PHP calling diff with two strings

Question

I'm able to call diff via exec() just fine with files, like so:

exec('diff -N -u '.escapeshellarg($oldfile).' '.escapeshellarg($newfile), $lines);

However, attempting to do this with arbitrary strings fails:

exec('diff -N -u <(echo '.escapeshellarg($oldstring).') <(echo '.escapeshellarg($newstring).')', $lines);

If I copy the command being run into bash, it works just fine. But when run in PHP I get no output. Even 2>&1 doesn't yield anything. Capturing the status code yields 1, which should indicate that diff found differences, but I still get 1 even in the case where $newstring === $oldstring.

So I'm not quite sure what's going on. I can only assume that, for some reason, exec doesn't like process substitutions? Any ideas?

exec runs the command with sh, which does not support process substitution. You'd need to run it with bash — that other guy
– that other guy, Commented Jan 21, 2018 at 18:39
@thatotherguy Good point, although exec('echo $SHELL'); yields /bin/bash. — Niet the Dark Absol
– Niet the Dark Absol, Commented Jan 21, 2018 at 18:53
@thatotherguy Interesting - I tried wrapping the whole thing in '/bin/bash -c '.escapeshellarg('diff ...') and suddenly it's working. That leaves more questions (like why $SHELL is /bin/bash) but at least it works! — Niet the Dark Absol
– Niet the Dark Absol, Commented Jan 21, 2018 at 18:56
$SHELL is not the currently running shell. It's the user's login shell, similar to how $EDITOR is the user's editor and $PAGER is their preferred pager. — that other guy
– that other guy, Commented Jan 21, 2018 at 20:16
@thatotherguy All righty, fair enough. I got it working thanks to your comment, so feel free to post as answer :) — Niet the Dark Absol
– Niet the Dark Absol, Commented Jan 21, 2018 at 20:27

that other guy · Accepted Answer · 2018-01-21 23:14:08Z

1

PHP's exec runs the command with /bin/sh, which does not support process substitution (even when sh is provided by bash).

You can instead run your command explicitly with bash -c.

Sadly, PHP has no convenience functions for safe and robust execv style execution, so the easiest way to do that to build your diff command and then escape the whole thing:

exec('bash -c ' . escapeshellarg('diff -N -u <(echo '.escapeshellarg($oldstring).') <(echo '.escapeshellarg($newstring).')'), $lines);

answered Jan 21, 2018 at 23:14

that other guy

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2 Comments

Niet the Dark Absol Over a year ago

Thank you. As it turns out, it doesn't like longer strings anyway (worked in testing, then tried with an 80Kb string and it died spectacularly XD) so I wound up going with temporary files and back to sh.

that other guy Over a year ago

Yep yep.. The limit is OS dependent but usually 128k-256k.

jzenlcuaj · Accepted Answer · 2018-01-21 18:53:47Z

0

What shell is being used? Make sure diff is in the $PATH, otherwise command will fail.

answered Jan 21, 2018 at 18:53

jzenlcuaj

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