[gentoo-server] backup and crontab problem

[Frank Nørvig]

Hi there,

I'm making a tape backup-script that looks like this:

#!/usr/bin/perl -w
WriteLog("Loading tape...");
   $exec = `mtx -f /dev/sg0 first 2>&1`;
   if ($exec) { WriteLog($exec); }

WriteLog("Rewinding tape...");
   $exec = `mt -f /dev/nst0 rewind 2>&1`;
   if ($exec) { WriteLog($exec); }

WriteLog("Backing up data...");
   $exec = `tar -c -p --exclude-from=/opt/backup/backup-exclude.txt -M 
-F /opt/backup/changetape.pl -f /dev/nst0 /var/data 2>&1`;
   if ($exec) { WriteLog($exec); }

... (rest is not important)

When running this script from bash it works like a charm. All data from 
/var/data is backed up onto tape and everyone is happy.

BUT when I put the script into my cronjob for automatic start in the 
night, it fails with following error:

[2004-09-21 05:43:58] Loading tape...
[2004-09-21 05:43:58] Rewinding tape...
[2004-09-21 05:44:58] Backing up data...
[2004-09-21 05:46:59] tar: /dev/nst0: Cannot open: Input/output error
tar: Error is not recoverable: exiting now

Why is this happening? And why is it only happening when run from the 
cronjob? Nothing else is run and no other program is using the 
tape-robot. The two tape-commands (mt and mtx) runs smoothly and loads + 
  rewinds the tape but the tar command fails everytime and has for over 
a month now. Every morning I have to run the script manually from bash 
and it runs perfectly then.

I'm not new to Linux (or Gentoo) and has been installing and maintaining 
servers for more than 4 years now but this problem is a killer :(

Frank Nørvig