Github Actions Error; cannot see git diff to Master

I am trying to find a way to lint only markdown files that have changed on the current branch. I wrote a bash script that runs fine locally, but breaks in Github Actions.

Expected Outcome: lint only markdown files that have been changed on the branch in GitHub Actions on pull-request.

#!/bin/bash
files=`git diff --name-only master`
for x in $files;
do
    if [ ${x: -3} == ".md" ]
    then
        node_modules/.bin/markdownlint $x
    fi
done

I call the script in package.json

"scripts": {
    "test": "bash mdlint.sh"
  }

I then call the bash script in GitHub Actions workflow:

name: CI

on: 
  pull_request:
    branches:
    - master

jobs:
  build:
    runs-on: ubuntu-latest

    steps:
    - uses: actions/checkout@v1
      with:
        fetch-depth: 1
    - uses: actions/setup-node@master
    - name: lint all markdownfiles
      run: |
        npm install
        npm run test

This is the Error in GitHub Actions:

  shell: /bin/bash -e {0}
npm WARN Gatsby_site No repository field.
npm WARN Gatsby_site No license field.

added 33 packages from 26 contributors and audited 43 packages in 0.952s
found 0 vulnerabilities


> @ test /home/runner/work/Gatsby_site/Gatsby_site
> bash mdlint.sh

fatal: ambiguous argument 'master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

Is there a way to make GitHub actions run the linter only on the files that changed on the current branch?

I think you can resolve it by associating master with origin master like this:

git fetch origin master:master
git diff --name-only master

Be aware that events that trigger on: pull_request workflows are not associated with a branch, they are associated with a merge commit SHA. So actions/checkout@v1 by default will checkout the merge commit specified by GITHUB_SHA, not the branch that is merging into the base. See the documentation here.

You can override this default behaviour and checkout the branch merging into the base like this:

      - uses: actions/checkout@master
        with:
          ref: ${{ github.head_ref }}

I'm not sure if this will be necessary for your use case, but there might be unintended effects when running diff on the merge commit.

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