shell.md

title
Shell Lab

Build & Run

make && ./sdriver.pl -t trace15.txt -s ./tshref -a "-p -v"
make && ./sdriver.pl -t trace15.txt -s ./tsh    -a "-p -v"

trace*.txt

1. Properly terminate on EOF.

Do nothing.

2. Process builtin quit command.

int builtin_cmd(char **argv) {
    if (!strcmp(argv[0], "quit"))
        exit(0);
    /* other builtin commands */
    return 0;
}

3. Run a FG job.

⚠️ addjob() and deletejob() has more arguments than the textbook version.

void eval(char *cmdline) 
{
    char *argv[MAXARGS];
    int bg;
    pid_t pid;
    sigset_t mask_all, mask_one, prev_one;

    bg = parseline(cmdline, argv);
    if (argv[0] == NULL)
        return; /* ignore empty cmdline */

    if (builtin_cmd(argv)) 
        return;

    Sigfillset(&mask_all);
    /* Block SIGCHLD */
    Sigemptyset(&mask_one);
    Sigaddset(&mask_one, SIGCHLD);
    Sigprocmask(SIG_BLOCK, &mask_one, &prev_one);
    if ((pid = Fork()) == 0) { /* Child Process */
        Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
        Setpgid(0, 0); /* Put the child in a new process group whose ID
                           == the child's PID. */
        if (execve(argv[0], argv, environ) < 0) {
            printf("%s: Command not found\n", argv[0]);
            exit(1);
        }
    }
    Sigprocmask(SIG_BLOCK, &mask_all, NULL); /* Block all */
    addjob(jobs, pid, 1+bg, cmdline);
    Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* recover */
    if (!bg)
        waitfg(pid);
}

In waitfg(), use sigsuspend() to wait:

void waitfg(pid_t pid)
{
    sigset_t mask_all, prev_all;
    pid_t fg_pid;

    Sigfillset(&mask_all);

    while (1) {
        Sigprocmask(SIG_BLOCK, &mask_all, &prev_all);
        fg_pid = fgpid(jobs);
        if (fg_pid == pid) {
            Sigsuspend(&prev_all);
            Sigprocmask(SIG_SETMASK, &prev_all, NULL);
        }
        else {
            Sigprocmask(SIG_SETMASK, &prev_all, NULL);
            break;
        }
    }

    if (verbose) {
        printf("waitfg: Process (%d) no longer the fg process\n", pid);
        fflush(stdout);
    }
}

4. Run a BG job.

void eval(char *cmdline) {
    /* ... */
    Sigprocmask(SIG_BLOCK, &mask_all, NULL); /* Block all */
    addjob(jobs, pid, 1+bg, cmdline);
    if (bg) {
        printf("[%d] (%d) %s", pid2jid(pid), pid, cmdline);
        fflush(stdout);
    }
    Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* recover */
    if (!bg)
        waitfg(pid);
}

5. Process jobs builtin command.

int builtin_cmd(char **argv) {
    sigset_t mask_all, prev_all;
    Sigfillset(&mask_all);

    /* ... */
    if (!strcmp(argv[0], "jobs")) {
        Sigprocmask(SIG_BLOCK, &mask_all, &prev_all);
        listjobs(jobs);
        Sigprocmask(SIG_SETMASK, &prev_all, NULL);
        return 1;
    }
    /* ... */
}

6. Forward SIGINT to FG job.

void sigint_handler(int sig) {
    int olderrno = errno;
    sigset_t mask_all, prev_all;
    pid_t pid;
    char buf[MAXLINE]; int pos = 0; /* local buffer for sprintf() */

    if (verbose)
        Sio_puts("sigint_handler: entering\n");

    Sigfillset(&mask_all);
    Sigprocmask(SIG_BLOCK, &mask_all, &prev_all);
    pid = fgpid(jobs);
    Sigprocmask(SIG_SETMASK, &prev_all, NULL);
    Kill(pid, sig); /* to be improved in #11 */

    if (verbose) {
        pos += sprintf(buf + pos, "sigint_handler: Job (%d) killed\n", pid);
        pos += sprintf(buf + pos, "sigint_handler: exiting\n");
        Sio_puts(buf);
    }

    errno = olderrno;
}

7. Forward SIGINT only to FG job.

Do nothing.

8. Forward SIGTSTP only to FG job.

Similar to #6, just one more thing:

void sigtstp_handler(int sig) {
    /* ... */
    job->state = ST; /* to be improved in #16 */
    /* ... */
}

9. Process bg builtin command

10. Process fg builtin command

11. Forward SIGINT to every process in FG process group

Similar to #6, just one change:

Kill(-pid, sig); /* forward signal to FG process group (see #6) */

12. Forward SIGTSTP to every process in FG process group

13. Restart every stopped process in process group

14. Simple error handling

15. Putting it all together

Main test.

16. Process signals from other processes

Move job->state = ST; from sigtstp_handler() to sigchld_handler().