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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—just don't use any of the job control primitives—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 <sys/types.h> |
| #include <termios.h> |
| #include <unistd.h> |
| |
| 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) < 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, &shell_tmodes); |
| } |
| } |
| </pre> |
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