Sunday, March 13, 2005

Perl 5.8 and FindBin

It appears that there is a bug in Perl 5.8.6's FindBin.pm whereby things don't work so well if it doesn't have access to the current working directory.

I picked this up trying to debug MRTG. The program would run just fine from the command line, but not from /etc/crontab. After playing with cron's path and env(1) to try and figure out what was going on, I found the following error from cron:

Use of uninitialized value in string at /usr/local/bin/mrtg line 78.
Empty compile time value given to use lib at /usr/local/bin/mrtg line 78
Use of uninitialized value in concatenation (.) or string at /usr/local/bin/mrtg line 79.
Use of uninitialized value in concatenation (.) or string at /usr/local/bin/mrtg line 703.
Use of uninitialized value in concatenation (.) or string at /usr/local/bin/mrtg line 703.
ERROR: Can't Execute '/rateup'

Looking on line 78 and 79 of the MRTG code I found:

use FindBin;
use lib "${FindBin::Bin}";

suggesting the problem was related to the FindBin Perl module.

Some googling lead me to a similar bug report for RedHat Linux. Their suggested fix was a patch to FindBin.pm.

Since I'm not wildly keen on altering bits of my Perl installation (I forsee problems later when I try and upgrade and have forgotten why I did it), I decided to try for a simpler, less invasive solution. Since it appears that the problem is related to permissions on the current working directory, and I'm not sure what cron's current working directory is by default, I decided to try setting it explicitly. So I changed my cron entry from

*/5 * * * * stats /usr/local/bin/mrtg /usr/local/etc/mrtg/mrtg.cfg

to

*/5 * * * * stats cd /tmp; /usr/local/bin/mrtg /usr/local/etc/mrtg/mrtg.cfg

and lo and behold things started to work properly again.

Anyway, I just thought I'd share this simple solution in case anyone else bumps their head against the same problem.

posted by guy at: 12:21 SAST | path: /systems | permanent link