Advanced backuping script

Advanced backuping script

I'm currently writing an advanced starting script for my minecraft server.
Everything works out as I want it too. But the problem is the backups. I
created a function for it. It works also great and does exactly what i
want it to do. But since the server has to be down during the backup and
the backup itself takes ages (I mean hours) I need to speed the backup up!
My current backup works like this:
tar -cjvf $BACKUP_PATH/$filename $MINECRAFT_PATH
As I said this takes ages!
So I had the idea to just make a tar file without compressing then start
the server and then compress the archive.
tar -cvf $BACKUP_PATH/$filename $MINECRAFT_PATH
START_SERVER
bzip2 -9 $BACKUP_PATH/$filename
rm $BACKUP_PATH/$filename
This should work faster. But then i had an idea. And this is the point
where i need you.
First I wanted to modify the actual backupping in a way that shows how
much is already backed up and an estimation how long it will approximately
take to finish. Therefor the output of tar would bes supressed, the
command would be run in the background and a (while) loop (that waits for
the command to finish) would calculate the percentage calculated and also
calculate an estimate of the remaining time.
To to this I would first make a call to du to get the size of all files
being compressed. This would be saved in a variable. the i would get the
current timestamp. Also saved in a variable. At this point the loop would
start. Then in the loop the percentage and the remaining time would be
calculated (simple...) and this would be printed in two lines that always
are updated. And this is my major problem. I would be very happy if
someone could help me with the loop!
Some basic code:
tar -cf $BACKUP_PATH/$filename $MINECRAFT_PATH &
backupsize=$(du -bs $MINECRAFT_PATH)
starttimestamp=$(date +%s)
# Here is the loop (how do I stay here while the tar command is still
running?
# And how do i update the two lines with the newly created numbers?
START_SERVER
bzip2 -9 $BACKUP_PATH/$filename
rm $BACKUP_PATH/$filename
If you are in the helpy mood you can help me getting this kind of thing
also to work for the bzip command. If it's not possible I will just leave
it! (bzip accepts the same flags as zip)