Do Bazel genrules offer a temp directory?

Question

Does Bazel offer a variable substitution for a temp directory in genrules?

Sometimes I need a staging area before creating the final output artefact.

I am imagining something like this:

genrule(
    name = "example",
    srcs = [ "a.txt" ],
    cmd = "cp $< $(TMP)/b.txt && cp $(TMP)/b.txt $@",
)

$(TMP) would be a folder generated for me by Bazel on each rule execution.

Answer 1

No it doesn't. (As of Bazel 0.23.1)

It does set $TMPDIR though (even with --incompatible_strict_action_env), so mktemp should work. But $TMPDIR is by no means a dedicated temp directory (it's often just /tmp), so be careful what you clobber.

Answer 2

I migrated my genrule to a full Starlark rule. There I can do

tmp = ctx.actions.declare_directory("TMP_" + ctx.label.name)

and just use that directory as my temp in further actions.

It is similar to what the Starlark tutorial shows, in https://docs.bazel.build/versions/2.0.0/skylark/rules-tutorial.html#creating-a-file. The difference is that I do not register that directory as an output. That is, I don't do something like

return [DefaultInfo(files = depset([tmp]))]