Which disk did I boot from?

I have multiple hard drives, each containing various partitions, with various bootable OS X versions on them.

This Apple support link, despite the title, Determine which disk started up your Mac, just tells me which hard disk is selected for a normal boot:

1. On your Mac, choose Apple menu > System Preferences.

2. Click Startup Disk.

   Your selected startup disk is shown at the top of the preferences pane.

However, if I booted whilst holding the Option key and selected a different disk, the Startup Disk control panel doesn't reflect which disk I actually booted from.

Disk Utility doesn't appear to explicitly show this information either, although it seems to imply that the active system disk is the pre-selected disk - upon opening the application.

How can I quickly determine which disk my Mac booted from (via the UI or command line)?

From the menu bar, select About this Mac, as shown below.

You should then get a popup window showing the startup disk, as shown below.

You can also get the startup disk by entering the command given below.

diskutil info / | grep "Volume Name"

Example output is given below.

   Volume Name:              Marlin

The version of macOS can be found in the by using the command below.

sw_vers

Example output is given below.

ProductName:    Mac OS X
ProductVersion: 10.13.6
BuildVersion:   17G14042