Although it shouldn't be overly complicated to make sure all needed environment variables are set before execution, I can see this could sometimes be useful and make configuration easier. I got a feeling that : is actually allowed in environment variable name (shell may not like it), but = shouldn't be so %{VAR=default} would be a bit better.

Are you interested to implement this and provide a PR? I can help if needed.

One of my project consists of lot of services, each of them running the same RF-tests repository (all of them have common keywords, resources, libraries, etc.) with different tags which helps to start the correct tests. The main issue is -- each service can be started on different envs and have lot of configuration differences (here are several dev teams creating these services) -- so defaults is very important for me.

As for PR -- i would like to try! Anyway, you can always say what i did wrong in PR. However, i didn't contribute in RF code earlier so if you have some suggestions -- i'd appreciate!

@pekkaklarck pls see #3384 PR
i'd also added two test-cases

Yeah, there definitely are use cases where this would help. Robot's own acceptance test runner sets a Python interpreter and OS specific TEMPDIR environment variable, and sometimes when running tests directly, without the runner, you need to explicitly set it as well. We could change the current usages of %{TEMPDIR} to %{TEMPDIR=${TEMPDIR}} to use the OS default temporary directory in these cases.

Implementing this would actually be pretty simple. The code related to finding environment variables is pretty simple and editing it to split the possible default value from the name and returning it if there's no match shouldn't be hard either. You'd need to also create some acceptance tests and update docs.

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Didn't see you earlier comment or the PR when I wrote my previous comment. Good start with the PR but some changes are needed.

@pekkaklarck pls review #3384 one more time...

This was implemented by the great PR #3384 by @VVasyaev!