[SOLVED] Why some binaries don't work with shell_exec?

Issue

I don't understand why, but I can't execute some binaries from a PHP page.

If I call this from a PHP page, I don't get any output:

<?php
echo shell_exec('/usr/bin/which ffmpeg');

If I execute it from CLI, it works:

$ sudo -u apache php -r 'echo shell_exec("/usr/bin/which ffmpeg");'
/usr/bin/ffmpeg

But if I try to call whereis instead of which from a PHP page, I do get an output although they are in the same directory:

<?php
echo shell_exec('/usr/bin/whereis ffmpeg');

I don't understand the logic behind...

---

I've verified the file permissions, and everything seems fine: -rwxr-xr-x. root:root, and there's is no ACL.

I'm using Fedora 28 (SELinux is set to permissive). These commands work on Debian and Ubuntu.

Solution

The which command will generate the output on STDOUT, when the program is found, but it will use STDERR when there is no such command.

$ strace which php
[...]
access("/usr/bin/php", R_OK)            = 0
fstat(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 0), ...}) = 0
write(1, "/usr/bin/php\n", 13) = 13
[...]

Notice the 1 at write(), which is the handler for STDOUT, but when the command is not found, a different handler is used:

$ strace which something_not_existing
[...]
stat("/usr/games/bin/something_not_existing", 0x7ffc20f046a0) = -1 ENOENT (No such file or directory)
write(2, "which: no something_not_existing"..., 150) = 150
[...]

Notice the 2 at write(), which is the handler for STDERR.

In the console, you will see both the output from STDOUT and the output from STDERR, but shell_exec() will only output/return the STDOUT stream.

Check http://www.php.net/shell_exec on how to capture the STDERR stream as well. You might even want to use other execution functions like proc_open() to control the STDIN/STDOUT/STDERR streams, depending on your requirement.

The whereis command will always write to STDOUT, thats because you see the output in php via shell_exec().

Answered By - Progman

Answer Checked By - Marilyn (WPSolving Volunteer)