There are a few ways to determine what version of OSX you are on.
In the GUI, you can click the on Apple menu () at the top left of your screen, and choose About This Mac. The version of OS X will be printed underneath the large bold Mac OS X title. Clicking on the Version XYZ text will reveal the Build number.
If you’re writing a script though, you may wish to access this programmatically. To do this, type:
$ system_profiler SPSoftwareDataType Software: System Software Overview: System Version: macOS 10.12.6 (16G29) Kernel Version: Darwin 16.7.0 Boot Volume: Macintosh HD Boot Mode: Normal Computer Name: Srini’s MacBook Pro User Name: Srini Karlekar (skarlekar) Secure Virtual Memory: Enabled System Integrity Protection: Enabled Time since boot: 21 days 8:30 $