How to use systemd for applications start

On the latest Debian distributions is installed by default systemd instead of sysvinit as initialization system. This article illustrates pratically how to use it.

Create a service

To launch a script at startup a proper service file have to be created and added to the systemd.

Let's create a service file for our blik.py program. Create as root a file called blink.service in /lib/systemd/system directory.

sudo nano /lib/systemd/system/blink.service

then save inside it this content:

[Unit]
Description=Blinking led
After=network.target

[Service]
Type=idle
ExecStart=/usr/bin/python /home/pi/blink.py
Restart=always
User=pi

[Install]
WantedBy=multi-user.target

Next systemd has to be configured, enabling the service so at the next reboot it will be started automatically. First off, though, the systemd itself has to be reloaded in order to detect the new service definition:

sudo systemctl daemon-reload

Next, enable the service

sudo systemctl enable blink

Check if it is enabled:

sudo systemctl list-unit-files | grep enabled

Of course, it can be started manually just now:

sudo systemctl start blink

The PID (process id) can be now checked:

ps -ef | grep blink
pi         817     1  0 09:19 ?        00:00:00 /usr/bin/python blink.py

it is 817 (the number will be different in your case). Please note that it has been started from systemd process, in fact the PPID (parent process id) is 1.

Test what happens if the program crashes and/or terminates.

In order to simulate a crash, we kill it manually:

sudo kill 817

Now we can check it was restarted looking again to the PID (process id):

ps -ef | grep blink
pi        1037     1  0 09:19 ?        00:00:00 /usr/bin/python blink.py

it is now 1037 (PPID always 1), so, as it is different, that means has been restarted automatically after the kill.

Use the systemd logging system

To check what is doing your service launch this command:

journalctl -f blink

It is possible, of course, to add our own messages to this log. In Python, for example, install the python-systemd package:

sudo apt-get update
sudo apt-get install python-systemd

add these lines to your code:

import logging
from systemd.journal import JournalHandler

log = logging.getLogger('blonk')
log.addHandler(JournalHandler())
log.setLevel(logging.INFO)

log.info("Blinking led started")

Open another terminal and launc journalctl with follow option (-f):

journalctl -u -f blink
...
May 13 11:49:24 roadrunner systemd[1]: Started Blinking led.
May 13 11:49:25 roadrunner /home/pi/blink.py[18081]: Blinking led started

Create a timer for your service

It is possible to use the systemd timers to launch a service instead of using cron

Example 1: Run a service every hour

Create a systemd service file in /lib/systemd/system/blink.service with this contents:

[Unit]
Description=Blinking led

[Service]
WorkingDirectory=/home/pi
ExecStart=python blink.py
User=pi

Create a systemd timer file for this service in /lib/systemd/system/blink.timer with this contents::

[Unit]
Description=Runs blink.py every hour

[Timer]
# Time to wait after booting before we run first time
OnBootSec=10min
# Time between running each consecutive time
OnUnitActiveSec=1h
Unit=blink.service

[Install]
WantedBy=multi-user.target

Reload the service definitions:

sudo systemctl daemon-reload

Then is your are using the same file of previous example disable the blink.service and enable just the blink.timer to start it after the bootstrap:

sudo systemctl disable blink.service
sudo systemctl enable blink.timer

Let's try it now:

sudo systemctl start blink.timer

Example 2: Run a service daily

[Unit]
Description=Runs blink.py daily at midnight

[Timer]
OnCalendar=*-*-* 00:00:00
Persistent=True 
Unit=blink.service

[Install]
WantedBy=multi-user.target

Changing the OnCalendar parameter is possible to start the blink.service in a lot of different way:

Minimal form                   Normalized form
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
Wed-Sat,Tue 12-10-15 1:2:3 ==> Tue-Sat 2012-10-15 01:02:03
*-*-7 0:0:0                ==> *-*-07 00:00:00
10-15                      ==> *-10-15 00:00:00
monday *-12-* 17:00        ==> Mon *-12-* 17:00:00
Mon,Fri *-*-3,1,2 *:30:45  ==> Mon,Fri *-*-01,02,03 *:30:45
12,14,13,12:20,10,30       ==> *-*-* 12,13,14:10,20,30:00
mon,fri *-1/2-1,3 *:30:45  ==> Mon,Fri *-01/2-01,03 *:30:45
03-05 08:05:40             ==> *-03-05 08:05:40
08:05:40                   ==> *-*-* 08:05:40
05:40                      ==> *-*-* 05:40:00
Sat,Sun 12-05 08:05:40     ==> Sat,Sun *-12-05 08:05:40
Sat,Sun 08:05:40           ==> Sat,Sun *-*-* 08:05:40
2003-03-05 05:40           ==> 2003-03-05 05:40:00
2003-03-05                 ==> 2003-03-05 00:00:00
03-05                      ==> *-03-05 00:00:00
hourly                     ==> *-*-* *:00:00
daily                      ==> *-*-* 00:00:00
monthly                    ==> *-*-01 00:00:00
weekly                     ==> Mon *-*-* 00:00:00
*:20/15                    ==> *-*-* *:20/15:00

Check the list of timers

systemctl list-timers --all
...
NEXT                          LEFT     LAST                          PASSED       UNIT                         ACTIVATES
Mon 2018-05-14 00:00:00 CEST  10h left Sun 2018-05-13 13:14:44 CEST  2min 17s ago blink.timer                  blink.service
Mon 2018-05-14 10:38:28 CEST  21h left Sun 2018-05-13 10:38:28 CEST  2h 38min ago systemd-tmpfiles-clean.timer systemd-tmpfiles-clean.service
n/a                           n/a      n/a                           n/a          apt-daily-upgrade.timer      apt-daily-upgrade.service
n/a                           n/a      n/a                           n/a          apt-daily.timer              apt-daily.service

Links

How to start a script right before shutdown

This is an example on how to start a Python script called shutdown.py at shutdown:

[Unit]
Description=Send a message on the Oled at shutdown

[Service]
Type=oneshot
RemainAfterExit=true
WorkingDirectory=/home/pi/Adafruit_Python_SSD1306/examples
ExecStop=/usr/bin/python shutdown.py
User=pi

[Install]
WantedBy=multi-user.target

Save is as /lib/systemd/system/oled.service

Reload the new service definition:

sudo systemctl daemon-reload

Enable the new service:

sudo systemctl enable oled.service

Then shutdown:

sudo halt

Links

SystemV the old way

A few pieces of old way are still present and working as for example /etc/rc.local where all the applications that implement the system functionality are started.

A typical rc.local contains:

cat /etc/rc.local

#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.

cd /home/acmesystems
/usr/bin/nohup /usr/sbin/python blink.py &
exit 0

In this example a Python program is started from application's home directory, and left in background (using the trailing ampersand). That works passably well till our python application never crashes: in such unfortunate case, our system only chance has, in order to regain the lost functionality, is to be manually restarted by user (probably cycling the power...).

Usually, the quick and dirty solution suggested is to add our application to the /etc/inittab file, specifyng a respawn in the line:

id:2345:respawn:/bin/sh /path/to/application/startup

in such case startup must be a proper script able to start out application (and probably it could be convenient to reuse this script as is even in /etc/rc.local).

Links

Andrea Montefusco
Currently employed as network architect, always Internet working man, real C/C++ programmer in the past, network and Unix system engineer as needed, HAM Radio enthusiast (former IW0RDI, now IW0HDV), aeromodeller (a person who builds and flies model airplanes) since 1976 (ex FAI10655).
http://www.montefusco.com - https://github.com/amontefusco - https://github.com/IW0HDV - andrew@montefusco.com
Sergio Tanzilli (Autore - Progettista hw & sw - Imprenditore - Musicista dilettante, classe 1964)
Fondatore di Area SX srl, Acme Systems srl e del TanzoLab, si occupa dal 1974 di elettronica digitale, dal 1978 di microprocessori, dal 1992 di microcontroller, dal 1995 di Linux e pubblicazioni su Web, dal 2005 di progettazione, produzione e commercializzazione di schede Linux Embedded.
http://www.tanzilli.com - http://www.acmesystems.it - https://github.com/tanzilli - sergio@tanzilli.com