Install Oracle 12c and NOT use pluggable databases

To check if database or listener is running on Linux, you could check if Oracle processes are running, for example ps aux | grep pmon(or grep lsnr for listener). Pmon is process monitor, process responsible for "housekeeping", like freeing buffer cache or some resources that are not in use anymore. It is always running if the instance is up ( but this process does not guarantee that you can connect, because the database could be started in mount mode, anyway it is a start of a troubleshooting ).

It seems that you didn't set ORACLE_SID, ORACLE_BASE, ORACLE_HOME and PATH variables. On link that you provided, environment variables are set at the runtime. You could permanently set them in your .bashrc or bash_profile file.

Add these lines to your .bashrc file(it is in your users home directory, for example /home/oracle/.bashrc).

ORACLE_SID=name_of_your_database

ORACLE_BASE=/oracle/base/

ORACLE_HOME=/oracle/base/dbhome_1

PATH=$PATH:$HOME/bin:$ORACLE_HOME/bin

export ORACLE_SID ORACLE_BASE ORACLE_HOME PATH

Reload new bash profile with command - source .bashrc and try again and read Oracle installation documentation for more detalis.

However, since Ubuntu is not supported OS for Oracle, there is no way to know for sure what the problem really is or was. If installation is for learning purpose, easier for you will be to install some supported OS in a VirtualBox for example, or to use Oracle prebuild VMs.

You'll want to prevent the OracleService<<SID>> from starting automatically. Assuming you used the default SID of ORCL, that would be OracleServiceORCL.

You'll likely also see a number of other services that start with Oracle that are for various other components (i.e. your listener, your MTS recovery service, etc.) You can set all those to not start automatically if you want. Given that those services tend to use many fewer resources, it's generally easiest to let them run to make it easier to start the database later on.