How to output cache key to GITHUB_OUTPUT in Github Cache action?

In my Github workflow, I am using cache across two jobs. The cache key is generated with the help of ${{ hashFiles(...) }} function. In the second job I need this cache key name in order to skip repeated action for retrieving arguments for hashFiles, and go straight to downloading the cache. But I can't do it, why doesn't this action output a cache key name?

Solution

why doesn't this action output a cache key name?

This question is well-suited for that action. You may create an issue on its repo (if there isn't one already) to ask this or maybe suggest it as a feature.

Apart from that, apparently hashFiles seems to work only in certain contexts i.e. it cannot be used in a separate step to calculate it once and then shared across jobs via an output parameter.

However, you may calculate it manually in a separate step without hashFiles as described in the docs:

This function calculates an individual SHA-256 hash for each matched file, and then uses those hashes to calculate a final SHA-256 hash for the set of files. If the path pattern does not match any files, this returns an empty string. For more information about SHA-256, see "SHA-2."

For example, in Bash you can do something like this using find and sha256sum utilities and then share the output between jobs:

FILENAME="<FILENAME>"
COMBINED_HASHES=$(echo $(find . -type f -name "$FILENAME" -exec bash -c "sha256sum {} | cut -d ' ' -f1" \;) | tr -d '\n')
HASH=$(sha256sum <<< "$COMBINED_HASHES" | cut -d ' ' -f1)

echo "HASH=$HASH" >> "$GITHUB_OUTPUT"

Depending on your use case, you may further refine it accordingly.

UPDATE (May 10th, 2023)

Another solution could be to use https://github.com/actions/github-script with glob which provides an implementation for hashFiles also.

Here's a working example:

name: github_script_hash_files_test

on: workflow_dispatch

jobs:
  hash:
    runs-on: ubuntu-latest

    outputs:
      result: '${{ steps.hash.outputs.result }}'

    steps:
      - name: Checkout
        uses: actions/checkout@v3

      - name: Hash
        id: hash
        uses: actions/github-script@v6
        with:
          result-encoding: string
          script: |
            return glob.hashFiles('**/README.md');

  check:
    runs-on: ubuntu-latest
    needs: [hash]

    steps:
      - name: Check
        env:
          HASH: '${{ needs.hash.outputs.result }}'
        run: |
          echo "HASH: [$HASH]"