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<h4 class="subsection">27.6.2 Initializing the Shell</h4>

<p><a name="index-job-control_002c-enabling-3213"></a><a name="index-subshell-3214"></a>
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.

   <p>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
<code>getpgrp</code> 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 <code>tcgetpgrp</code> function).

   <p>If the subshell is not running as a foreground job, it must stop itself
by sending a <code>SIGTTIN</code> 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.

   <p><a name="index-job-control_002c-enabling-3215"></a>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
<code>setpgid</code> to put itself into its own process group, and then
calling <code>tcsetpgrp</code> to place this process group into the
foreground.

   <p>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
<code>SIG_IGN</code>.

   <p>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&mdash;just don't use any of the job control primitives&mdash;but
you must remember to make the shell do it.

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

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