..mutable-in-out-dir-bazel-example.md

.envrc

use nix -p bazel_7

.gitignore

.direnv/
/bazel-*
MODULE.bazel.lock

build.bash

#!/usr/bin/env bash

# $1: src dir
# $2: bld dir
#
# reads $src/inputs/a
# reads $bld/existing_outputs/b
# produces $bld/my_outputs/*

set -euo pipefail

# set -x # to show progress (debugging)

readonly src="$1" bld="$2"

input_a="${src}/inputs/a"
input_b="${bld}/existing_outputs/b"

out_dir="$bld/my_outputs"
rm -rf "$out_dir"
mkdir -p "$out_dir"

cat "$input_a" > "${out_dir}/a"
cat "$input_b" > "${out_dir}/b"
cat "$input_a" "$input_b" > "${out_dir}/both"

BUILD.bazel

# NOTE: Normally would need to use `new_local_repository`..
filegroup(
    name = "src",
    srcs = ["//:src/inputs/a"], # can use glob
)
filegroup(
    name = "bld",
    srcs = ["//:bld/existing_outputs/b"], # can use glob
)

genrule(
    name = "test",
    tools = [
        ":build.bash",
    ],
    srcs = [
        ":src",
        ":bld",

        # for the purpose of getting at the dir locations..
        ":src/inputs/a",
        ":bld/existing_outputs/b",
    ],
    outs = ["out.tar.gz"],

    # NOTE: naive approach; doesn't work because we cannot modify
    # `$(location :bld)`.
    # cmd = "; ".join([
    #     "$(location :build.bash) $(location :src) $(location :bld)"
    # ])

    cmd = "; ".join([
        # set up a temp dir:
        "tmp=$$(mktemp -d)",

        # get path to just `src` and just `dst` — not `src/inputs/a`, etc.
        "src=$$(dirname $$(dirname $(location :src/inputs/a)))",
        "bld=$$(dirname $$(dirname $(location :bld/existing_outputs/b)))",

        # copy inputs to our temp dir to accommodate the script that mutates the
        # `bld` dir which has both inputs and outputs:
        'cp -R "$$src" "$$tmp/src"',
        'cp -R "$$bld" "$$tmp/bld"',

        # run the script, pointing at our copied directories:
        # '$(location :build.bash) "$$tmp/src" "$$tmp/bld" 2>&1',

        # NOTE: in this case we actually don't to make a copy of `src`: it is
        # not modified by the `build.bash` script! i.e. this works too:
        '$(location :build.bash) "$$src" "$$tmp/bld" 2>&1',

        # for debugging:
        'tree $$tmp',

        # NOTE: normally we'd do this to copy back the outputs for Bazel...
        # unfortunately we can't because genrules are not allowed to have
        # directory outputs.
        # 'cp -R $$bld/my_outputs $(location :out)'

        # Instead we'll make a tarball with out outputs of interest:
        'out="$(location :out.tar.gz)"',
        'mkdir -p "$$(dirname "$$out")"',
        'tar cvf "$$out" -C "$$tmp" "bld/my_outputs"',
    ])
)

MODULE.bazel