How to programmatically find out the name of the user logged into the system on Mac OS, Windows, Linux?

Question

linux

Primarily interested in Mac OS

Answer 1

whoami?

Answer 2

macOS/Linux:echo "$USER"

MacBook-Pro-doublench:Debug doublench$ echo "$USER"
doublench

Answer 3

If the question is about Java, I guess the author wanted to see this
System.getProperty("user.name")

Answer 4

linux, macOS:
windows:
echo %USERNAME%