Catching Errors with External Commands in PowerShell and Azure DevOps
When calling an external command with PowerShell, how do you capture a printed error message?

Published on Thursday, March 5, 2020

Here's a quick tip for a problem I ran into within my Azure DevOps pipeline. I have a job task that executes a PowerShell script (Inline) and that script invokes a git push command to an Azure Kudu Git endpoint that deploys my site:

git push -u kudu HEAD:$env:GITBRANCH

I have multiple stages in my pipeline where GITBRANCH environment variable gets set to dev, master, etc. depending on the target environment I'm deploying to.

The Git command will print this kind of output within the task log:

remote: Updating branch 'master'.
remote: Updating submodules.
remote: Preparing deployment for commit id 'xxx'.
remote: Running custom deployment command...
remote: Running deployment command...
remote: Handling .NET Web Application deployment.

The problem becomes that even when the remote build fails, the git push command technically executes successfully with exit code 0 so PowerShell's $LASTEXITCODE check doesn't fail.

##[debug]$LASTEXITCODE: 0
##[debug]Exit code: 0

Since the command executed successfully, Azure does not fail the pipeline and I don't get notified when my builds actually fail.

To fix this, we need to somehow capture the output (but still preserve the logs) and check for a specific error string in the output to manually fail the pipeline.

We're looking for a log message like this in the output:

remote: An error has occurred during web site deployment.
remote:
remote: Error - Changes committed to remote repository but deployment to website failed.

If the string An error has occurred during web site deployment is present in the command output, we can fail the build.

So in our PowerShell script, we can take advantage of a neat cmdlet called Tee-Object which I had never heard of, inspired by this StackOverflow answer.

cmd /c "git push" '2>&1' | Tee-Object -Variable pushOutput

if ($null -ne ($pushOutput | ? { $_ -match 'An error has occurred during web site deployment' })) {
  Write-Error 'Build failed'
} else {
  Write-Verbose 'Build succeeded'
}

What Tee-Object does is redirect output to two places (like a T, get it? 🐺) so we get our output both logged and stored in a variable (a PowerShell string array).

What we can then do is operate against our variable $pushOutput and match any lines that contain our target string. If there's a match, it will not equal $null so the condition will pass and we can write an error.

⚠ HUGE ISSUE ALERT: Normally you would not need to include the cmd /c and '2>&1' portions of the script to execute the git push but because life is unfair, you can actually run into a specific edge case with git commands where Tee-Object does not create a variable when all the output directed to it is stderr output (which is unintuitively the case with a git push to Azure Kudu). It took me several hours of trial and error with different commands to stumble upon this StackOverflow answer that mentioned the edge case. By using cmd /c and then utilizing its redirection with 2>&1 it turns it all into stderr into stdout for PowerShell's consumption.

This will properly fail the Azure pipeline now if our remote build fails, hurray! 🤩

Hopefully this helps the one person running into the same issues I did!

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About Kamran

I'm a technologist, speaker, and Pluralsight author and I specialize in building full-stack solutions with a focus on modern web technology and cloud native architecture.

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