Assignment 5: Unix Shell


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. 


As usual, you may work in a group of up to two people in solving the problems for this assignment.  Any clarifications and revisions to the assignment will be posted to the Blackboard discussion forum. 


For the "trivia" assignment:  12:00pm, Thursday, April 5
For the main assignment:  11:59pm, Monday, April 16. 

Hand Out Instructions

Copy the file /u/cs252/labs18/shlab/shlab-handout.tar to the protected 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 team member names and CSUG IDs 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.  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 (more harshly) ctrl-C to kill your shell. 

General Overview of Unix Shells

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"
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 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. 

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 separated 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 foreground job, as well as any descendents of that job (e.g., any child processes that it forked).  If there is no foreground job (i.e., tsh is in the foreground), then the signal should have no effect. 
  • 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 small 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 <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. 
    • The 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. 

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 identical to the reference solution (modulo 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 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> ./ -h
      Usage: [-hv] -t <trace> -s <shellprog> -a <args>
        -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> ./ -t trace01.txt -s ./tsh -a "-p"
    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> ./ -t trace01.txt -s ./tshref -a "-p"
    unix> make rtest01
And you can run all the tests on the reference shell by typing
    unix> make rtests
For your reference, tshref.out gives the output of the reference solution on all 16 traces.  This might be more convenient for you than running the shell driver. 

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
      ./ -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


  • Read Chapter 8 (Exceptional Control Flow) in the textbook carefully. 
  • 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 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. 
  • 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 /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. 
  • 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 solution:  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.  If the child is to run in the foreground, it should then call tcsetpgrp(0, my_pid).  This ensures that when you type ctrl-C, the resulting SIGINT goes either to your shell or to the appropriate foreground job (more precisely, the process group that contains the foreground job), as appropriate.  Note that you will need to call tcsetpgrp again after reaping a foreground job.  If you read the man page, you'll notice that a process gets a SIGTTOU signal whenever it calls tcsetpgrp from the background; to avoid this, you will need to call signal once in your shell, during startup, to explicitly ignore SIGTTOU. 
  • You may notice that tcsetpgrp fails when testing your shell with the given test script.  This is a known problem with the script.  It should not, however, prevent you from correctly running any of the trace*.txt files, as they take no user input.  Your shell must, however, display correct behavior on programs, both foreground and background, that do take user input—including correct handling of ctrl-C and ctrl-Z events entered at the keyboard.  We will be testing these behaviors manually.  If you are not sure what the “correct” behavior ought to be, try the same operations in your favorite Linux shell. 


Your score will be computed out of a maximum of 90 points based on the following distribution: 

  • [80] Correctness:  16 trace files at 5 points each.
  • [10] Style points.  We expect you to have good comments (5 pts) and to check the return value of every system call (5 pts).
Your solution shell will be tested for correctness on a CSUG Linux machine (likely cycle1), 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. 
We reserve the right to run additional checks if we suspect that your program generates the “right” output for the wrong reason. 

“Trivia” Assignment

Trivia assignment is available on Blackboard as a test. Below is the exact same five questions, just for reference purposes.

  1. Are you working alone or in a team of two?  If the latter, who is your partner? 
  2. What output do you get when you run the “trace driver” on the “reference shell”?  Use trace02.txt as the trace file. 
  3. What is the difference between the fork and execve system calls? 
  4. How are command line arguments retrieved in a program (in C)?  Name the funtion(s) or variable(s) used. 
  5. Explain the meaning and purpose of the WNOHANG and WUNTRACED options to the waitpid system call. 

Turn In Instructions

The main assignment will be submitted using the script /u/cs252/bin/TURN_IN.  From the directory you wish to turn in type: 

    /u/cs252/bin/TURN_IN .
It will ask you for your partner's netid. If you are working alone, simply press enter. Otherwise, put your partner's netid, and press enter. Note that there's a dot in that command! Watch the Blackboard discussion forum for details, and for any clarifications or revisions to the assignment. 

Before running the TURN_IN script, be sure that you have

  • Included your full name and email address in the comment at the top of tsh.c. 
  • Removed any extraneous print statements. 
  • Included any appropriate commentary on your code in a separate README file or as C comments in tch.c.