How can I pass a build setting to a genrule?

[njlr]

Suppose I have a simple genrule like this:

genrule(
  name = "fruit",
  outs = [ "fruit.txt" ],
  cmd = "echo \"fruit: apple\" > $@",
)

Now I want to make this configurable from the outside.

I can use --action_env like this:

genrule(
  name = "fruit",
  outs = [ "fruit.txt" ],
  cmd = "echo \"fruit: $$FRUIT\" > $@",
)

bazel build --action_env=FRUIT=banana //:fruit

However, I don't want to use --action_env because it affects all rules and it is bad for caching.

There is --define, but the docs describe it as legacy:

--define is an alternative legacy syntax for custom flags (for example --define foo=bar). This can be expressed either in the values attribute (values = {"define": "foo=bar"}) or the define_values attribute (define_values = {"foo": "bar"}). --define is only supported for backwards compatibility. Prefer Starlark build settings whenever possible.

So that leaves build settings.

However, it is not clear how to get the value of a build setting into my genrule.

I am looking for something like this:

# Not real code 

config_setting(
  name = "favourite_fruit",
)

genrule(
  name = "fruit",
  outs = [ "fruit.txt" ],
  cmd = "echo \"fruit: $(value :favourite_fruit)\" > $@",
)

[meteorcloudy]

Maybe examples under https://github.com/bazelbuild/examples/tree/main/configurations can help?

/cc @gregestren @katre

[njlr]

Maybe examples under https://github.com/bazelbuild/examples/tree/main/configurations can help?

/cc @gregestren @katre

Thanks for the links.

Those appear to require implementing a custom rule. Is it possible to simply use genrule so as to avoid this?

[gregestren]

Hi @njlr,

Build settings work best with select() and with custom rules that are designed to know how to read them. You have a case where --define is more powerful: build settings don't automatically work with make variable substitution like --define does.

I can think of 3 approaches:

1. Just use --define in this case, in spite of the legacy documentation
2. Write some Starlark that tells Bazel how to connect your build setting into a cmd = $(foo) variable, then register that setting in your genrule's toolchains attribute (also see here)
3. Have your genrule use a select() like this

Each approach has its pluses and minuses. If you just want something quick and easy and don't want to have to add any extra boilerplate, I'd just stick with --define.

Otherwise I'd use option 3 if it works, with the biggest weakness that you need a config_setting for every possible value the flag might take.

Option 2 should work for any value but has the most boilerplate.