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Perl Cookbook by Nathan Torkington, Tom Christiansen

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Sending a Signal

Problem

You want to send a signal to a process. This could be sent to your own process or to another on the same system. For instance, you caught SIGINT and want to pass it on to your children.

Solution

Use kill to send a signal by name or number to the process IDs listed in the remaining arguments:

kill  9     => $pid;                    # send $pid a signal 9
kill -1     => $pgrp;                   # send whole job a signal 1
kill  USR1  => $$;                      # send myself a SIGUSR1
kill  HUP   => @pids;                   # send a SIGHUP to processes in @pids

Discussion

Perl’s kill function is an interface to the system call of the same name. The first argument is the signal to send, identified by number or by name; subsequent arguments are process IDs to send the signal to. It returns the count of processes successfully signaled. You can only send signals to processes running under the same real or saved UID as your real or effective UID—unless you’re the superuser.

If the signal number is negative, Perl interprets remaining arguments as process group IDs and sends that signal to all those groups’ processes using the killpg (2) system call.

A process group is essentially a job. It’s how the operating system ties related processes together. For example, when you use your shell to pipe one command into another, you’ve started two processes, but only one job. When you use Ctrl-C to interrupt the current job, or Ctrl-Z to suspend it, this sends the appropriate signals to the entire job, which may be more than one process.

kill can also check whether ...

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