Explain who command's ':0' value in 4th column's output

Question

I have a graphical machine of rhel7, when i run who command , one of the outputs in 2nd column is ':0'. What does ':0' mean ?

Answer 1

If you read the man page for who it tells you that most of the documentation is available as a Textinfo manual and that:

info coreutils 'who invocation'

will display it. An extract of that documentation is:

If given no non-option arguments, 'who' prints the following information for each user currently logged on: login name, terminal line, login time, and remote hostname or X display.

Therefore, the :0 is the X display in use.

Answer 2

It is the value used by X Windows system to direct the display traffic in your machine. It has the following format:

hostname:D.S

empty hostname is localhost. D is the sequence number and 0 means only one display in your system. There could also be S number which is the screen number. A display can have more than one screen. But usually there's only one screen and its number is 0 (the default). So in your case the traffic is directed to local host and the only display you have.