I am trying out systemd script along in Docker environment.
Consider this:
command mentioned in ExecStartPre updating environment file and ExecStart actually making use of environment variable mentioned in env. file.? (all in the same systemd file).
like this:
[Unit]
Description=test service
Before=memcached.service
[Service]
Type=oneshot
EnvironmentFile=-/etc/sysconfig/testfile
ExecStartPre=/usr/local/bin/update_sysconfig_testfile.sh
ExecStart=/usr/bin/testmebinary $VOLUMES
[Install]
WantedBy=multi-user.target
Here, $VOLUMES is defined inside testfile and it is updated by update_sysconfig_testfile.sh script.
Will systemd aware about the change made by ExecStartPre (or) it just loads whatever value in testfile?
If there is any better approach, please share.
Since at least systemd 237, you can write an file in ExecStartPre= that's read by EnvironmentFile= before ExecStart= runs as long as you prefix the file path with a dash (EnvironmentFile=-/some/path/.env).
This appears to due to the timing of when the environment is evaluated. poettering says:
the env vars are determined only at execution time
So it's an error if EnvironmentFile= is not defined when ExecStartPre= is executed which is why it needs to be optional, but when the "execution time" of ExecStart= comes, the EnvironmentFile is apparently re-checked and the environment variables set.
This could be better documented in man systemd.exec.
You can also use an alternative approach which is suggested in man systemd.exec where Environmentfile= is documented:
Use one systemd service to write the environment file and a second one to consume it. Using either a Before= or After= relationship in the [Unit] section, you can ensure that the service that writes the environment file is started first in the boot sequence.