"Trivia" email to TA by 11:59pm, Monday, March 23.
Assignment due by 11:59pm, Monday, March 30.
The purpose of this assignment is to become more familiar with the concepts of process control and signaling. You'll do this by writing a simple Unix shell program that supports job control.
The managing TA for this assignment is Sharanyan Srikanthan (srikanth@cs.rochester.edu). Please direct your questions about this assignment to the managing TA. The TA's office hours for this assignment are:
You can form a group of two for this assignment. You can also choose to work alone. You will be able to choose to work in a group of two for four assignments (assignments #2/#3/#4/#5). However, it is our policy that you can NOT work with the same team partner for more than two assignments. If you do, we will apply 50% penalty on your third and fourth assignments with the same grouping.
Copy the file /u/cs252/labs15/shlab-handout.tar
to the protected
directory in which you plan to do your work. Then do the following:
tar xvf shlab-handout.tar
to expand the tarfile.make
to compile and link some test routines.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. 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. [80 lines]builtin_cmd
: Recognizes and interprets the built-in commands quit
, fg
, bg
, and jobs
. [30 lines]do_bgfg
: Implements the bg
and fg
built-in commands. [50 lines]waitfg
: Waits for a foreground job to complete. [25 lines]sigchld_handler
: Catches SIGCHILD signals. [75 lines]sigint_handler
: Catches SIGINT (ctrl-C
) signals. [20 lines]sigtstp_handler
: Catches SIGTSTP (ctrl-Z
) signals. [20 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]
If you want to exit your shell, but have not implemented the appropriate
commands, you can type ctrl-D
or ctrl-C
to kill your shell.
A shell is an interactive command-line interpreter that runs programs on behalf of the user. A shell repeatedly 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 prompting for 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 ls
begins executing its main
routine:
int main(int argc, char *argv[])the
argc
and argv
arguments have the following
values:
argc == 3
argv[0] == "/bin/ls"
argv[1] == "-l"
argv[2] == "-d"
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 background 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 <job>
: Change a stopped background job to a running background job.fg <job>
: Change a stopped or running background job to a running foreground job.kill <job>
: Terminate a job.tsh
Specification:
Your tsh
shell should have the following features:
tsh>
".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 >
).C
(ctrl-Z
) should
cause a SIGINT (SIGTSTP) signal to be sent to the current foreground job,
as well as any descendants of that job (e.g., any child processes that it
forked). If there is no foreground job, then the signal should have no effect.tsh
should run the job in the background. Otherwise, it should run the job in
the foreground.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:
quit
command terminates the shell.jobs
command lists all background jobs.bg <job>
command restarts <job>
by
sending it a SIGCONT signal, and then runs it in the background. The
<job>
argument can be either a PID or a JID.fg <job>
command restarts <job>
by
sending it a SIGCONT signal, and then runs it in the foreground.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.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 identical to the reference solution
(except for PIDs, of course, which change from run to run).
Past experience indicates that obtaining exact correspondence is
difficult. In some cases one might argue that different output
would be equally “good;” even so, for the sake of the
TA's sanity, you will receive full credit only for matching the reference solution.
Shell driver. The sdriver.pl
program (a perl script)
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.
unix> ./sdriver.pl -h Usage: sdriver.pl [-hv] -t <trace> -s <shellprog> -a <args> Options: -h Print this message -v Be more verbose -t <trace> Trace file -s <shell> Shell program to test -a <args> 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:
unix> ./sdriver.pl -t trace01.txt -s ./tsh -a "-p"
or
unix> make test01
(The -a "-p"
argument tells your shell not to emit a prompt.)
And, if you want to run all of the tests on your shell, you can type:
unix> make tests
Similarly, 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
And you can run all the tests on the reference shell by typing
unix> make rtests
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: Interrupt 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: Stopped 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 & tsh> fg %1 tsh> quit unix>
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.psignal
, waitpid
, kill
,
fork
, execve
, setpgid
,
sigprocmask
, and tcsetpgrp
functions will come
in very handy. The WUNTRACED and WNOHANG options to waitpid
will also be useful.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 /bin/ls
, /bin/ps
, and
/bin/echo
. If you run one of the others by accident, you
may be able to get back to normal by typing stty reset
.
In the worst case, you may need to terminate your terminal session and log
back in again.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.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).Your score will be computed out of a maximum of 90 points based on the following distribution:
/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.By 11:59pm on Monday, March 23, send email to TA Sharanyan Srikanthan (srikanth@cs.rochester.edu) with the subject line "[CSC252] Assign#3 Trivia - uname1 uname2" (without the quotes, where uname is your login name) containing answers to the following questions (a single email per team is expected):
trace02.txt
as the trace file.fork
and execve
system calls do?WNOHANG
option to the waitpid
system call.You should electronically turn in your solution through Blackboard. Before turn-in, be sure that you have
Late turn-ins will be accepted for up to three days, with 10% penalty for each late day. No turn-ins more than three-day late will be accepted.