Systemd

Systemd: a linux service management system. It manages all the process from the start of the system.

There are standard methods to call systemd:

enable: service will automatically be started by its starting logic specified
disable: reverse of enable
start: start the service regardless
stop: stop the service regardless
restart: stop and start (if not running, just start)
reload: faster version of restart but unstable
status: check status

To config .service file, you can modify user-wide /usr/lib/systemd or systemwide /etc/systemd/system/*.service, or permissioned-systemd /etc/systemd/user. Or you can make it default with /lib/systemd/system/*.service Usually we put in /etc/systemd/system/*.service, for more info, visit stackoverflow

Below is a template that can help you write *.system file.

[Unit]
Description=Description
Requires=dependency1.service dependency2.service
Wants=dependency-that-may-fail-and-still-ok.service
After=only-execute-after.service

[Service]
Type=simple/oneshot
RemainAfterExit=yes/no (only specified when oneshot)
User=www-data
Group=www-data
WorkingDirectory=/absolute/directory
ExecStart=/path/to/zsh ./start-gunicorn-http.sh
ExecStart=/bin/bash -c "echo you can have none or multiple ExecStart is you use oneshot, otherwise only one"
ExecStop=/bin/bash -c "echo something && echo other things"

[Install]
WantedBy=if-this-service-launch-I-will-launch-but-it-is-okay-for-me-to-fail.target
RequiredBy=if-this-service-launch-I-will-launch-but-it-is-bad-for-me-to-fail.target

Common Mistakes

You can't use any && and < commands directly in ExecStart because the commands is not executed in any shell
You can't directly execute program like tmux or zsh because environment variable is not loaded
You should make sure the permission of User and Group is correct (not with super power root but with sufficient and reasonable power)
You should make sure the command you try to execute has resources defined by After and by Requires.

Service Type

This section is (non-exact) copy of original article here

Simple service and follow-up units timing

Oneshot service and follow-up units timing

Situration: Something Depends on Simple Type

simple-test.service

[Unit]
Description=Simple service test

[Service]
Type=simple
ExecStart=/bin/bash -c "echo Simple service - start && sleep 60 && echo Simple service - end"

dep-simple-test.service

[Unit]
Description=Dependent service
After=simple-test.service
Requires=simple-test.service

[Service]
ExecStart=/bin/bash -c "echo Dependent service - running"

Starting dep-simple-test.service will start simple-test.service first (because of the After/Requires directives), and the logging shows:

Jun 19 20:28:16 thstring20200619162314 systemd[1]: Started Simple service test.
Jun 19 20:28:16 thstring20200619162314 systemd[1]: Started Dependent service.
Jun 19 20:28:16 thstring20200619162314 bash[1238]: Simple service - start
Jun 19 20:28:16 thstring20200619162314 bash[1239]: Dependent service - running
Jun 19 20:28:16 thstring20200619162314 systemd[1]: dep-simple-test.service: Succeeded.
Jun 19 20:29:16 thstring20200619162314 bash[1238]: Simple service - end
Jun 19 20:29:16 thstring20200619162314 systemd[1]: simple-test.service: Succeeded.

You can see the dependency runs first but the depended program will start immediately around the same time without waiting for the dependency to finish execute.

Situration: Something Depends on Oneshot Type

oneshot-test.service

[Unit]
Description=Oneshot service test

[Service]
Type=oneshot
ExecStart=/bin/bash -c "echo Oneshot service - start && sleep 60 && echo Oneshot service - end"

dep-oneshot-test.service

[Unit]
Description=Dependent service
After=oneshot-test.service
Requires=oneshot-test.service

[Service]
ExecStart=/bin/bash -c "echo Dependent service - running"

Jun 19 20:31:46 thstring20200619162314 systemd[1]: Starting Oneshot service test...
Jun 19 20:31:46 thstring20200619162314 bash[1420]: Oneshot service - start
Jun 19 20:32:46 thstring20200619162314 bash[1420]: Oneshot service - end
Jun 19 20:32:46 thstring20200619162314 systemd[1]: oneshot-test.service: Succeeded.
Jun 19 20:32:46 thstring20200619162314 systemd[1]: Started Oneshot service test.
Jun 19 20:32:46 thstring20200619162314 systemd[1]: Started Dependent service.
Jun 19 20:32:46 thstring20200619162314 bash[1440]: Dependent service - running
Jun 19 20:32:46 thstring20200619162314 systemd[1]: dep-oneshot-test.service: Succeeded.

You can see the program wait for the dependency to finish execution.

Type	Before	During	After
Simple	inactive (dead)	active (running)	inactive (dead)
Oneshot	inactive (dead)	activating (start)	inactive (dead)
Oneshot (RemainAfterExit)	inactive (dead)	activating (start)	active (exited)

Situration: Something Depends on Oneshot Type and RemainAfterExit=yes

oneshot-execstop-remainafterexit.service

[Unit]
Description=Oneshot service test with ExecStop and RemainAfterExit

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/bash -c "echo Oneshot service - start && sleep 60 && echo Oneshot service - end"
ExecStop=/bin/bash -c "echo Oneshot service - stop"

Running systemctl status on this service at it has run, we can see the difference:

● oneshot-execstop-remainafterexit.service - Oneshot service test with ExecStop and RemainAfterExit
   Loaded: loaded (/etc/systemd/system/oneshot-execstop-remainafterexit.service; static; vendor preset: enabled)
   Active: active (exited) since Fri 2020-06-19 21:07:54 UTC; 8s ago
  Process: 1708 ExecStart=/bin/bash -c echo Oneshot service - start && sleep 60 && echo Oneshot service - end (code=exited, status=0/SUCCESS)
 Main PID: 1708 (code=exited, status=0/SUCCESS)

Jun 19 21:06:54 thstring20200619162314 systemd[1]: Starting Oneshot service test with ExecStop and RemainAfterExit...
Jun 19 21:06:54 thstring20200619162314 bash[1708]: Oneshot service - start
Jun 19 21:07:54 thstring20200619162314 bash[1708]: Oneshot service - end
Jun 19 21:07:54 thstring20200619162314 systemd[1]: Started Oneshot service test with ExecStop and RemainAfterExit.

You can see with RemainAfterExit=yes, oneshot program will show active even after program finish execution.

If we run systemctl stop oneshot-execstop-remainafterexit.service, then it will execute ExecStop=/bin/bash -c "echo Oneshot service - stop":

Jun 19 21:07:54 thstring20200619162314 systemd[1]: Started Oneshot service test with ExecStop and RemainAfterExit.
Jun 19 21:08:58 thstring20200619162314 systemd[1]: Stopping Oneshot service test with ExecStop and RemainAfterExit...
Jun 19 21:08:58 thstring20200619162314 bash[1900]: Oneshot service - stop
Jun 19 21:08:58 thstring20200619162314 systemd[1]: oneshot-execstop-remainafterexit.service: Succeeded.
Jun 19 21:08:58 thstring20200619162314 systemd[1]: Stopped Oneshot service test with ExecStop and RemainAfterExit.

This is in contrast to RemainAfterExit=no (default behavior) as shown below:

Situration: Something Depends on Oneshot Type and RemainAfterExit=no

oneshot-execstop.service

[Unit]
Description=Oneshot service test with ExecStop

[Service]
Type=oneshot
RemainAfterExit=no
ExecStart=/bin/bash -c "echo Oneshot service - start && sleep 60 && echo Oneshot service - end"
ExecStop=/bin/bash -c "echo Oneshot service - stop"

● oneshot-execstop.service - Oneshot service test with ExecStop
   Loaded: loaded (/etc/systemd/system/oneshot-execstop.service; static; vendor preset: enabled)
   Active: inactive (dead)

Jun 19 21:04:10 thstring20200619162314 systemd[1]: Starting Oneshot service test with ExecStop...
Jun 19 21:04:10 thstring20200619162314 bash[1480]: Oneshot service - start
Jun 19 21:05:10 thstring20200619162314 bash[1480]: Oneshot service - end
Jun 19 21:05:10 thstring20200619162314 bash[1604]: Oneshot service - stop
Jun 19 21:05:10 thstring20200619162314 systemd[1]: oneshot-execstop.service: Succeeded.
Jun 19 21:05:10 thstring20200619162314 systemd[1]: Started Oneshot service test with ExecStop.

We can see that the ExecStop ran immediately when the ExecStart was done.

Oneshot Service's Multiple ExecStart

oneshot service has special power:

It does not have to have ExecStart (but then you must have ExecStop and RemainAfterExit=yes)
It can have multiple ExecStart

When multiple ExecStart are defined, the service fail if one of the ExecStart fails:

oneshot-multiple-execstart-failure.service

[Unit]
Description=Oneshot service test with multiple ExecStart and failure

[Service]
Type=oneshot
ExecStart=/bin/bash -c "echo First"
ExecStart=/bin/bash -c "false && echo Second"
ExecStart=/bin/bash -c "echo Third"

$ sudo systemctl start oneshot-multiple-execstart-failure.service
Job for oneshot-multiple-execstart-failure.service failed because the control process exited with error code.
See "systemctl status oneshot-multiple-execstart-failure.service" and "journalctl -xe" for details.

$ sudo journalctl -u oneshot-multiple-execstart-failure.service
-- Logs begin at Mon 2020-06-22 13:24:01 UTC, end at Mon 2020-06-22 13:37:16 UTC. --
Jun 22 13:36:53 thstring20200622092223 systemd[1]: Starting Oneshot service test with multiple ExecStart and failure...
Jun 22 13:36:53 thstring20200622092223 bash[1441]: First
Jun 22 13:36:53 thstring20200622092223 systemd[1]: oneshot-multiple-execstart-failure.service: Main process exited, code=exited, status=1/FAILURE
Jun 22 13:36:53 thstring20200622092223 systemd[1]: oneshot-multiple-execstart-failure.service: Failed with result 'exit-code'.
Jun 22 13:36:53 thstring20200622092223 systemd[1]: Failed to start Oneshot service test with multiple ExecStart and failure.

However, if you want to prevent this behavior, you can add - character in front of the executable.

oneshot-multiple-execstart-failure-success.service

[Unit]
Description=Oneshot service test with multiple ExecStart and failure

[Service]
Type=oneshot
ExecStart=/bin/bash -c "echo First"
ExecStart=-/bin/bash -c "false && echo Second"
ExecStart=/bin/bash -c "echo Third"

-- Logs begin at Mon 2020-06-22 13:24:01 UTC, end at Mon 2020-06-22 13:39:04 UTC. --
Jun 22 13:38:59 thstring20200622092223 systemd[1]: Starting Oneshot service test with multiple ExecStart and failure...
Jun 22 13:38:59 thstring20200622092223 bash[1553]: First
Jun 22 13:38:59 thstring20200622092223 bash[1555]: Third
Jun 22 13:38:59 thstring20200622092223 systemd[1]: oneshot-multiple-execstart-failure-success.service: Succeeded.
Jun 22 13:38:59 thstring20200622092223 systemd[1]: Started Oneshot service test with multiple ExecStart and failure.

When you are deciding which service type to choose between simple and oneshot, here is some guidance:

Does your service need to complete before any follow-up services run? Use oneshot.
Do your follow-up services need to be running while this service does? Use simple.
Is this a long-running service? Probably use simple.
Do you need to run this service only at shutdown? Use oneshot.
Do you need to have multiple separate commands to run? Use oneshot.

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