How do I assign the result of curl to a systemd variable?

mattm asked:

I am using an Amazon Web Service server, and am trying to setup a systemd job that takes as an input the AWS external IP retrieved by curl.

Description=My example with AWS external IP

ExecStart=/path/to/myexample $EXTERNAL_IP


The verify tool systemd-analyze verify myexample.service indicates there is a problem with curl and the environment assignment.

Invalid environment assignment, ignoring: EXTERNAL_IP=$(/usr/bin/curl

If I replace the curl result with a hard-coded IP string, my daemon starts. How can I use the curl result in my systemd job?

My answer:

(First, I presume you are using coturn or one of its predecessors.)

To begin with, systemd units are not shell scripts and aren’t parsed by a shell. This is why you can’t just call a command and use its output as an environment variable.

However, you can call a shell from ExecStart=, which might be the fastest way to get up and running.

ExecStart=/bin/bash -c '/usr/local/bin/turnserver -X $(/usr/bin/curl'

(In this configuration you should not daemonize turnserver with -o.)

You could also write a script, which you can call from ExecStartPre=, which fetches your metadata and then alters turnserver.conf, which would occur before the daemon is started.

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