18-213/18-613, Summer 2024
Shell Lab: Writing Your Own Linux Shell
Assigned: Mon, July 8, 2024
Due: Thurs, Mon, July 22, 2024 at 11:59PM
Last Possible Handin: Thu, July 25th, 2024 at 11:59PM
1 Introduction
The purpose of this assignment is to help you become more familiar with the concepts of process control and
signalling. You’ll do this by writing a simple Linux shell program, tsh (tiny shell), that supports a simple
form of job control and I/O redirection. Please read the whole writeup before starting.
2 Logistics
This is an individual project. All handins are electronic. You must do this lab assignment on a class shark
machine.
To get your lab materials, click "Download Handout" on Autolab. Clone your repository on a Shark machine
by running:
git
3 Overview
Looking at the tsh.c ffle, you will see that it contains a skeleton of a simple Linux shell. It will not, of
course, function as a shell if you compile and run it now. To help you get started, we’ve provided you with a
helper ffle, tsh_helper.{c,h}, which contains the implementation of routines that manipulate a job list,
and a command line parser. Read the header ffle carefully to understand how to use it in your shell.
Your assignment is to complete the remaining empty functions listed below.
• eval: Main routine that parses, interprets, and executes the command line.
• sigchld_handler: Handles SIGCHLD signals.
• sigint_handler: Handles SIGINT signals (sent by Ctrl-C).
• sigtstp_handler: Handles SIGTSTP signals (sent by Ctrl-Z).
When you wish to test your shell, type make to recompile it. To run it, type tsh to the command line:
1linux> ./tsh
tsh> [type commands to your shell here]
4 General Guidelines for Writing Your Shell
This section provides an overview of how you can start writing your shell. You should read Section 4: The
tsh Speciffcation, for a list of everything your shell should support and the format of all shell output.
• 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.
Each command consists of one or more words, the ffrst of which is the name of an action to perform.
This may either be the path to an executable ffle (e.g., tsh> /bin/ls), or a built-in command—
a word with special meaning to the shell—(e.g., tsh> quit). Following this are command-line
arguments to be passed to the command.
• Built-in commands run within the shell’s process. Looking at the handout code, you may notice that
it’s difffcult to exit the program. Try making it respond to the word quit.
• So as not to corrupt its own state, the shell runs each executable in its own child process. You should
recall from lecture the sequence of three library calls necessary to create a new process, run a particular
executable, and wait for a child process to end. Try to make your shell correctly respond to /bin/ls,
without breaking the existing quit command. If this works, try passing ls a particular directory to
make sure your shell is passing the arguments along.
• The child processes created as a result of interpreting a single command line are known collectively as
a job. We just saw one type of job, a foreground job. However, sometimes a user wants to do more
than one thing at once: in this case, they can instruct the shell not to wait for a command to terminate by
instead running it as a background job. Looking back at the sequence of calls you made to implement
foreground jobs, what do you think you would do differently to spawn a background job?
• Given that your shell will need to support both types of job, consider refactoring your existing code to
minimize the amount of duplication that will be necessary.
• Try implementing the execution of background jobs, which your shell should do whenever the command
line ends with an & character. To test this feature, try executing tsh> /usr/bin/sleep 5 and
comparing against tsh> /usr/bin/sleep 5 &. In the latter case, the command prompt should
appear immediately after running the command. Now you can run multiple sleeps at once!
• When children of your shell die, they must be reaped within a bounded amount of time. This means
that you should not wait for a running foreground process to ffnish or for a user input to be entered
before reaping. The sigchld_handler might be a good place to reap all your child processes.
• The shell might want to track in-ffight jobs and provide an interface for switching their status i.e.
background to foreground, etc. Now might be a good time to read the api in tsh_helper.{c,h} and
start maintaining a job list.
• Typing Ctrl-C or Ctrl-Z causes a SIGINT or SIGTSTP signal, respectively. Your shell should catch
the signals and forward them to the entire process group that contains the foreground job. If there is no
foreground job, then these signals should have no effect.
2• When you run your shell from the standard Linux 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 created by your
shell. Obviously, this isn’t correct.
Here is a possible workaround: After the fork, but before the execve, you may want to think of a
way to put the child in a new process group whose group ID is identical to the child’s PID. This would
ensure that there will be only one process, your shell, in the foreground process group. Hint: man
setpgid.
1
• Remember that signal handlers run concurrently with the program and can interrupt it anywhere, unless
you explicitly block the receipt of the signals. Be very careful about race conditions on the job list. To
avoid race conditions, you should block any signals that might cause a signal handler to run any time
you access or modify the job list.
Aside from these guidelines, you should use the trace ffles to guide the development of your shell. The trace
ffles are in order of difffculty so it might not be the best to attempt a trace before passing all traces up to it.
5 The tsh Speciffcation
Your tsh shell should have the following features:
• Each job can be identiffed by either a process ID (PID) or a job ID (JID). The latter is a positive integer
assigned by tsh. JIDs are denoted on the command line with the preffx “%”. For example, “%5” denotes
a JID of 5, and “5” denotes a PID of 5.
• tsh should support the following built-in commands:
– The quit command terminates the shell.
– The jobs command lists all background jobs.
– The bg job command resumes 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.
– The fg job command resumes job by sending it a SIGCONT signal, and then runs it in the
foreground. The job argument can be either a PID or a JID.
• 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. When starting a background job, tsh should print
out the command line, prepended with the job ID and the process ID. For example:
[1] (**757) /bin/ls &
• Your shell should be able to handle SIGINT and SIGTSTP appropriately. If there is no foreground job,
then these signals should have no effect.
1With a real shell, the kernel will send SIGINT or SIGTSTP directly to each child process in the terminal foreground process
group. The shell manages the membership of this group using the tcsetpgrp function, and manages the attributes of the terminal
using the tcsetattr function. These functions are outside of the scope of the class, and you should not use them, as they will break
the autograding scheme.
3• tsh should reap all of its zombie children. If any job terminates or stops because it receives a signal
that it didn’t catch, then tsh should recognize that event and print a message with the job’s JID and
PID, and the offending signal number. For example,
Job [1] (1778) terminated by signal 2
Job [2] (1836) stopped by signal 20
• tsh should support I/O redirection (See Appendix C for more details). For example:
tsh> /bin/cat < foo > bar
Your shell must support both input and output redirection in the same command line.
• Your shell should be able to redirect the output from the built-in jobs command. For example,
tsh> jobs > foo
should write the output of jobs to the foo ffle. The reference shell supports output redirection for all
built-ins, but you are only required to implement it for jobs.
• Your shell does not need to support pipes.
6 Checking Your Work
Running your shell. The best way to check your work is to run your shell from the command line. Your
initial testing should be done manually from the command line. Run your shell, type commands to it, and see
if you can break it. Use it to run real programs!
Reference solution. The 64-bit Linux executable tshref is the reference solution for the shell. Run this
program (on a 64-bit machine) 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, which change from run
to run. (See the Evaluation section.)
Once you are conffdent that your shell is working, then you can begin to use some tools that we have provided
to help you check your work more thoroughly. These are the same tools that the autograder will use when
you submit your work for credit.
Trace interpreter. We have provided a set of trace ffles (trace*.txt) that validate the correctness of your
shell. Each trace ffle tests a different shell feature. For example, does your shell recognize a particular built-in
command? Does it respond correctly to the user typing a Ctrl-C?
The runtrace program (the trace interpreter) interprets a set of shell commands in a single trace ffle:
linux> ./runtrace -h
Usage: runtrace -f <file> -s <shellprog> [-hV]
Options:
-h Print this message
-s <shell> Shell program to test (default ./tsh)
-f <file> Trace file
-V Be more verbose
The neat thing about the trace ffles is that they generate the same output you would have gotten had you run
your shell interactively (except for an initial comment that identiffes the trace). For example:
4linux> ./runtrace -f trace05.txt -s ./tsh
#
# trace05.txt - Run a background job.
#
tsh> ./myspin1 &
[1] (15849) ./myspin1 &
tsh> quit
The lower-numbered trace ffles do very simple tests, while the higher-numbered trace ffles do increasingly
more complicated tests. The appendix contains a description of each of the trace ffles, as well as each of the
commands used in the trace ffles.
Please note that runtrace creates a temporary directory runtrace.tmp, which is used to store the output
of redirecting commands, and deletes it afterwards. However, if for some reason the directory is not deleted,
then runtrace will refuse to run. In this case, it may be necessary to delete this directory manually.
Shell driver. After you have used runtrace to test your shell on each trace ffle individually, then you are
ready to test your shell with the shell driver. The sdriver program uses runtrace to run your shell on each
trace ffle, compares its output to the output produced by the reference shell, and displays the diff if they
differ.
linux> ./sdriver -h
Usage: sdriver [-hV] [-s <shell> -t <tracenum> -i <iters>]
Options
 -h Print this message.
-i <iters> Run each trace <iters> times (default 4)
-s <shell> Name of test shell (default ./tsh)
-t <n> Run trace <n> only (default all)
-V Be more verbose.
Running the driver without any arguments tests your shell on all of the trace ffles. To help detect race
conditions in your code, the driver runs each trace multiple times. You will need to pass each of the runs to
get credit for a particular trace:
linux> ./sdriver
Running 3 iters of trace00.txt
1. Running trace00.txt...
2. Running trace00.txt...
3. Running trace00.txt...
Running 3 iters of trace01.txt
1. Running trace01.txt...
2. Running trace01.txt...
3. Running trace01.txt...
Running 3 iters of trace02.txt
1. Running trace02.txt...
2. Running trace02.txt...
3. Running trace02.txt...
...
Running 3 iters of trace31.txt
1. Running trace31.txt...
2. Running trace31.txt...
3. Running trace31.txt...
5Running 3 iters of trace**.txt
1. Running trace**.txt...
2. Running trace**.txt...
3. Running trace**.txt...
Summary: 33/33 correct traces
Use the optional -i argument to control the number of times the driver runs each trace file:
linux> ./sdriver -i 1
Running trace00.txt...
Running trace01.txt...
Running trace02.txt...
...
Running trace31.txt...
Running trace**.txt...
Summary: 33/33 correct traces
Use the optional -t argument to test a single trace file:
linux> ./sdriver -t 06
Running trace06.txt...
Success: The test and reference outputs for trace06.txt matched!
Use the optional -V argument to get more information about the test:
linux> ./sdriver -t 06 -V
Running trace06.txt...
Success: The test and reference outputs for trace06.txt matched!
Test output:
#
# trace06.txt - Run a foreground job and a background job.
#
tsh> ./myspin1 &
[1] (10276) ./myspin1 &
tsh> ./myspin2 1
Reference output:
#
# trace06.txt - Run a foreground job and a background job.
#
tsh> ./myspin1 &
[1] (10285) ./myspin1 &
tsh> ./myspin2 1
7 Hints
• Start early! Leave yourself plenty of time to debug your solution, as subtle problems in your shell are
hard to find and fix.
6• There are a lot of helpful code snippets in the textbook. It is OK to use them into your program, but
make sure you understand every line of code that you are using. Please do not build your shell on top
of code you do not understand!
• Read the manual pages for all system calls that you make. Be sure to understand what their arguments
and return/error values are.
• Signal Blocking and Unblocking. Child processes inherit the blocked vectors and handlers of their
parents, so the child must be sure to then unblock any signals before it execs the new program, and
also restore the default handlers for the signals that are ignored by the shell.
• Busy-waiting. It is forbidden to spin in a tight loop while waiting for a signal (e.g. “while (1);”).
Doing so is a waste of CPU cycles. Nor is it appropriate to get around this by calling sleep inside a
tight loop. Instead, you should use the sigsuspend function, which will sleep until a signal is received.
Refer to the textbook or lecture slides for more information.
• Reaping child processes. You should not call waitpid in multiple places. This will set you up for
many potential race conditions, and will make your shell needlessly complicated. The WUNTRACED and
WNOHANG options to waitpid will also be useful. Use man and your textbook to learn more about each
of these functions.
• Saving/restoring errno. Signal handlers should always properly save/restore the global variable
errno to ensure that it is not corrupted, as described in Section 8.5.5 of the textbook. The driver checks
for this explicitly, and it will print a warning if errno has been corrupted.
• Async-signal-safety. Many commonly used functions, including printf, are not async-signal-safe;
i.e., they should not be invoked from within signal handlers. Within your signal handlers, you must
ensure that you only call syscalls and library functions that are themselves async-signal-safe.
For the printf function specifically, the CS:APP library provides sio_printf as an async-signalsafe
replacement, which you may wish to use in your shell. (See Section 8.5.5 in the textbook for
information on async-signal-safety, and see the appendix for information about the functions provided
by the CS:APP library.)
• Error Handling. Your shell needs to handle error conditions appropriately, which depends on the error
being handled. For example, if malloc fails, then your shell might as well exit; on the other hand,
your shell should not exit just because the user entered an invalid filename. (See the section on style
grading.)
• Programs such as top, 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/cat, /bin/ls,
/bin/ps, and /bin/echo.
• Don’t use any system calls that manipulate terminal groups (e.g. tcsetpgrp), which will break the
autograder.
8 Evaluation
Your score will be computed out of a maximum of 103 points based on the following distribution:
799 Correctness: 33 trace files at 3 pts each. In addition, if your solution passes the traces but is not actually
correct (you hacked a way to get it to pass the traces, or there are race conditions), we will deduct
correctness points (up to 20 percent!) during our read through of your code.
The most common thing we will be looking for is race conditions that you have simply plastered over,
often using the sleep call. In general, your code should not have races, even if we remove all sleep
calls.
4 Style points. We expect you to follow the style guidelines posted on the course website. For example,
we expect you to check the return value of system calls and library functions, and handle any error
conditions appropriately (see Appendix B for exemptions).
We expect you to break up large functions such as eval into smaller helper functions, to enhance
readability and avoid duplicating code. We also expect you to write good comments. Some advice
about commenting:
• Do begin your program file with a descriptive block comment that describes your shell.
• Do begin each routine with a block comment describing its role at a high level.
• Do preface related lines of code with a block comment.
• Do keep your lines within 80 characters.
• Don’t comment every single line of code.
You should also follow other guidelines of good style, such as using a consistent indenting style (don’t
mix spaces and tabs!), using descriptive variable names, and grouping logically related blocks of code
with whitespace.
Your solution shell will be tested for correctness on a 64-bit shark machine (the Autolab server), using the
same driver and trace files that were included in your handout 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 trace26.txt and trace27.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.
The driver deals with all of these subtleties when it checks for correctness.
9 Hand In Instructions
To receive a score, you will need to upload your submission to Autolab. The Autolab servers will run the
same driver program that is provided to you. There are two ways you can submit your code to Autolab.
1. Running the make command will generate a tar file, tshlab-handin.tar. You can upload this file to
the Autolab website.
2. If you are running on the Shark machines, you can submit from the command line by typing:
$ make submit
8Keep in mind the following:
• You may handin as often as you like until the due date. However, you will only be graded on the last
version you hand in.
• After you hand in, it takes a minute or two for the driver to run through multiple iterations of each trace
file.
• Do not assume your submission will succeed! You should ALWAYS check that you received the
expected score on Autolab. You can also check if there were any problems in the autograder output,
which you can see by clicking on your autograded score in blue.
• As with all our lab assignments, we’ll be using a sophisticated cheat checker. Please don’t copy another
student’s code. Start early, and if you get stuck, come see your instructors for help.
Good luck!
9Appendix A: Trace Files
The trace driver runs an instance of your shell in a child process and communicates with the shell interactively
in a way that mimics the behavior of a user. To test the behavior of your shell, the trace driver reads in
trace files that specify shell line commands that are actually sent to the shell, as well as a few special
synchronization commands that are interpreted by the driver when handling the shell process. The trace files
may also reference a number of shell test programs to perform various functions, and you may refer to the
code and comments of these test programs for more information.
The format of the trace files is as follows:
• The comment character is #. Everything to the right of it on a line is ignored.
• Each trace file is written so that the output from the shell shows exactly what the user typed. We do
this by using the /bin/echo program, which not only tests the shell’s ability to run programs, but also
shows what the user typed. For example:
/bin/echo -e tsh\076 ./myspin1 \046
Note: \076 is the octal representation of >, and \046 is the octal representation of &. These are special
shell metacharacters that need to be escaped in order to be passed to /bin/echo. This command will
echo the string tsh> ./myspin1 &.
• There are also a few special commands which are used to synchronize the job (your shell) and the
parent process (the driver) and to send Linux signals from the parent to the job. These are handled in
your shell by the wrapper functions in wrapper.c.
A wrapper is a function injected at link time around calls to a function. For instance, where your code
calls fork, the linker will replace this call with an invocation of __wrap_fork, which in turn calls the
real fork function. Some of those wrappers are configured to signal the driver and resume execution
only when signaled.
WAIT Wait for a sync signal from the job over its synchronizing UNIX domain socket.
SIGNAL Send a sync signal to the job over its synchronizing UNIX domain socket.
NEXT Read and print all responses from the shell until you see the next shell prompt.
This command is essential for synchronizing with the shell and mimics the way
people wait until they see the shell prompt until they type the next string. It also
automatically signals the shell when receiving a signal from the shell.
SIGINT Send a SIGINT signal to the job.
SIGTSTP Send a SIGTSTP signal to the job.
SHELLSYNC function
Sets
an environment to indicate that synchronization in function is enabled. Currently
supported values of function are: kill, get_job_pid, and waitpid. See
wrapper.c for details.
SHELLWAIT Wait for a wrapper in the shell to signal runtrace over the shell synchronizing
domain socket.
SHELLSIGNAL Tell the wrapper to resume execution over the shell synchronizing domain socket.
PID name fg/bg Calls the shell builtin command fg or bg, passing the PID of the process name.
10The following table describes what each trace file tests on your shell against the reference solution.
NOTE: this table is provided so that you can quickly get a high level picture about the testing traces. The
explanation here is over-simplified. To understand what exactly each trace file does, you need to read the
trace files.
trace00.txt Properly terminate on EOF.
trace01.txt Process built-in quit command.
trace02.txt Run a foreground job that prints an environment variable.
trace03.txt Run a synchronizing foreground job without any arguments.
trace04.txt Run a foreground job with arguments.
trace05.txt Run a background job.
trace06.txt Run a foreground job and a background job.
trace07.txt Use the jobs built-in command.
trace08.txt Check that the shell can correctly handle reaping multiple process
trace09.txt Send fatal SIGINT to foreground job.
trace10.txt Send SIGTSTP to foreground job.
trace11.txt Send fatal SIGTERM (15) to a background job.
trace12.txt Child sends SIGINT to itself.
trace13.txt Child sends SIGTSTP to itself.
trace14.txt Run a background job that kills itself
trace15.txt Forward SIGINT to foreground job only.
trace16.txt Forward SIGTSTP to foreground job only.
trace17.txt Forward SIGINT to every process in foreground process group.
trace18.txt Forward SIGTSTP to every process in foreground process group.
trace19.txt Exit the child in the middle of sigint/sigtsp handler
trace20.txt Signal a job right after it has been reaped.
trace21.txt Forward signal to process with surprising signal handlers.
trace22.txt Process bg built-in command (one job).
trace23.txt Process bg built-in command (two jobs).
trace24.txt Check that the fg command waits for the program to finish.
trace25.txt Process fg builtin command (many jobs, with PID and JID, test error message)
trace26.txt Signal and end a background job in the middle of a fg command
trace27.txt Restart every stopped process in process group.
trace28.txt I/O redirection (input).
trace29.txt I/O redirection (output)
trace30.txt I/O redirection (input and output).
trace31.txt I/O redirection (input and output, different order, permissions)
trace**.txt Error handling
11Appendix B: CS:APP library and Error handling
B.1 CS:APP library
The csapp.c provides the SIO series of functions, which are async-signal-safe functions you can use to print
output. This code will be linked with your code, and so you can make use of any of these functions. The
main function of interest is the sio_printf function that you can use to print formatted output, which you
can use the same way you use the printf function. However, it only implements a subset of the format
strings, which are as follows:
• Integer formats: %d, %i, %u, %x, %o, with optional size specifiers l or z
• Other formats: %c, %s, %%, %p
For this lab, we have removed the sio_puts and sio_putl functions that are used in the textbook. Instead,
we encourage you to use the sio_printf family of functions for async-signal-safe I/O, which should help
you write more readable code.
B.2 Error handling
Using wrapper functions to handle errors can be useful. However, in systems programming, abruptly exiting
the program is rarely the right way to handle errors. For example, even if your shell is unable to start new
processes, it should still continue to run so that the user’s existing background jobs can be managed.
For this reason, we have removed all of the “Stevens-style wrapper functions” used in the CS:APP textbook.
While you are welcome to write your own, we strongly discourage doing so, as opposed to thinking carefully
about how to handle each error on an individual basis.
We expect you to check for and appropriately handle errors for any system calls or library functions that you
invoke. However, you do not need to check for error for the following calls (you can assume they always
succeed):
getpgid, getpid, getppid, sigaddset, sigdelset, sigemptyset, sigfillset,
sigismember, sigprocmask, setpgid, sigsuspend
B.3 errno
System calls and library functions generally indicate the presence of an error by their return value. For
example, fork() returns -1 on error, and malloc() returns NULL on error.
However, many of these functions can also return information about the type of error that was encountered
through the “global variable” errno (see man errno for more information). The types of errors that a
function can return are documented in its man page. For instance, man fork shows that ENOMEM is one of
the errors that can be returned by fork().
When handling errors, you should use the perror or strerror functions, which provide user-readable
strings for errno values.
12Appendix C: Unix I/O Redirection
The conventional Unix shell accepts inputs provided from a keyboard and displays outputs to the terminal
window. In particular, we refer to the keyboard input as stdin and the output to the terminal window as
stdout. In many cases we may wish to alter the source of the input or output of our commands. This can be
done through I/O redirection.
Standard Output
By default, a Unix shell will display output content to the terminal as defined by stdout. In the event that
we want to change the output destination, we can redirect stdout to another location such as a file using the
“>” character. For example:
tsh> ls > dir.txt
should write the output of the ls command to the file dir.txt.
Standard Input
Similarly, a Unix shell will read input content from the keyboard as defined by stdin. In the event that we
want to change the input source, we can redirect stdin from another location such as a file using the “<”
character. For example:
tsh> grep < foo.txt -i bar
should read the input of the file foo.txt into the grep command.
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