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How to use systemd to start a daemon only at a specific time. It seems that there is unexpected demand, but I could not find a related article, so I will write down trial and error as a memorandum.

Thing you want to do

I want to start the daemon (hereinafter referred to as Mr.daemon, also known as Daemon) only between 8:00 and 23:00. I want to stop other times to reduce power consumption. By the way, the daemon I want to operate is my own, and it keeps streaming audio using HLS with ffmpeg during startup. As an aside, when I investigated what is different between services and daemons, it seems that both are resident programs, services are used in Windows systems, and daemons are used in Unix systems.

The first method I came up with

I will write about the problems later, but for the time being.

Start-up

I think there are two main ways to start Daemon using systemd.

-** Automatic startup ** systemctl enable {Mr.daemon} -** Manual start ** systemctl start {Mr.daemon}

However, you cannot specify the time with these methods. With automatic startup, it always starts without any questions, and it goes without saying that manual startup is necessary. Therefore, I think that the following method can be used to specify the time.

-** Timer ** {Mr.daemon} .timer

cron 0 8 * * * systemctl start {Mr.daemon}

`systemd` timer

You can start Daemon at a specific time by using the timer function of systemd.

`bash:{Mr.daemon}.timer`


[Unit]
#Any description
Description=Run {Mr.daemon}.service

[Timer]
#Time you want to start
OnCalendar=*-*-* 8:00:00 #year-month-date hour:minute:second
#If the OS is sleeping at the time specified by OnCalender, the daemon will be started at the next startup.
Persistent=true 

[Install]
WantedBy=timers.target

After writing the unit file like this, enable the timer with sudo systemctl enable {Mr.daemon} .timer.

Administrator privileges `cron`

You can do the same with administrator-privileged cron by running systemctl start at a specified time.

sudo crontab -e
Add right 0 8 * * * systemctl start {Mr.daemon}

Stop

It seems that the systemd timer can start but cannot stop. So use cron anyway to stop it. Just add the following to the cron with administrator privileges.

0 23 * * * systemctl stop {Mr.daemon}

Problems with the above method

Well, when I implemented it by the above method, there were some inconveniences. That is, the daemon will not start after rebooting between 8:00 and 23:00. Of course, only the start and end triggers are set, so if you restart at 9 o'clock for maintenance, for example, Daemon will stay asleep until 8 o'clock the next day unless you start it manually. This is not enough for a daemon that runs only at specific times.

Then what should we do

I decided to play with the daemon startup script.

`stream.sh`


###Postscript part
NOW=`date +%H` #Get the current time
if [ $NOW -ge 23 -o $NOW -lt 8 ]; then #The current time is 23:00~If it's between 8 o'clock the next day
    systemctl stop {Mr.daemon} #Stop the daemon
    exit 0 #Successful completion
fi
###

function fork() {
    #processing
}
fork > /dev/null 2>&1 </dev/null &
echo $! > /run/{Mr.daemon}.pid

In the postscript part, if this script runs between 23:00 and 8:00 the next day, it will exit the process. In this state, set sudo systemctl enable {Mr.daemon} to always start automatically. It starts automatically between 8:00 and 23:00, but the automatic start is canceled between 23:00 and 8:00 the next day, so Daemon stays asleep. If you combine the systemd timer that you tried first with the cron that stops the daemon, you will have Daemon running at a specific time.

Commentary

Initially, there was only ʻexit in ʻif, but in that state, an error was thrown when the startup failed. It was the same even if I returned normal termination as ʻexit 0. It was until then that there was no problem, but I felt uncomfortable, so when I wrote systemctl stop {Mr.daemon}`, it was treated as startup cancellation and no error occurred.

In `cron`?

I haven't tried it, but if you don't mess with the daemon startup script, you can do the same with cron.

@reboot if [ $NOW -lt 23 -a $NOW -ge 8 ]; then systemctl start {Mr.daemon}; fi

If you add it to the administrator authority cron in this way, Daemon will also start when the OS is started between 23:00 and 8:00. I feel that cron is smarter because it does not start wastefully, but it was troublesome to adjust the order related to systemd such as ʻAfterand dependencies, so I decided to start it automatically withsystemd`. did.

Finally

None of these methods seem smart, so if anyone knows a good method, please teach me.

I want to manage systemd by time zone! !!