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String Operations in GitHub Actions

Expressions in GitHub Actions pipelines have some gaps that can make simple operations rather frustrating. I've been bitten more than once while attempting to concatenate a string, or join an array of strings. Here's some string operations tips to add to your Actions toolbox.

Concatenating several strings #

Ever tried something like this?

  - run: |
    working-directory: ${{ join([env.TEMP_DB_FOLDER, env.PROJECT_NAME, 'artifacts'], '/') }}

    # => Unexpected symbol: '['

The problem here is not the join() function itself, it's that GitHub Actions expression syntax does not have any way to express an array literal. To call join() successfully you need to obtain an array from some other supported method... for example, by parsing it from a JSON string:

  - run: |
      echo ${{ join(fromJSON('["1", "2", "3"]'), ' and ') }}

      # => 1 and 2 and 3

However, this doesn't help much in our original example, because we need to inject these dynamic variables into the array. For simple cases like this it's probably easiest to avoid the function call altogether by breaking it into multiple expressions.

  - run: |
    working-directory: ${{ env.TEMP_DB_FOLDER }}/${{ env.PROJECT_NAME }}/artifacts

The format function #

In the example above, I showed how you could "concatenate" strings just by cutting them into multiple expressions. One situation where you can't do that is if you are already stuck inside an expression (perhaps a fake ternary operator expression).

The format() function can help out for simple string concatenation in this case. For example, here's the "working directory" example from above expressed as a format call:

  - run: |
    working-directory: ${{ format('{0}/{1}/{2}', env.TEMP_DB_FOLDER, env.PROJECT_NAME, 'artifacts') }}

Here's an example I've run into more than once, using a fake ternary operator that wraps a call to format().

    BASE_REF: ${{ github.base_ref && format('origin/{0}', github.base_ref) || github.event.merge_group.base_sha }}

In this case, we're checking if github.base_ref exists; if it does, I know this event is a pull request and I want to prepend it with origin/. If it doesn't exist, I know the event is a merge group event instead, and I don't prepend origin/ because the base will be a temporary commit SHA instead of a branch name.

Using join on splats (object filters) #

One special case where join() is actually useful is when you can perform a splat operation on a natural GitHub array, like a list of jobs you are dependent on.

For example, a common construct I write in workflows:

  # Run if any previous job failed
  if: ${{ contains(join(needs.*.result, ','), 'failure') }}

JSON-to-JSON property selection #

Again using a splat, we can actually "select" a property out of a JSON blob using just the JSON string functions. For example, suppose we have a job called get-projects that produces an output projects that looks like this:

    "project_folder": "apps/my-app",
    "infra_folder": "apps/my-app/infra/terraform"
    "project_folder": "apps/your-app",
    "infra_folder": "apps/your-app/infra/terraform"

I can "select out" an array of infra_folder entries and send that JSON array to another job:

    infra_folders: '${{ toJSON(fromJSON(needs.get-projects.outputs.projects).*.infra_folder) }}'

Checking if an array is empty #

GitHub Actions will generate an error if you attempt to run a matrix job and there are no jobs to run. This is one use case where it's useful to check if there are any entries in the array. Since there's no first-class support for arrays, we can abuse string operations for this once again:

  if: always() && fromJSON(needs.get-projects.outputs.projects)[0]

The easiest way to ask "is an array empty?" is to just access the first element, which (if it's null) will be falsy.