Through the man who appears the following for the -T option
-T, -w, --mesg add user's message status as +, - or ? According with some tutorials, if is executed who -T should appears these symbols.
In my case - appears for all the users logged through tty and + for all the users logged through ssh - nothing about ?
What do the - + ? symbols represent with the -T option?
The "message status" tells you whether the user accepts messages on that particular TTY.
Here, my shell session is running on /dev/pts/3:
$ tty /dev/pts/3 My message status on this TTY, as well as on the other TTYs that I'm using, is +, meaning I accept messages:
$ who -T kk + pts/0 2022-04-23 14:32 (192.168.1.107) kk + pts/1 2022-04-23 13:02 (tmux(2971).%0) kk + pts/2 2022-04-23 13:02 (tmux(2971).%1) kk + pts/3 2022-04-23 13:02 (tmux(2971).%2) kk + pts/4 2022-04-23 14:32 (tmux(2971).%3) I then turn off the ability for other users to send messages to me (using e.g. write; see man write and man mesg):
$ mesg n This affects the message status:
$ who -T kk + pts/0 2022-04-23 14:32 (192.168.1.107) kk + pts/1 2022-04-23 13:02 (tmux(2971).%0) kk + pts/2 2022-04-23 13:02 (tmux(2971).%1) kk - pts/3 2022-04-23 13:02 (tmux(2971).%2) kk + pts/4 2022-04-23 14:32 (tmux(2971).%3) If the message status is ?, this means the terminal device is unknown.
The following is from the GNU who info manual (info '(coreutils)who invocation'), relating to the -T option:
After each login name print a character indicating the user’s message status: ‘+’ allowing ‘write’ messages ‘-’ disallowing ‘write’ messages ‘?’ cannot find terminal device
The who utility is a POSIX utility, and in the POSIX standard, the -T option adds a terminal state with four possible values: +, -, ? and (space). The specification describes these like so:
+ The terminal allows write access to other users. - The terminal denies write access to other users. ? The terminal write-access state cannot be determined. <space> This entry is not associated with a terminal.
