I'm trying to use a part of current branch name as a tag for publishing docker image in CI pipeline. The problem is that in my org, there is a convention to name branches like feature/foo, and docker tags can't contain slashes. So I want to split the string on '/' symbol and take the last part.

The full branch name, with slashes, should be in variable github.ref_name. Here is the expression I'm trying to evaluate: ${{ github.ref_name.split('/')[-1] }}.

I get this error: Unable to interpolate expression 'format('{0}', github.ref_name.split('/')[-1])': Failed to parse: unexpected token "(" while parsing arguments of function call. expecting ",", ")"

What are the options for manipulating strings in github actions expression? I didn't find it in docs https://docs.github.com/en/actions/learn-github-actions/expressions

IMPROVED:

This technique doesn't need to bring in anything from the actions marketplace.

Add this to steps: in an action:

      - name: Split branch name
        env:
          BRANCH: ${{ github.ref_name }}
        id: split
        run: echo "fragment=${BRANCH##*/}" >> $GITHUB_OUTPUT

It captures the output of a shell command. The only necessary actions are setting github.ref_name to an environmental variable and using parameter expansion to get the part of the branch name you want.

• ## greedily removes the subsequent pattern off the front
• */ matches anything followed by a /
• So, ##*/ removes everything up to and including the final forward slash.

Because id is split, and name=fragment, you then reference your split string with steps.split.outputs.fragment. For example,

      # Another step
      - name: Test variable
        run: |
          echo ${{ steps.split.outputs.fragment }}

Other parameter expansion features would be useful here, like #, %, and %% in addition to ##.

220829: Final update

Originally I spawned another process with $(echo ${BRANCH##*/}) but that is unnecessary. The variable can be directly referenced.

---

ORIGINAL:

I'm doing something essentially the same and using an action was the easiest temporary workaround, however I also would have preferred something built-in like a string-splitting expression. For now here is what worked.

      - uses: jungwinter/split@master
        id: branch
        with:
          msg: ${{ github.ref_name }}
          separator: "/"
          maxsplit: -1

From its repo:

Outputs

• _0, _1, ..., _n: Each result of a split
  • According to metadata syntax of outputs, it has _ prefix
  • Currently, support only 100 splits
• length: Length of the splits

So, then I can reference each portion of the string with steps.branch.outputs._n, with branch being the id and n being the split index.

I was looking for the first instead of the last value in the branch name, so I could set maxsplit to 1 and just take _0. In your case you might need to do some magic with checking length and choosing the index from there.