27.6.2 Initializing the Shell

When a shell program that normally performs job control is started, it has to be careful in case it has been invoked from another shell that is already doing its own job control.

A subshell that runs interactively has to ensure that it has been placed in the foreground by its parent shell before it can enable job control itself. It does this by getting its initial process group ID with the getpgrp function, and comparing it to the process group ID of the current foreground job associated with its controlling terminal (which can be retrieved using the tcgetpgrp function).

If the subshell is not running as a foreground job, it must stop itself by sending a SIGTTIN signal to its own process group. It may not arbitrarily put itself into the foreground; it must wait for the user to tell the parent shell to do this. If the subshell is continued again, it should repeat the check and stop itself again if it is still not in the foreground.

Once the subshell has been placed into the foreground by its parent shell, it can enable its own job control. It does this by calling setpgid to put itself into its own process group, and then calling tcsetpgrp to place this process group into the foreground.

When a shell enables job control, it should set itself to ignore all the job control stop signals so that it doesn't accidentally stop itself. You can do this by setting the action for all the stop signals to SIG_IGN.

A subshell that runs non-interactively cannot and should not support job control. It must leave all processes it creates in the same process group as the shell itself; this allows the non-interactive shell and its child processes to be treated as a single job by the parent shell. This is easy to do—just don't use any of the job control primitives—but you must remember to make the shell do it.

Here is the initialization code for the sample shell that shows how to do all of this.

     /* Keep track of attributes of the shell.  */
     
     #include <sys/types.h>
     #include <termios.h>
     #include <unistd.h>
     
     pid_t shell_pgid;
     struct termios shell_tmodes;
     int shell_terminal;
     int shell_is_interactive;
     
     
     /* Make sure the shell is running interactively as the foreground job
        before proceeding. */
     
     void
     init_shell ()
     {
     
       /* See if we are running interactively.  */
       shell_terminal = STDIN_FILENO;
       shell_is_interactive = isatty (shell_terminal);
     
       if (shell_is_interactive)
         {
           /* Loop until we are in the foreground.  */
           while (tcgetpgrp (shell_terminal) != (shell_pgid = getpgrp ()))
             kill (- shell_pgid, SIGTTIN);
     
           /* Ignore interactive and job-control signals.  */
           signal (SIGINT, SIG_IGN);
           signal (SIGQUIT, SIG_IGN);
           signal (SIGTSTP, SIG_IGN);
           signal (SIGTTIN, SIG_IGN);
           signal (SIGTTOU, SIG_IGN);
           signal (SIGCHLD, SIG_IGN);
     
           /* Put ourselves in our own process group.  */
           shell_pgid = getpid ();
           if (setpgid (shell_pgid, shell_pgid) < 0)
             {
               perror ("Couldn't put the shell in its own process group");
               exit (1);
             }
     
           /* Grab control of the terminal.  */
           tcsetpgrp (shell_terminal, shell_pgid);
     
           /* Save default terminal attributes for shell.  */
           tcgetattr (shell_terminal, &shell_tmodes);
         }
     }