systemd script - environment file updated by ExecStartPre

Question

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.

Answer 1

As an alternate approach, consider a script like with_testfile_vars doing the following:

#!/bin/sh
export foo=bar   # export the same calculated values you would otherwise write to the file
export baz=qux
exec "$@"        # then invoke your "real" program

...with a service file using that wrapper:

[Unit]
Description=test service
Before=memcached.service

[Service]
Type=oneshot
ExecStart=/usr/local/with_testfile_vars /usr/bin/testmebinary $VOLUMES

[Install]
WantedBy=multi-user.target

No EnvironmentFile needed at all, no ordering dependencies around how exactly it's interpreted, no concerns about how systemd's parsing differs from a shell's, etc.