Ubuntu – Where is catalina_base and catalina_home located for tomcat6

Why are these two variables separated?

catalina.home points to the location of the common information.
catalina.base points to the directory where all the instance specific information are held.

So you have 1 home and can have more than 1 base.

When should they be separated? When should these two variables be the same?

If you have 1 tomcat you can set them to the same value but good practice would suggest you plan ahead and keep them separate: you never know if you need more than one tomcat. Ubuntu started following this way of thinking due to it being more logical: it makes it easier to run 2+ tomcats but does not disable running 1 tomcat where setting them both to the same value would require everyone to edit the base value. Makes more sense to keep them different.

What does this allow the administrator to do?

Allow for more than 1 tomcat to run at the same time where each tomcat instance can have their own apps.

What do you then place into each directory?

Home contains the binairy.
Base contains conf, logs, webapps, work and temp. 1 for every tomcat instance.

You'll have to download the tar file, decompress and move to a directory of your choice. You can choose a mirror here and get a different download link.

My steps:

$ wget http://ftp.unicamp.br/pub/apache/tomcat/tomcat-6/v6.0.53/bin/apache-tomcat-6.0.53.tar.gz
$ tar xvzf apache-tomcat-6.0.47.tar.gz
$ sudo mkdir /usr/local/tomcat
$ sudo mv apache-tomcat-6.0.47 /usr/local/tomcat

I'm almost sure that Tomcat 6 works in Java 6, so additional steps are required if your Ubuntu is using, for instance, Java 8.

As Java 6 is no longer distributed for Ubuntu, you'll need to download a legacy package and install manually, according to this answer.

$ chmod a+x jdk-6u45-linux-x64.bin
$ sudo ./jdk-6u45-linux-x64.bin
$ mv jdk1.6.0_45 java-6-oracle
$ sudo mv java-6-oracle /usr/lib/jvm

And now, you can switch java version using either update-alternatives or galternatives.

Additionally, modify your .bashrc file to set JAVA_HOME variable:

export JAVA_HOME=/usr/lib/jvm/java-6-oracle