If you want to make a backup file of a specific mysql database via the ssh shell command line, for example if it's too large to backup via phpmyadmin tool. Here's the command to do that.
First off, you will need to be logged into your dedicated server or vps (virtual private server) via SSH. Once you are logged into the ssh command line terminal, issue this command as follows:
mysqldump db_name > db_backup.sql
db_name would be the name of the database for which you want to make the backup for. For example: john_xcartdb
db_backup.sql is the backup file name you wish to make the dump backup to. This will create the dump file in the present directory you are in. For example, if you just login to your server via ssh, and issue the dump command above. It will create this dump file in: /root/db_backup.sql