Systemd – How to Override Unit File Settings

It depends a little on what option you're trying to configure. If it's a folder location (like the home directory, runtime directory, etc.), systemd provides this through variables.

Otherwise, you can use an environment file to specify the options, and then have the service unit include the variables in those files. For example, the service file would contain the following (in addition to the usual parameters):

[Service]
Environment=ENV_OPTION_1=0 ENV_OPTION_2=linux
EnvironmentFile=/etc/default/program-name
ExecStart=/usr/bin/program $ENV_OPTION_1 ${ENV_OPTION_2}

EnvironmentFile points to a simple file that contains key-value pairs, like so:

ENV_OPTION_1=8913
ENV_OPTION_2=ubuntu

Environment are default options you may want to have.

The environment values are then used in ExecStart as shown above. Note that if the value may be space-separated, you need to use the ${} syntax.

As explained by @StephenHarris the problem is at the moment systemd tries to read the unit, the file that's symlinked isn't available yet

To just have systemd reload the units after it has mounted :

[Unit]
Description=reloads units stored in /mnt/data
DefaultDependencies=no
After=mnt-data.mount
Requires=mnt-data.mount

[Service]
Type=oneshot
ExecStart=/bin/systemctl daemon-reload

[Install]
WantedBy=local-fs.target

This will cause the units to become available, because this time this time the target of the symlinks is mounted.

But by that time, the list of jobs needing to be run to reach the default.target is already built, and the service won't be started.

To have it also restart your service:

[Unit]
Description=restart unit stored in /mnt/data
Requires=mnt-data.mount

[Service]
Type=oneshot
ExecStart=/bin/systemctl daemon-reload
ExecStartPost=/bin/systemctl start sprinterd.service

[Install]
WantedBy=multi-user.target

Alternatives:

• I've tested with ExecStart= & ExecStartPost=, but it should obviously work with ExecStartPre= & ExecStart=
• if it's all about 1 single unit, you might as well : ExecStart=/bin/systemctl enable /mnt/data/sprinterd.service instead of daemon-reload
• if there are multiple services, do the daemon-reload, then start a single unit that uses ConsistsOf= or PartOf= to load all the multiple services.
• If its NFS (or other networked system), of course local-fs.target isn't your best Installation option, obviously.

For a more old-school SysVinit-style approach, put the systemctl commands inside /etc/rc.local and chmod +x that file.

And then go smuggly post on Devuan's mailing list how you needed SysVInit to fix b0rked SystemD ;-)