How can one detect if a github actions runner is running in ubuntu or windows

Question

Is it possible to detect which OS a Github actions runner is using? For example, in node one can run process.platform to get the OS. Is there something analogous inside of Github actions?

Answer 1

There is an environment variable $RUNNER_OS containing the operating system; you can specify the shell for a run: independent of the runner operating system, i.e., you can use Bash everywhere.

For example, this workflow

name: OS test

on:
  workflow_dispatch:

jobs:
  printos:
    name: Print OS name for each OS
    strategy:
      matrix:
        os:
          - ubuntu-20.04
          - windows-2022
          - macos-11
    runs-on: ${{ matrix.os }}
    steps:
      - name: Print OS name
        shell: bash
        run: |
          echo "$RUNNER_OS"

produces three jobs, with outputs Linux, Windows, and macOS, respectively.

The value is also available in the runner context, which can be used in, e.g., if conditionals:

      - name: Windows-specific step
        if: runner.os == 'Windows'
        shell: bash
        run: |
          echo "I am a Windows runner!"