How to set the program as daemon after $DISPLAY is set? - bash

Question

I want to set my screen as screensave status every 50minutes (3000 seconds).
cat /home/rest.sh
while true;do
sleep 3000
xscreensaver-command --lock 1>/dev/null
done
sh /home/rest.sh & can make it run.
Now i want to set it as a daemon.
sudo vim /etc/systemd/system/screensave.service
[Unit]
Description=screensave
[Service]
User=root
ExecStart=/bin/bash /home/rest.sh
StandardError=journal
[Install]
WantedBy=multi-user.target
To set it and enable as daemon.
systemctl enable screensave.service
I find that the service is not running as a daemon.
sudo journalctl -u screensave
Jan 24 12:16:50 user systemd[1]: Started screensave.
Jan 24 12:17:22 user bash[621]: xscreensaver-command: warning: $DISPLAY is not set: defaulting to ":0.0".
Jan 24 12:17:22 user bash[621]: No protocol specified
Jan 24 12:17:22 user bash[621]: xscreensaver-command: can't open display :0.0
How to run it as a daemon after $DISPLAY is set ?

Answer 1

This is a very common FAQ. A system daemon cannot easily connect to the X session of any individual user. On a multi-user system, how do you tell which user's session to connect to, anyway? On a single-user system, what should the daemon do if no session is running (as it often isn't at the time the daemon starts up)?
Trying to run a system daemon as any particular user won't work, and giving individual users access to a system daemon is a recipe for security problems. It can be done, but the solution is complex, and probably not something you want to attempt on your own. (Briefly, have the daemon listen to commands on a socket; create a user-space program which knows how to talk to the socket, and build some sort of authorization and authentication so the daemon knows whom it's talking to and can verify that this user is allowed to connect to this display.)
The drop-dead simple solution is to run this from your desktop environment's startup scripts instead. Most desktops have something like "session start-up items" or "autorun on login" hooks.

Answer 2

I'm not running linux and can't check now but the steps to daemonize a process are to close stdin stdout stderr change current working directory to / and to fork twice and setsid so that current process is a new session leader.
adding something like this at the beginning, before running, first thing to check is exec command creates a new session leader process with ps -Cbash -o sid,pgid,pid,ppid,comm,args
# checking if current process is a session leader to avoid infinite call
if [[ $(ps -p $$ -osid=) != $$ ]]; then
( cd / ; exec setsid /bin/bash /home/rest.sh & ) </dev/null 1>&0 2>&0 &
exit
fi