How to get the short sha for the github workflow?

You can also set an environment variable with the short sha:

    - name: Add SHORT_SHA env property with commit short sha
      run: echo "SHORT_SHA=`echo ${GITHUB_SHA} | cut -c1-8`" >> $GITHUB_ENV

SHORT_SHA can then be used like any other environment variable, e.g. like this:

    - name: My step
      run: myscript ${SHORT_SHA}

Found a possible solution here, using bash's parameter substitution

- name: Step
  run: echo ${GITHUB_SHA::7}

As VonC mentioned, you can just compute the string yourself in a previous step.

      - name: Set outputs
        id: vars
        run: echo "::set-output name=sha_short::$(git rev-parse --short HEAD)"
      - name: Check outputs
        run: echo ${{ steps.vars.outputs.sha_short }}