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Just a tech.

How does job control work?

Do you often use multiple programs at a single terminal? For instance, I typically stop my text editor with Ctrl+Z to return to the shell and compile my project. If compilation fails, I can go back to my text editor with fg to make some changes. Perhaps you also use bg to continue a command in the background, leaving the shell in the foreground. Many shell scripts append & at the end of a command to start it in the background.

Job control is the mechanism behind all of these use-cases.

But what is job control?

Each time I start a command from my shell, a job is created. For instance, if I type ls | grep asdf, the shell creates one job with two processes inside: ls and grep asdf.

As a shell user, I want to manipulate jobs, not processes. If I type Ctrl+C during a job’s execution, I want all processes belonging to the job to receive a SIGINT signal to ask them to exit. If I type Ctrl+Z, I want all processes to get stopped.

Also, when I have multiple jobs running at the same time, I want my key presses to go to the foreground job. If I have dd running in the background and my text editor running in the foreground, I want to type in my text editor. The job which will receive input events has exclusive access to my terminal.

In short, job control solves two problems:

  • Multiple processes need to be manipulated at once (jobs)
  • The shell needs to decide which job has exclusive access to the terminal (the foreground job)

The shell’s toolbelt

To implement job control, the shell has several tools at its disposal:

  • waitpid: this function can collect child processes’ status. From this, the shell can figure out whether a child has terminated or has been stopped.
  • We want all processes in a job to receive SIGINT when Ctrl+C is pressed. To achieve this, we need to put all processes belonging to a job in the same process group. setpgid can be used for this.
  • We want a single job to have exclusive access to the terminal. tcsetpgrp allows the shell to tell which process group has control over the terminal.
  • Processes can change some attributes of the terminal, for instance whether characters typed in the terminal are echoed. tcgetattr can be used to get the current attributes and tcsetattr can be used to set them.

Putting it all together

Starting a job

When starting a job, the shell first forks and then execs each program. Error handling left out for brevity:

void launch_process(char *argv[]) {
	pid_t pid = fork();
	if (pid == 0) {
		// This is the child
		execvp(argv[0], argv);
	}
	// This is the parent
}

We’ll need to do two things: put the child process in its own process group and give it control over the terminal. We only want to do the latter if we’re starting a foreground job (e.g. not when running ls &).

To create a new process group, we can set the process group of a process to its own PID.

void launch_process(char *argv[], bool foreground) {
	pid_t pid = fork();
	if (pid == 0) {
		// Create a new process group
		pid_t pgid = getpid();
		setpgid(getpid(), pgid);
		// Give control over the terminal
		if (foreground) {
			tcsetpgrp(STDIN_FILENO, pgid);
		}

		execvp(argv[0], argv);
	}

	// Create a new process group
	pid_t pgid = pid;
	setpgid(pid, pgid);
	// Give control over the terminal
	if (foreground) {
		tcsetpgrp(STDIN_FILENO, pgid);
	}
}

Note that we call setpgid and tcsetpgrp both in the parent and in the child with the same parameters: this prevents race conditions where the parent thinks the process group has been created but the child hasn’t done so already.

However there is one issue with the launch_process function: if the shell executes ls | grep asdf, this will start two processes, but each will be in its own process group. We want both processes to share the same process group. When running ls, we need to create a new process group, but when running grep we want to put the new child process into ls’s process group. We’ll need to call setpgid(grep_pid, ls_pgid).

To fix this issue, let’s change the function to take a pointer to the process group ID. The first time we call launch_process, *pgid will be zero and it’ll get updated with the newly created process group ID. Next time it gets called, the new process will be put in the existing process group.

void launch_process(char *argv[], pid_t *pgid, bool foreground) {
	pid_t pid = fork();
	if (pid == 0) {
		if (*pgid == 0) {
			*pgid = getpid();
		}
		setpgid(getpid(), *pgid);
		if (foreground) {
			tcsetpgrp(STDIN_FILENO, *pgid);
		}
		execvp(argv[0], argv);
	}

	if (*pgid == 0) {
		// This is the first process of the job, create a new process group
		*pgid = pid;
	}
	setpgid(pid, *pgid);
	if (foreground) {
		tcsetpgrp(STDIN_FILENO, *pgid);
	}
}

Stopping a job

We get this one for free: when Ctrl+Z is pressed, the terminal will send SIGTSTP (terminal stop) to the process group controlling it. This will stop all processes in the job.

The shell still needs to figure out the job has been stopped. To do so, it can call waitpid with WUNTRACED and check the process’ status with WIFSTOPPED.

int status;
pid_t pid = waitpid(-1, &status, WUNTRACED);
bool stopped = WIFSTOPPED(status);

In this case, the shell needs to get back control over the terminal (with tcsetpgrp).

Because child processes may have changed the terminal attributes, the shell also needs to restore its own attributes (with tcsetattr). Right before doing that, it saves the current attributes (with tcgetattr): this will be useful if it needs to restore them when putting the job in the foreground again.

// Give back control to the shell
tcsetpgrp(STDIN_FILENO, shell_pgid);

// Save the job's terminal attributes
tcgetattr(STDIN_FILENO, &job_attrs);
// Restore the shell's terminal attributes
tcsetattr(STDIN_FILENO, TCSADRAIN, &shell_attrs);

Putting a job in the foreground/background

To continue a job in the background, we just need to send SIGCONT. We don’t need to give control over the terminal (only the foreground job has it).

To continue a job in the foreground, we need to send SIGCONT and give control over the terminal to the job’s process group. We also need to restore the job’s saved terminal attributes with a tcsetattr call.

Complete shell

That’s about it! With all these ingredients, we can bake a little shell.

The GNU libc job control manual describes how to write a complete shell with job control support. I’ve put together a runnable version if you’re interested.

When I was working on adding job control support to my shell mrsh, a pain point was to figure out how to plug all of this new logic into the shell’s existing interpreter.

Additionally, job control can be a little bit tricky to debug because there are no clear errors: when you do something wrong, things can continue to work as expected until they become completely messed up later. For instance if your shell starts randomly exiting for no reason, you probably gave control over the terminal to a terminated process.

I hope this helps shedding some light on this somewhat obscure Unix feature. Happy hacking!


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