Pass an environment variable from current shell to a systemd unit

Question

I am trying to pass an environment variable defined in the current shell to one of the systemd unit I am writing.

DB_URL=databus.dev.mysite.io:8080

I am using this in a python script which is running as a service. My systemd unit will run this script as a unit making use of the variable for its working.

[Unit]
Description=device-CompStatus: Computes device availability status

[Service]
Type=simple    
ExecStart=/usr/bin/bash -c "/usr/bin/python /opt/deviceCompStatus/deviceCompStatusHandler.py"
Restart=always

[Install]
WantedBy=default.target

The way am using the variable in Python script would be

if os.environ.get('DB_URL') is not None:
    dbEndPoint = "http://" + os.environ['DB_URL']

The problem is am not able to use the variable when running the script in systemd. I looked up couple of resources Using environment variables in systemd units, it says to use assignment under [Service] directly as

[Service]
Environment=DB_URL=databus.dev.mysite.io:8080

As you can see, my DB_URL could change depending upon the environment I am deploying my machine, it could be a developer, or a production setup, in which the URLs would vary.

How do I do this dynamically? i.e. pass whatever value available to DB_URL to systemd environment?

I also tried using the EnvironmentFile= option to define a file and pass it to service. But the same problem again, my variable could be dynamic and cannot be hardcoded.

Update

After using the option

systemctl import-environment DB_URL

I am able to see the variable available in the environment of systemd which I confirmed by

systemctl show-environment
DB_URL=databus.dev.mysite.io:8080
LANG=en_US.UTF-8
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin

but still the value is not reflected in the python application which I run. Is os.environ('DB_URL') a wrong way to access the variable?

Answer 1

You can affect the global systemd environment for all future commands by using

 sudo systemctl set-environment var=value

or if you already have var exported in your environment, you can use

 sudo systemctl import-environment var

After starting your unit you can remove the variable with unset-environment similarly.

As this is global in effect you would be better off simply writing the line DB_URL=databus.dev.mysite.io:8080 into some file /some/file and setting EnvironmentFile=/some/file in your unit.

An alternative method is to use a template unit myunit@.service which is started with systemctl start myunit@'databus.dev.mysite.io:8080'. You can then recover this parameter as %i inside the unit, for example in the [Service] section with a line like:

Environment=DB_URL=%i