What the heck is $< in a Bazel genrule?

While learning about Bazel and making progress on achieving my desired build configuration I kept encountering $< in genrule command attributes. Whenever the command would appear I mentally skipped over it because it appeared to be a shell variable unknown to me. Being curious, I did some digging on google under this assumption and didn’t find much. It’s difficult to search for symbol expressions and find meaningful results. Eventually I stumbled on some of the Bazel documentation which explains it’s usage.

When you construct a genrule target you see that if there is a singular source defined it can be referenced via $< in the command. These special variables come from the Makefile rules expansion. Here is an example of the rule in action when there is only one source file and one output file.

genrule(
    name = "copy-file",
    srcs = [
        "//some:file",
    ],
    outs = ["concatenated.txt"],
    cmd = "cp $< $@",
)

The above snippet is just shorthand for using $(locations ...) expansion as below.

# Same as above but using location expansion
genrule(
    name = "copy-file",
    srcs = [
        "//some:file",
    ],
    outs = ["concatenated.txt"],
    cmd = "cp $(location //some:file) $@",
)

If you have multiple sources listed in a genrule target you cannot reference them using $<. Instead, they need to be referenced with the longhand form so that the file is referenced unambiguously. The long form is $(locations //something/listed/in/srcs) as below. Notice that this is using the plural, locations, to find the file.

genrule(
    name = "concat_all_files",
    srcs = [
        "//some:files",  # a filegroup with multiple files in it ==> $(locations)
        "//other:gen",   # a genrule with a single output ==> $(location)
    ],
    outs = ["concatenated.txt"],
    cmd = "cat $(locations //some:files) $(locations //other:gen) > $@",
)

Lastly, you’ll see that a singular output is referenced as $@.

If you would like to see what other environment variables are available you can run $bazel info --show_make_env.

Hopefully this is helpful for you.