Assignment 4: stsh -- Stanford Shell

Kudos to Randal Bryant and Dave O'Hallaron of Carnegie Mellon for assignment inspiration and for parts of this handout. Huge thanks to Truman Cranor for writing the command line parser using tools you'll learn in CS143, which you should all take someday because it's amazing.

Due Date: Wednesday, February 12th at 11:59PM

Learning Goals

This assignment gives you a chance to implement a core tool that you've relied on all quarter, and in CS107 - a fully-functional shell! This assignment leverages the simpler shells from lecture and extends them to support process control, job lists, signals, pipelines, and I/O redirection--all while managing the interprocess concurrency problems that make a shell's implementation a genuinely advanced and fascinating systems programming project. There's lots of neat code to write, and we're confident you can pull it off. This assignment will help you:

Starter Code

All coding should be done on a myth cluster machine, as that's where we'll be testing all assign4 submissions. Clone the starter project using the command

    git clone /afs/ir/class/cs110/repos/assign4/$USER assign4

Note that this repo includes a soft link to a fully functional solution. When in doubt about how something should work, just run the solution to see what it does.

The starter project contains the following - there are a lot of files, but the main ones you should focus on are the header files. For the most part, you do not need to worry about how they are implemented, just how to use them:

Getting Started

Inspect the stsh.cc file we give you. This is the only file you should need to modify. The core main function you're provided looks like this:

int main(int argc, char *argv[]) {
  pid_t stshpid = getpid();
  installSignalHandlers();
  rlinit(argc, argv); // configures stsh-readline library so readline works properly
  while (true) {
   string line;
   if (!readline(line)) break;
   if (line.empty()) continue;
   try {
    pipeline p(line);
    bool builtin = handleBuiltin(p);
    if (!builtin) createJob(p); // createJob is initially defined as a wrapper around cout << p;
   } catch (const STSHException& e) {
    cerr << e.what() << endl;
    if (getpid() != stshpid) exit(0); // if exception is thrown from child process, kill it
   }
  }

  return 0;
}

For shells it's important to define the notion of a job. A job is a collection of 1 or more processes that execute as a result of a single shell command. For instance, ls | sort is a single shell command, and executing it creates a single job with two processes - one for ls, and another for sort. At any time, there can be at most one foreground job, and zero or more background jobs.

The general idiom here is the same used in the shell examples from lecture (first-shell.c, second-shell.c and third-shell.c), except that this version is in C++, and it'll support many more features. The readline function prompts the user to enter a command and a pipeline record is constructed around it. readline and pipeline (which is different from the pipeline function you implemented for Assignment 3) are implemented via a suite of files in the stsh-parser subdirectory, and for the most part you can ignore those implementations. You should, however, be familiar with the type definitions of the command and pipeline types:

const size_t kMaxCommandLength = 32;
const size_t kMaxArguments = 32;
struct command {
  char command[kMaxCommandLength + 1]; // '\0' terminated
  char *tokens[kMaxArguments + 1]; // NULL-terminated array, C strings are all '\0' terminated
};

struct pipeline {
  std::string input;  // empty if no input redirection file to first command
  std::string output; // empty if no output redirection file from last command
  std::vector<command> commands;
  bool background;

  pipeline(const std::string& str);
  ~pipeline();
};

Check out what the initial version of stsh is capable of before you add any new code.

Builtin stsh Commands

Your Assignment 4 shell needs to support a collection of builtin commands, many of which relate to manipulating jobs, that should execute without creating any new processes. These builtins are:

quit, exit, and jobs are already implemented for you. You're responsible for implementing the others and ensuring the global job list is appropriately updated.

Setting Milestones

The best approach to implementing anything this complex is to invent a collection of milestones that advance you toward your final goal. Never introduce more than a few lines of code before compiling and confirming that the lines you added do what you expect. To repeat: Never introduce more than a few lines of code before compiling, testing, and confirming that the additional lines do what you expect. View everything you add as a slight perturbation to a working system that slowly evolves into the final product. Understand every single line you add, why it's needed, and why it belongs where you put it.

Here is a sequence of milestones We'd like you to work through in order to get started:

  1. Descend into the stsh-parser directory, read through the stsh-readline.h and stsh-parse.h header files for data type definitions and function/method prototypes, type make, and play with stsh-parse-test to gain a sense of what readline and the pipeline constructor do for you. In general, the readline function is like getline, except that you can use your up and down arrows to scroll through your history of inputs (neat!). The pipeline record defines a bunch of fields that store all of the various commands that chain together to form a pipeline. For example, the text cat < /usr/include/stdio.h | wc > output.txt would be split into two commands--one for the cat and a second for the wc--and populate the vector<command> in the pipeline with information about each of them. The input and output fields would each be nonempty, and the background field would be false.
  2. Get a pipeline of just one command (e.g. stsh> sleep 5) to run in the foreground until it's finished. Rely on a call to waitpid to stall stsh until the foreground job finishes. Ignore the job list, ignore the SIGCHLD handler, don't worry about background jobs, pipelining, or redirection. Don't worry about programs like emacs just yet. Focus on these executables instead: ls, date, sleep, as their execution is well understood and predictable.
  3. Establish the process group ID of the job to be the PID of the process by investigating the setpgid system call. When you run stsh from the standard Unix shell, note that stsh is running in the foreground. If your shell then creates a child process, by default that child will also be a member of the foreground process group, and you don't want that. 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, and you don't want that. The notion of a group ID isn't all that important yet, because at the moment, a process group consists of only one process. But we'll eventually be dealing with jobs comprised of many processes, and we'll want a single process group id to represent all of them.
  4. Read through stsh-job-list.h, stsh-job.h, and stsh-process.h to learn how to add a new foreground job to the job list, and how to add a process to that job. Add code that does exactly that to the stsh.cc file, right after you successfully fork off a new process. After your waitpid call returns, remove the job from the job list. If it helps, inline cout << joblist; lines in strategically chosen locations to confirm your new job is being added after fork and being removed after waitpid.
  5. Implement the SIGCHLD handler to reap the resources of a foreground process after it exits, and suspend stsh's main thread of execution using a sigset_t, sigprocmask, sigsuspend, and related functions until an examination of the job list proves that the foreground job is no longer the foreground job. Your call to waitpid should be moved into the SIGCHLD handler, and that should be the only place in your entire solution--even the final one you submit--with a waitpid call.
  6. Install functions to activate ctrl-z and ctrl-c on your keyboard to stop and kill the foreground process instead of the shell itself. If, for instance, a sleep 500 is running as the foreground, you may want to kill the process by pressing ctrl-c. When you press ctrl-c, the OS sends a SIGINT to your shell, which unless handled will terminate the shell itself. If, however, you install a function to handle SIGINT, then that handler can (and should) forward the SIGINT on to the foreground job, should one exist. The story for ctrl-z is similar, except ctrl-z prompts the OS to send your shell a SIGTSTP. If you install a custom handler to intercept a SIGTSTP, you can forward the SIGTSTP on to the foreground job, should one exist. In these cases, the kill function is your friend and will contribute to your implementation of both of these handlers.
  7. Implement the fg builtin, which takes a stopped process--stopped presumably because it was running in the foreground at the time you pressed ctrl-z--and prompts it to continue, or it takes a process running in the background and brings it into the foreground. The fg builtin takes a job number, translates that job number to a process group ID, and, if necessary, forwards a SIGCONT on to the process group via a call to kill(-groupID, SIGCONT). Right now, process groups consist of just one process, but once you start to support pipelines (e.g. cat words.txt | sort | uniq | wc -l), you'll want fg to bring the entire job into the foreground, and if all relevant processes are part of the same process group, you can achieve this with a single kill call. Of course, if the argument passed to fg isn't a number, or it is but it doesn't identify a real job, then you should throw an STSHException that's wrapped around a clear error message saying so.
  8. Update the SIGCHLD handler to detect state changes in all processes under stsh's jurisdiction. Processes can exit gracefully, exit abnormally because of some signal, or be terminated by ctrl-c. Processes can halt because of a signal or a ctrl-z. And processes can continue because of a signal or an fg builtin. The job list needs to remain in sync with the state of your shell's world, and waitpid is the perfect function to tell you about changes. You're already familiar with WNOHANG, WUNTRACED, WIFEXITED, WEXITSTATUS, etc. Read through waitpid's man page (man 2 waitpid) to get word on some Linux-specific flags and macros that tell you about processes that have started executing again. Buzzwords include WCONTINUED and WIFCONTINUED, so read up on those.
  9. Add support for background jobs. The pipeline constructor already searches for trailing &'s and records whether or not the pipeline should be run in the background. If it does, then you still add information about the job to the job list, but you immediately come back to the shell prompt without waiting.
  10. Add support for slay, halt, cont (the process-oriented commands) and bg (which prompts all processes in a single job to continue in the background), and use some of the sample user programs we include in your repo (int, fpe, tstp, spin, split) to test all of these.

The following are additional milestones you need to hit on your way to a fully functional stsh.

Each of these bullet points represents something larger.

Shell Driver

The stsh-driver program in samples executes stsh as a child process, sends it commands and signals as directed by a trace file, and allows the shell to print to standard output and error as it normally would. The stsh process is driven by the stsh-driver, which is why we call stsh-driver a driver.

Go ahead and type ./stsh-driver -h to learn how to use it:

Usage: ./stsh-driver [-hv] -t <trace> -s <shell> [-a <args>]
Options:
  -h     Print this message
  -v     Output more information
  -t <trace> Trace file
  -s <shell> Version of stsh to test
  -a <args> Arguments to pass through to stsh implementation
myth62$

We've also provided several trace files that you can feed to the driver to test your stsh. If you drill into your repo's samples symlink, you'll arrive at /usr/class/cs110/samples/assign4, which includes not only a copy of our own stsh solution, but also a directory of shared trace files called scripts. Within scripts , you'll see simple, intermediate, and advanced subdirectories, each of which contains one or more trace files you can use for testing.

Run the shell driver on your own shell using trace file bg-spin.txt by typing this:

myth62$ ./stsh-driver -t ./samples/scripts/simple/bg-spin.txt -s ./stsh -a "--suppress-prompt --no-history”

(the -a "--suppress-prompt --no-history" argument tells stsh to not emit a prompt or to use the fancy readline history stuff, since it confuses the sanitycheck and autograder stuff).

Similarly, to compare your results with those generated by my own solution, you can run the driver on ./stsh_soln shell by typing:

myth62$ ./stsh-driver -t ./samples/scripts/simple/bg-spin.txt -s ./samples/stsh_soln -a "--suppress-prompt --no-history"

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 identifying the output as something generated via stsh-driver). For example:

myth62$ ./stsh-driver -t ./samples/scripts/advanced/simple-pipeline-1.txt -s ./samples/stsh_soln -a "--suppress-prompt --no-history"
# Trace: simple-pipeline-1
# ------------------------
# Exercises support for pipes via a foreground pipeline with
# just two processes.
stsh> /bin/echo abc | ./conduit --count 3
aaabbbcccdddeeefffggghhhiiijjj

The process IDs listed as part of a trace's output will be different from run to run, but otherwise your output should be exactly the same as that generated by my solution.

Tips and Tidbits

stsh> sleep 10 | sleep 10 | sleep 10 &
[1] 27684 27685 27686

Submitting And Grading

Once you're done, you should test all of your work as you normally would and then run the submit script by typing ./tools/submit.

Your assignments will be rigorously tested using the tests we expose via sanitycheck plus a whole set of others. I reserve the right to add tests and change point values if during grading I find some features aren't being exercised, but we're fairly certain the breakdown presented below will be a very good approximation regardless.

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