Mail status-reports from daemon fails

Question

Hi all,
I'm using a daily backup-script «Backup-Bisang.sh» controlled by launchd:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>com.bisang.backup-daily</string>
<key>Program</key>
<string>/opt/local/sbin/Backup-Bisang.sh</string>
<key>ProgramArguments</key>
<array>
<string>/opt/local/sbin/Backup-Bisang.sh</string>
</array>
<key>ServiceDescription</key>
<string>Daily backup from server</string>
<key>StartCalendarInterval</key>
<dict>
<key>Hour</key>
<integer>16</integer>
<key>Minute</key>
<integer>25</integer>
</dict>
<key>UserName</key>
<string>ruedibis</string>
</dict>
</plist>
The script itself should email me a status-report:
rsync $MY_OPTIONS >> /tmp/backupscripttmpfile
Mailx -s "Backup Report `date "+%d.%m.%y %H:%M:%S"`" $MAILADDR </tmp/backupscripttmpfile
This works fine, if I test the script via terminal (all report data in the message body). If the script executes via launchd (as daemon), an email is sent, but with an empty body! Although the backupscripttmpfile is correct. What may cause this strange behavior?

Answer 1

If I pipe my tmp_file like this:
cat /tmp/backupscripttmpfile | Mailx -s "Backup Report `date "+%d.%m.%y %H:%M:%S"`" $MAILADDR
then it works fine. Why?