man shutdown at:
... The first argument may be a time string (which is usually "now"). Optionally, this may be followed by a wall message to be sent to all logged-in users before going down. ... Note that to specify a wall message you must specify a time argument, too. ...
Note this post is about the wall message to be broadcast to all the logged users.
Therefore using the following pattern as follows:
shutdown <time> ["something to share"]
<time> can be +1, ... +5, ... +15, therefore from the
+15 range - it as follows:
shutdown +1 ["something to share"] ... shutdown +5 ["something to share"] ... shutdown +15 ["something to share"]
I can confirm the other users that are logged receive - appears in their
tty - the broadcast message (including or not the custom message according the case). Therefore as the documentation indicates, the message is broadcast. Until here no reason to create this post.
Situation: the reason, I did realise - if
<time> is equals or greater than 16 minutes - used through either
hh:mm the message is not broadcast to the other logged users in their
tty. For example if
date command returns
00:10 then with
shutdown 00:26 ... this situation happens. Of course, if is used
shutdown 00:15 all work as expected.
- Why the wall message is not broadcast to all logged users if the time is equals or greater than 16 minutes? - Is it normal or is a bug?
Through either the
man shutdown or
shutdown --help theoretically the wall message must be always broadcast to all the logged users, it does not matter the range of time about "from now" +15, +20 etc ...
Note I have this situation in many Virtual Machines - for different hosts - for Ubuntu Server 20.04