Where is the MySQL JDBC jar file in Ubuntu?

First, apt-get install libmysql-java

Then mysql.jar is located in /usr/share/java/mysql.jar.

Third, in Eclipse, do this as: Project -> Properties -> Java Build Path -> Libraries -> Add External JARs -> select /usr/share/java/mysql.jar

In ubuntu 19.10 the following works:

apt-get install libmariadb-java

If you're using a version of Ubuntu prior to 19.04, you'll need to install libmysql-java, the MySQL JDBC driver, because the connector is not in the Ubuntu-packaged MySQL package that you've already installed. Versions after that do not publish that package.

Once it's installed, you'll have the file /usr/share/java/mysql.jar, which is an indirect symlink to the actual jar file.

Alternatively, install the mysql-apt-config package, update, and you'll be able to install all currently supported MySQL software with apt.