How to set environment variable in systemd service?

I have an Arch Linux system with systemd and I've created my own service. The configuration service at /etc/systemd/system/myservice.service looks like this:

[Unit]
Description=My Daemon

[Service]
ExecStart=/bin/myforegroundcmd

[Install]
WantedBy=multi-user.target

Now I want to have an environment variable set for the /bin/myforegroundcmd. How do I do that?

Times change and so do best practices.

The current best way to do this is to run systemctl edit myservice, which will create an override file for you or let you edit an existing one.

In normal installations this will create a directory /etc/systemd/system/myservice.service.d, and inside that directory create a file whose name ends in .conf (typically, override.conf), and in this file you can add to or override any part of the unit shipped by the distribution.

For instance, in a file /etc/systemd/system/myservice.service.d/myenv.conf:

[Service]
Environment="SECRET=pGNqduRFkB4K9C2vijOmUDa2kPtUhArN"
Environment="ANOTHER_SECRET=JP8YLOc2bsNlrGuD6LVTq7L36obpjzxd"

Also note that if the directory exists and is empty, your service will be disabled! If you don't intend to put something in the directory, ensure that it does not exist.

For reference, the old way was:

The recommended way to do this is to create a file /etc/sysconfig/myservice which contains your variables, and then load them with EnvironmentFile.

For complete details, see Fedora's documentation on how to write a systemd script.

The answer depends on whether the variable is supposed to be constant (that is, not supposed to be modified by user getting the unit) or variable (supposed to be set by the user).

Since it's your local unit, the boundary is quite blurry and either way would work. However, if you started to distribute it and it would end up in /usr/lib/systemd/system, this would become important.

Constant value

If the value doesn't need to change per instance, the preferred way would be to place it as Environment=, directly in the unit file:

[Unit]
Description=My Daemon

[Service]
Environment="FOO=bar baz"
ExecStart=/bin/myforegroundcmd

[Install]
WantedBy=multi-user.target

The advantage of that is that the variable is kept in a single file with the unit. Therefore, the unit file is easier to move between systems.

Variable value

However, the above solution doesn't work well when sysadmin is supposed to change the value of the environment variable locally. More specifically, the new value would need to be set every time the unit file is updated.

For this case, an extra file is to be used. How — usually depends on the distribution policy.

One particularly interesting solution is to use /etc/systemd/system/myservice.service.d directory. Unlike other solutions, this directory is supported by systemd itself and therefore comes with no distribution-specific paths.

In this case, you place a file like /etc/systemd/system/myservice.service.d/local.conf that adds the missing parts of unit file:

[Service]
Environment="FOO=bar baz"

Afterwards, systemd merges the two files when starting the service (remember to systemctl daemon-reload after changing either of them). And since this path is used directly by systemd, you don't use EnvironmentFile= for this.

If the value is supposed to be changed only on some of the affected systems, you may combine both solutions, providing a default directly in the unit and a local override in the other file.