I need some help with a `bash` script. 
The script "backup.sh" looks like this: 

```
  rm -f $(ls -t /backups/mongo/ | awk 'NR>14')
  rsync -pvztr  --progress /backups/*...........

```

I want to remove old files in /backups/mongo/ folder, keeping no more than 15 files. The script works fine if I run ./backup.sh, but if I make it run as a cronjob, the remove part (rm) does not work, the rests work. 

Can someone help me spot the problem?