Question

I’m trying to invoke a long-running shell command inside a PHP CLI script with exec(). But I can’t for the life of me figure out how to interrupt the PHP script and kill the spawned child process(s). It seems like as soon as I call exec(), my signal handler is ignored. The following code works as I would expect; if I send SIGTERM to the process, it echoes SIGTERM and exits immediately.

<?php
  declare(ticks = 1);

  function sig_handler($signo) {
    switch ($signo) {
      case SIGTERM:
        echo 'SIGTERM' . PHP_EOL;
        flush();
        break;
      default:
    }
  }

  pcntl_signal(SIGTERM, 'sig_handler', false);

  sleep(60);
?>

However, if I replace sleep(60); with exec('sleep 60');, I don’t reach my signal handler until the sleep finishes. I have two questions:

1. How can I get signals to work with exec (or shell_exec or proc_open)?
2. Upon trapping a signal, how can I kill any child processes spawned by exec?

Answer 1

The documentation does say that:

If a program is started with this function, in order for it to continue running in the background, the output of the program must be redirected to a file or another output stream. Failing to do so will cause PHP to hang until the execution of the program ends.

(Emphasis mine.) I guess the hanging applies to signal handlers, too.

To be able to control the child process it looks like you have to execute them in the old-fashioned way, with fork+exec:

switch ($pid = pcntl_fork()) {
  case -1: // failed to create process
     die('Fork failed');
  case 0: // child
     pcntl_exec($path,$args);
     die('Exec failed');
}

Once the parent process has the child’s process id in $pid, it could send SIGINT to the child with posix_kill($pid, SIGINT).

Update: apparently you could also use proc_open and proc_terminate.