Shell_exec is not working

I am trying to execute a command with shell_exec but this command does not work, other commands work

<?php
    $output = shell_exec("tail /var/log/syslog");
    echo "<pre>$output</pre>";
    ?>

Please execute and post the error from your PHP error.log file.

When we code in PHP, we always run a tail on the PHP error.log file so we can see errors as the occur.

[Fri Sep 20 16:29:05.960464 2019] [mpm_prefork:notice] [pid 1090] AH00163: Apache/2.4.29 (Ubuntu) configured -- resuming normal operations
[Fri Sep 20 16:29:05.960506 2019] [core:notice] [pid 1090] AH00094: Command line: '/usr/sbin/apache2'
tail: cannot open '/var/log/syslog' for reading: Permission denied

So...?

So, it pretty obvious that since your core PHP process cannot read /var/log/syslog ; hence the basic problem is not shell_exec , the problem is that this file is not readable by your LAMP server process.

Please execute these commands:

ls -l /var/log/syslog

and also

ps aux | grep -i apache | tail -10

... and cut-and-paste the results (using our code tags) into your reply.