Environment directive
Environment="ONE=one" 'TWO=two two'
ExecStart=/bin/echo $ONE $TWO ${TWO}
EnvironmentFile directive
EnvironmentFile similar to Environment directive but reads the environment variables from a text file. The text file should contain new-line-separated variable assignments.This environment file can then be sourced and its variables used
Example file : /run/metadata/coreos
COREOS_DIGITALOCEAN_IPV4_ANCHOR_0=X.X.X.X COREOS_DIGITALOCEAN_IPV4_PRIVATE_0=X.X.X.X COREOS_DIGITALOCEAN_HOSTNAME=test.example.com COREOS_DIGITALOCEAN_IPV4_PUBLIC_0=X.X.X.X COREOS_DIGITALOCEAN_IPV6_PUBLIC_0=X:X:X:X:X:X:X:X
[Unit]
Requires=coreos-metadata.service
After=coreos-metadata.service
[Service]
EnvironmentFile=/run/metadata/coreos
ExecStart=
ExecStart=/usr/bin/etcd2 \
--advertise-client-urls=http://${COREOS_DIGITALOCEAN_IPV4_PUBLIC_0}:2379 \
--initial-advertise-peer-urls=http://${COREOS_DIGITALOCEAN_IPV4_PRIVATE_0}:2380 \
--listen-client-urls=http://0.0.0.0:2379 \
--listen-peer-urls=http://${COREOS_DIGITALOCEAN_IPV4_PRIVATE_0}:2380 \
--initial-cluster=%m=http://${COREOS_DIGITALOCEAN_IPV4_PRIVATE_0}:2380
References
https://stackoverflow.com/questions/37864999/referencing-other-environment-variables-in-systemd
https://coreos.com/os/docs/latest/using-environment-variables-in-systemd-units.html