Issuing workflow commands from the Windows shell in GitHub Actions

It's a little-known fact that the default shell in for GitHub Actions is different depending on the operating system on which you run. And that the syntax to set a variable, for example, differs too.

The docs explain the syntax for Bash and PowerShell:
Workflow commands for GitHub Actions - GitHub Docs

Below is an example of the different syntax required to set an output variable in each of the most common shells:

jobs: windows: runs-on: windows-latest steps: # Runs on PowerShell Core - name: Set an output variable on Windows run: | echo foo=bar >> $env:GITHUB_OUTPUT linux: runs-on: ubuntu-latest steps: # Runs on Bash - name: Set an output variable on Linux run: | echo foo=bar >> $GITHUB_OUTPUT 

When you always target the same OS, it won't matter much, but if you have multi-platform jobs, matrix jobs, reusable templates, and composite actions, it pays to be explicit. That way your bash script won't suddenly be executed by PowerShell or vice versa.

You aren't limited to pwsh on windows or bash on linux and mac either. As long as they are installed on the Runner, you can run either on each OS.

jobs: windows: runs-on: windows-latest steps: # Runs on Bash - name: Set an output variable on Windows run: | echo foo=bar >> $GITHUB_OUTPUT shell: bash - name: Set an output variable on Windows run: | echo foo=bar >> $env:GITHUB_OUTPUT shell: pwsh - name: Set an output variable on Windows # Requires explicitly setting the output encoding run: | "foo=bar" | Out-File -FilePath $env:GITHUB_OUTPUT -Encoding utf8 -Append shell: powershell linux: runs-on: ubuntu-latest steps: # Runs on Bash - name: Set an output variable on Linux run: | echo foo=bar >> $GITHUB_OUTPUT - name: Set an output variable on Linux run: | echo foo=bar >> $env:GITHUB_OUTPUT shell: pwsh 

Today someone asked what the correct syntax would be to do this on the shell: cmd old Windows Command Shell. I couldn't find that in the docs.

To be honest, since I wrote half of that doc, I was certain I didn't add that information...

But after some experimentation I found out you need to do the same things you'd do for the old PowerShell. Set the encoding to utf-8 and use the correct environment variable syntax:

jobs: windows: runs-on: windows-latest steps: - shell: cmd run: | @chcp 65001>nul echo foo=bar >> %GITHUB_OUTPUT% 

First published here.