Linux – using systemd timers instead of cron

arch linuxlinuxsystemdsystemd-timer

I am on Arch Linux where I am trying to create a systemd timer as a cron alternative for hibernating my laptop on low battery. So I wrote these three files:

/etc/systemd/system/battery.service

[Unit]
Description=Preko skripte preveri stanje baterije in hibernira v kolikor je stanje prenizko

[Service]
Type=oneshot
ExecStart=/home/ziga/Dropbox/workspace/operacijski/archlinux/hibernate/hibernatescript
User=nobody
Group=systemd-journal

/etc/systemd/system/battery.timer

[Unit]
Description=Periodical checking of battery status every two minutes

[Timer]
OnUnitActiveSec=2min 

[Install]
WantedBy=timers.target

/home/ziga/Dropbox/workspace/operacijski/archlinux/hibernate/hibernatescript

#!/bin/sh
/usr/bin/acpi -b | /usr/bin/awk -F'[,:%]' '{print $2, $3}' | (
        read -r status capacity
        if [ "$status" = Discharging ] && [ "$capacity" -lt 50 ]; then
                /usr/bin/systemctl hibernate
        fi                                                                                                                
)

And then to enable timer I executed:

sudo systemctl enable battery.timer
sudo systemctl start battery.timer

And somehow it isn't working. Script works on its own. This means that if I execute command below, my computer hibernates just fine.

/home/ziga/Dropbox/workspace/operacijski/archlinux/hibernate/hibernatescript

ADD1:

After enabling and starting timer I ran some checks and this is what I get:

[ziga@ziga-laptop ~]$ systemctl list-timers
NEXT                          LEFT       LAST                          PASSED    UNIT                         ACTIVATES
n/a                           n/a        n/a                           n/a       battery.timer                battery.serv
Tue 2016-06-28 00:00:00 CEST  42min left Mon 2016-06-27 00:01:54 CEST  23h ago   logrotate.timer              logrotate.se
Tue 2016-06-28 00:00:00 CEST  42min left Mon 2016-06-27 00:01:54 CEST  23h ago   shadow.timer                 shadow.servi
Tue 2016-06-28 00:00:00 CEST  42min left Mon 2016-06-27 00:01:54 CEST  23h ago   updatedb.timer               updatedb.ser
Tue 2016-06-28 22:53:58 CEST  23h left   Mon 2016-06-27 22:53:58 CEST  23min ago systemd-tmpfiles-clean.timer systemd-tmpf

and

[ziga@ziga-laptop ~]$ systemctl | grep battery
battery.timer  loaded active elapsed   Periodical checking of battery status every two minutes

ADD2:

After applying solution from Alexander T my timer starts (check the code below) but script doesn't hibernate my laptop while it hibernates it if I execute it directly.

[ziga@ziga-laptop ~]$ systemctl list-timers
NEXT                          LEFT          LAST                          PASSED       UNIT                         ACTIVATES
Tue 2016-06-28 19:17:30 CEST  1min 43s left Tue 2016-06-28 19:15:30 CEST  16s ago      battery.timer                battery.service

Best Answer

An answer to this question is to swap User=nobody not with User=ziga but with User=root in /etc/systemd/system/battery.service. Somehow even if user ziga has all the privileges of using sudo command it can't execute systemctl hibernate inside of the bash script. I really don't know why this happens. So the working files are as follows:

/etc/systemd/system/battery.service

[Unit]
Description=Preko skripte preveri stanje baterije in hibernira v kolikor je stanje prenizko

[Service]
Type=oneshot
ExecStart=/home/ziga/Dropbox/workspace/operacijski/archlinux/hibernate/hibernatescript
User=root
Group=systemd-journal

/etc/systemd/system/battery.timer

[Unit]
Description=Periodical checking of battery status every two minutes

[Timer]
OnBootSec=2min
OnUnitActiveSec=2min 

[Install]
WantedBy=battery.service

/home/ziga/Dropbox/workspace/operacijski/archlinux/hibernate/hibernatescript

#!/bin/sh
/usr/bin/acpi -b | /usr/bin/awk -F'[,:%]' '{print $2, $3}' | (
        read -r status capacity
        if [ "$status" = Discharging ] && [ "$capacity" -lt 7 ]; then
                /usr/bin/systemctl hibernate
        fi
)

I tried it and it allso works with User=ziga or User=nobody but we need to change /usr/bin/systemctl hibernate into sudo /usr/bin/systemctl hibernate in the last script. So it looks like User variable somehow doesn't even matter... Oh and you can as well remove absolute names from the last script and change first line from #!/bin/sh to #!/bin/bash. I also changed WantedBy=timers.target to WantedBy=battery.service in /etc/systemd/system/battery.timer.

There you go. The best cron alternative to hibernate laptops on low battery. =)

Related Solutions

Cron vs systemd timers

Here are some points about those two:

checking what your cron job really does can be kind of a mess, but all systemd timer events are carefully logged in systemd journal like the other systemd units based on the event that makes things much easier.
systemd timers are systemd services with all their capabilities for resource management, IO CPU scheduling, ...
There is a list :
- systemcall filters
- user/group ids
- membershipcontrols
- nice value
- OOM score
- IO scheduling class and priority
- CPU scheduling policy CPU
- affinity umask
- timer slacks
- secure bits
- network access and ,...
with the dependencies option just like other systemd services there can be dependencies on activation time.
Units can be activated in different ways, also combination of them can be configured. services can be started and triggered by different events like user, boot, hardware state changes or for example 5mins after some hardware plugged and ,...
much easier configuration some files and straight forward tags to do variety of customizations based on your needs with systemd timers.
Easily enable/disable the whole thing with:
```
systemctl enable/disable 
```
and kill all the job's children with:
```
systemctl start/stop
```
systemd timers can be scheduled with calenders and monotonic times, which can be really useful in case of different timezones and ,...
systemd time events (calendar) are more accurate than cron (seems 1s precision)

systemd time events are more meaningful, for those recurring ones or even those that should occur once, here is an example from the document:

Sat,Thu,Mon-Wed,Sat-Sun → Mon-Thu,Sat,Sun *-*-*00:00:00
  Mon,Sun 12-*-* 2,1:23 → Mon,Sun 2012-*-* 01,02:23:00
                Wed *-1 → Wed *-*-01 00:00:00
        Wed-Wed,Wed *-1 → Wed *-*-01 00:00:00
             Wed, 17:48 → Wed *-*-* 17:48:00

From the CPU usage view point systemd timer wakes the CPU on the elapsed time but cron does that more often.
Timer events can be scheduled based on finish times of executions some delays can be set between executions.
The communication with other programs is also notable sometimes it's needed for some other programs to know timers and the state of their tasks.

Why x0vncserver is not starting at boot

As x0vncserver will only start after xsession, and as mine is an LXDE environment, the easier way is to create a script.sh file like following

#!/bin/bash
x0vncserver -display :0 -rfbport 5900 -passwordfile ~/.vnc/passwd

and make it executable by sudo chmod +x script.sh and add a line in ~/.config/lxsession/LXDE/autostart file like following

path_to_the_script_file/script.sh

and the x0vncserver starts as my LXDE desktop session starts and that's what I need.

Best Answer

Related Solutions

Cron vs systemd timers

Why x0vncserver is not starting at boot

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