PHP - Blocking File Read

Question

I have a file that is getting added to remotely (file.txt). From SSH, I can call tail -f file.txt which will display the updated contents of the file. I'd like to be able to do a blocking call to this file that will return the last appended line. A pooling loop simply isn't an option. Here's what I'd like:

$cmd = "tail -f file.txt";
$str = exec($cmd);

The problem with this code is that tail will never return. Is there any kind of wrapper function for tail that will kill it when once it has returned content? Is there a better way to do this in a low overhead way?

Answer 1

tail -n 1 file.txt will always return you the last line in the file, but I'm almost sure what you want instead is for PHP to know when file.txt has a new line, and display it, all without polling in a loop.

You will need a long running process anyway if it will check for new content, be it with a polling loop that checks for file modification time and compares to the last modification time saved somewhere else, or any other way.

You can even have php be run via cron to do the check if you don't want it running in a php loop (probably best), or via a shell script that does the loop and calls the php file if you need sub 1-minute runs that are cron's limit.

Another idea, though I haven't tried it, would be to open the file in a non-blocking stream and then use the quite efficient stream_select on it to have the system poll for changes.

Answer 2

The only solution I've found is somewhat dirty:

<?php
$descriptorspec = array(
   0 => array("pipe", "r"),  // stdin
   1 => array("pipe", "w"),  // stdout
   2 => array("pipe", "w")   // stderr
);

$process = proc_open('tail -f -n 0 /tmp/file.txt',$descriptorspec,$pipes);
fclose($pipes[0]);
stream_set_blocking($pipes[1],1);
$read = fgets($pipes[1]);
fclose($pipes[1]);
fclose($pipes[2]);
//if I try to call proc_close($process); here, it fails / hangs untill a second line is
//passed to the file. Hence an inelegant kill in the next 2 line:
$status = proc_get_status($process);
exec('kill '.$status['pid']);
proc_close($process);
echo $read;