from your shell. Stick with simple text-based programs such as /bin/ls,
/bin/ps, and /bin/echo.
• When you run your shell from the standard Unix shell, your shell is running in the foreground process
group. If your shell then creates a child process, by default that child will also be a member of the
foreground process group. Since typing ctrl-c sends a SIGINT to every process in the foreground
group, typing ctrl-c will send a SIGINT to your shell, as well as to every process that your shell
created, which obviously isn’t correct.
Here is the workaround: After the fork, but before the execve, the child process should call
setpgid(0, 0), which puts the child in a new process group whose group ID is identical to the
child’s PID. This ensures that there will be only one process, your shell, in the foreground process
group. When you type ctrl-c, the shell should catch the resulting SIGINT and then forward it
to the appropriate foreground job (or more precisely, the process group that contains the foreground
job).
Evaluation
Your score will be computed out of a maximum of 65 points based on the following distribution:
48 Correctness by testing: 16 trace files at 3 points each. However, keep in mind that there are potentially
race conditions in the program, where the signal handler and the main program will access/modify
the same data structures. If you don’t have correct handling of the race conditions, your code could
sometimes provide correct result and sometimes wrong result.
10 Manual testing: we will have some extra testing of your code to intentionally create race conditions (by
adding delays in either the parent process or the child processes) and check if your code has dealt with
these race conditions properly.
7 Style points. We expect you to have good comments (4 pts) and to check the return value of EVERY
system call (3 pts).
Your solution shell will be tested for correctness on a Linux machine, using the same shell driver and trace
files that were included in your lab directory. Your shell should produce identical output on these traces as
the reference shell, with only two exceptions:
• The PIDs can (and will) be different.
• The output of the /bin/ps commands in trace11.txt, trace12.txt, and trace13.txt
will be different from run to run. However, the running states of any mysplit processes in the
output of the /bin/ps command should be identical.
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Hand In Instructions
• Make sure you have included your name and PSU ID in the header comment of tsh.c.
• Submit your tsh.c file to Gradescope.
Good luck!
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