Who

https://www.thegeekstuff.com/2009/03/4-ways-to-identify-who-is-logged-in-on-your-linux-system/

who command is used to get the list of the usernames who are currently logged in. Output of the who command contains the following columns: user name, tty number, date and time, machine address.

$ who
ramesh pts/0        2009-03-28 22:57 (dev-db-server)
jason  pts/1        2009-03-28 23:01 (dev-db-server)
john   pts/2        2009-03-28 23:04 (dev-db-server)

To get a list of all usernames that are currently logged in, use the following:

$ who | cut -d' ' -f1 | sort | uniq
john
jason
ramesh