27.6.4 Foreground and Background

Now let's consider what actions must be taken by the shell when it launches a job into the foreground, and how this differs from what must be done when a background job is launched.

When a foreground job is launched, the shell must first give it access to the controlling terminal by calling tcsetpgrp. Then, the shell should wait for processes in that process group to terminate or stop. This is discussed in more detail in Stopped and Terminated Jobs.

When all of the processes in the group have either completed or stopped, the shell should regain control of the terminal for its own process group by calling tcsetpgrp again. Since stop signals caused by I/O from a background process or a SUSP character typed by the user are sent to the process group, normally all the processes in the job stop together.

The foreground job may have left the terminal in a strange state, so the shell should restore its own saved terminal modes before continuing. In case the job is merely stopped, the shell should first save the current terminal modes so that it can restore them later if the job is continued. The functions for dealing with terminal modes are tcgetattr and tcsetattr; these are described in Terminal Modes.

Here is the sample shell's function for doing all of this.

     /* Put job j in the foreground.  If cont is nonzero,
        restore the saved terminal modes and send the process group a
        SIGCONT signal to wake it up before we block.  */
     
     void
     put_job_in_foreground (job *j, int cont)
     {
       /* Put the job into the foreground.  */
       tcsetpgrp (shell_terminal, j->pgid);
     
       /* Send the job a continue signal, if necessary.  */
       if (cont)
         {
           tcsetattr (shell_terminal, TCSADRAIN, &j->tmodes);
           if (kill (- j->pgid, SIGCONT) < 0)
             perror ("kill (SIGCONT)");
         }
     
       /* Wait for it to report.  */
       wait_for_job (j);
     
       /* Put the shell back in the foreground.  */
       tcsetpgrp (shell_terminal, shell_pgid);
     
       /* Restore the shell's terminal modes.  */
       tcgetattr (shell_terminal, &j->tmodes);
       tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes);
     }

If the process group is launched as a background job, the shell should remain in the foreground itself and continue to read commands from the terminal.

In the sample shell, there is not much that needs to be done to put a job into the background. Here is the function it uses:

     /* Put a job in the background.  If the cont argument is true, send
        the process group a SIGCONT signal to wake it up.  */
     
     void
     put_job_in_background (job *j, int cont)
     {
       /* Send the job a continue signal, if necessary.  */
       if (cont)
         if (kill (-j->pgid, SIGCONT) < 0)
           perror ("kill (SIGCONT)");
     }