The mysqldump command is used to create textfile “dumps” of databases managed by MySQL. These dumps are just files with all the SQL commands needed to recreate the database from scratch.
If you want to back up a single database, you merely create the dump and send the output into a file, like so:
mysqldump database_name > database_name.sql
Multiple databases can be backed up at the same time:
mysqldump --databases database_one database_two > two_databases.sql
It is also simple to back up all of the databases on a server:
mysqldump --all-databases > all_databases.sql
Restoring a Backup
Since the dump files are just SQL commands, you can restore the database backup by telling mysql to run the commands in it and put the data into the proper database.
mysql database_name < database_name.sql
If you are trying to restore a single database from dump of all the databases, you have to let mysql know like this:
taken from: http://www.liquidweb.com/kb/how-to-back-up-mysql-databases-from-the-command-line/
mysql --one-database database_name < all_databases.sql