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There are several commands that you can use to find out what version of Ubuntu you are running. The lsb_release utility displays LSB (Linux Standard Base) information about the Linux distribution.

Use the lsb_release -a command to display the Ubuntu version.

$ lsb_release -a

Then the following output will display and its showing Ubuntu version as 18.04

No LSB modules are available.
Distributor ID:    Ubuntu
Description:    Ubuntu 18.04.5 LTS
Release:    18.04
Codename:    bionic

Instead of printing all of the above information, you can display the description line, which shows your Ubuntu version passing the -d switch.

$ lsb_release -d

Then you will see the output as below only displaying description.

Description:    Ubuntu 18.04.5 LTS