i can back up db using following command
mysql -u root -p123 db | bzip2 -c > /db.sql

and how can i use tar for backup ?

I use " tar -czvf $mydate-sqlback.tar.gz backup.sql " in my script that takes a backup of the mysql db's with a timestamp date (after doing a mysqldump flushing logs) and FTP's it to an offsite server.

and can i use this like above command

Sure just like you use the pipe command above " | " to join the two different commands or you can use separate lines.

mysql -u root -p123 db | bzip2 -c > /db.sql

is the same as

mysql -u root -p123 db
bzip2 -c /db.sql

now I could be wrong (its been awhile since I researched this can came up with the "best solution") but you more then likely want to do a mysqldump of your db then zip that up.

Here is the simple version of a script that I use to do full backups then FTP them to another server (which you dont' have to do). Your account also has to have a relatively updated version of LFTP on it since the older ones are problematic.

I also have a few things being checked here that you don't have to but I found that it eliminated most of the script errors that happened on occasion.

#! /bin/bash
mydate=$(date +%m%d%Y%I%M%p)
PATH=$PATH:/usr/local/bin
export PATH

/usr/bin/mysqldump --flush-logs --opt -u USERNAME -pPASSWORD DBNAME > /dir1/dir2/backup.sql
cd /dir1/dir2/

if [ -e backup.sql ]
then
tar -czvf $mydate-sqlback.tar.gz backup.sql
fi

then
lftp <<EOF
open FTP SERVER
user USERNAME PASSWORD
pwd
put $mydate-sqlback.tar.gz
bye
EOF
fi
if [ -e backup.sql ]
then rm /dir1/dir2/backup.sql