Working with Bash in Bazel

Bazel has two out-of-the-box rules, sh_test and genrule, that can be used for one-shot custom build targets. If there is some reusable logic behind your target, consider building a new rule in Skylark instead.

Platform

There are a number of limitations in the way Bazel will set up the environment for these Bash scripts when running them, and they differ based on the OS. The instructions given here will hopefully help you get to a point where your rules run on all platforms.

Dependencies

Bazel promises hermeticity, but Bash scripts have access to whatever they want. As far as I can tell they are not run in any kind of chroot or container (at least on macOS), although they are run with a restricted PATH (on Linux) and in a specially set-up folder.

As such, in order to get all of your dependencies available, you need to specify them in the relevant attributes:

srcs for (source) files required during build; note that in the case of sh_test, this must be a single-element list that contains just the script you intend to run.
data for files required at run time (sh_test only).
deps for results of other Bazel rules required at run time.
tools for executables (genrule only).

`genrule`: substitutions

The code of a genrule directive is the cmd attribute, which is a multiline string embedding a Bash script. Within that script, you need to use $$ to produce a single $ in the Bash script itself, as $() will be expanded by Bazel's own string interpolation. You can use the location and locations functions to get paths for Bazel build results. You should note that the paths these return is relative to the starting $PWD of the script, so if your Bash script cds somwhere, you have to take that into account.

You can include local files (files present in your workdir that are not the result of another Bazel rule) as part of your srcs by using the glob function, as in:

    srcs = glob([
        "package.json",
        "syntaxes/*",
    ]) + [
        "//some:rule",
    ],

The script will start running at the root of your workspace. While local files specified in srcs are specified using paths relative to the package root in the glob directive (i.e. relative to the containing directory of the BUILD.bazel file); they are added to the run time directory of the script using their package-qualified path. If the above snippet came from my/package/BUILD.bazel, the package.json file would be at my/package/package.json.

Note that tools and data dependencies will also be given as relative (to the script's starting directory) paths through the location and locations functions.

Conditionals

Bazel supports simple conditionals in its rules, though I have not found a way to use them in string substitutions. As some rules have different names under Windows, this creates a bit of a problem. Aliases provide a way around that; for example:

alias(
    name = "yarn",
    actual = "@nodejs//:bin/yarn.cmd" if is_windows else "@nodejs//:bin/yarn",
)

would let you use, in the cmd entry of a script:

        $$DIR/$(location :yarn)

to invoke the correct yarn executable regardless of platform.

Links and speed

Bazel relies a lot on symbolic links to provide what should appear like a new, isolated environment to each build rule without having to pay the cost of copying files over each time. However, this means that

Everything path-related is different on Windows, because symlinks there are not well-supported, and
While Bazel can claim some nice speed for creating the environment, every read your rule does from a Bazel-provided file takes the hit of going through the symlink indirection. That second point can be very costly when you have to read many small files (e.g. node dependencies).

`sh_test`: "runfiles"

Whereas genrule takes an inline Bash script as its cmd attribute, the sh_test script will run a Bash script defined in a separate file and specified as the single entry in the srcs attribute. This adds the complication that location substitution is not available.

A partial workaround for that is to pass the relevant paths to the script as the args attribute of the sh_rule; those will be available as traditional arguments in Bash ($@, $#, $1, etc.).

However, in order for the test script to work under Windows (where symlinks are not well supported), another level of indirection is introduced by Bazel: the paths returned by the location function cannot be used directly by the script given to sh_test, and must further be translated to "real", absolute paths through the use of an rlocation Bash function.

To make this function available to your Bash script, the sh_test rule must include an additional dependency:

deps = ["@bazel_tools//tools/bash/runfiles"],

Then, anything your script needs access to, including executables, needs to be passed in as arguments (using location in the BUILD.bazel file). In the script itself, before using any of the arguments, you have to paste the following snippet from the Bazel runfiles:

    # Copy-pasted from the Bazel Bash runfiles library v2.
    set -uo pipefail; f=bazel_tools/tools/bash/runfiles/runfiles.bash
    source "${RUNFILES_DIR:-/dev/null}/$f" 2>/dev/null || \
      source "$(grep -sm1 "^$f " "${RUNFILES_MANIFEST_FILE:-/dev/null}" | cut -f2- -d' ')" 2>/dev/null || \
      source "$0.runfiles/$f" 2>/dev/null || \
      source "$(grep -sm1 "^$f " "$0.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
      source "$(grep -sm1 "^$f " "$0.exe.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
      { echo>&2 "ERROR: cannot find $f"; exit 1; }; f=; set -e
    # --- end runfiles.bash initialization v2 ---

After that, in your Bash script, you can construct the absolute paths by taking the values Bazel gave you and expanding them like (assuming the first argument specified in the BUILD.bazel file was a $(location ...) call) :

MY_FIRST_ARG="$(rlocation "$TEST_WORKSPACE/$1")"

The TEST_WORKSPACE variable is set by Bazel to the current WORKSPACE name. Note that you should use this name even for dependencies from external namespaces.

Note that you should use the above method to access all of your dependencies, including "local" files from the same package as the sh_test. Trying to access those files directly by their expected path, as you can do in genrules, will not work with sh_test on Windows. You may have to wrap your local files in a filegroup to have a valid target for the $(location ...) call you should pass as an args to the sh_test.

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