[LINUX] How to use variables in systemd Unit definition files

We will show you how to use ** variables ** in the Unit definition file when starting the service with the systemctl command.

environment

• OS：CentOS Linux release 7.7.1908

1. Specify variables in the Unit definition file

You can specify variables using ʻEnvironmentin the Unit definition file. Here, create a Unit definition file oftest.service` and check it.

• Set the variable TEST_VALUE1 to display1 and the variable TEST_VALUE2 to display2.
• ʻEcho` to display the variable.

/etc/systemd/system/test.service

[Unit]
Description=TestService

[Service]
Environment=TEST_VALUE1=display1
Environment=TEST_VALUE2=display2
ExecStart=/bin/echo ${TEST_VALUE1} ${TEST_VALUE2}

Start with systemctl start test.

[root@CENTOS7 ~]# systemctl start test
[root@CENTOS7 ~]#

Check with journalctl -u test.

[root@CENTOS7 ~]# journalctl -u test
--Logs begin at day 2019-12-29 17:19:15 JST,end at day 2019-12-29 18:07:04 JST.
December 29 18:07:04 CENTOS7 systemd[1]: Started TestService.
December 29 18:07:04 CENTOS7 echo[1398]: display1 display2

The values of the variables TEST_VALUE1 and TEST_VALUE2, display1 and display2 are displayed.

2. Specify environment variables in the Unit definition file

You can use ʻEnvironmentFilein the Unit definition file to specify an environment variable file. Here, create a Unit definition file oftest2.service` and check it.

/etc/sysconfig/test2

TEST_VALUE1=display1
TEST_VALUE2=display2

• ʻEnvironmentFile is set to the environment variable file / etc / sysconfig / test2`.
• ʻEcho` to display the variable.

/etc/systemd/system/test2.service

[Unit]
Description=Test2Service

[Service]
EnvironmentFile=/etc/sysconfig/test2
ExecStart=/bin/echo ${TEST_VALUE1} ${TEST_VALUE2}

Start with systemctl start test2.

[root@CENTOS7 ~]# systemctl start test2
[root@CENTOS7 ~]#

Check with journalctl -u test2.

[root@CENTOS7 ~]# journalctl -u test2
--Logs begin at day 2019-12-29 17:19:15 JST,end at day 2019-12-29 18:19:45 JST.
December 29 18:19:45 CENTOS7 systemd[1]: Started Test2Service.
December 29 18:19:45 CENTOS7 echo[1444]: display1 display2

The values of the variables TEST_VALUE1 and TEST_VALUE2, display1 and display2 set in / etc / sysconfig / test2 are displayed.

Supplement

Even if you specify TEST_VALUE1 = $ PATH etc. in the environment variable file / etc / sysconfig / test2, $ PATH will not be expanded as shown below.

/etc/sysconfig/test2

TEST_VALUE1=display1
TEST_VALUE2=＄PATH

[root@CENTOS7 ~]# systemctl start test2
[root@CENTOS7 ~]# journalctl -u test2
--Logs begin at day 2019-12-29 17:19:15 JST,end at day 2019-12-29 18:25:04 JST.
December 29 18:25:04 CENTOS7 systemd[1]: Started Test2Service.
December 29 18:25:04 CENTOS7 echo[1457]: display1 $PATH

that's all