backup of postgres scheduled with cron

Hello all!

I've a small bash script backup.sh for creating dumps on my Postgre db:

#!/bin/bash
time=`date '+%d'-'%m'-'%y'`
cd /home/swkm/services/test
pg_dump mydb > mydb_dump_$time.out

I've edited crontab and added a line:

00 4 * * * swkm /home/swkm/services/test/backup.sh

to execute the backup.sh as user swkm daily at 4 am.

The user swkm is the user I use to create backups manually. The script
itself is executed fine if run manually but run on cron scheduler I got an
mydb_dump_$time.out file empty (of 0 kb)

Do you have any idea about what's wrong?

Thanks
Sorin