How To Get Exit Code From PowerShell Script

Question

I'm trying to write a PowerShell script which sends a Slack notification whenever our build system in unable to push Docker images to Docker Hub. How can I detect and handle the Process exited with code 1 from the docker push command? I've tried the following, but the catch block isn't called.

try {
  docker push $dockerImage;
}catch{
  #Send error details to Slack
}

Does this answer your question? PowerShell and process exit codes — iRon
– iRon, Commented May 28, 2021 at 17:38

mklement0 · Accepted Answer · 2024-08-27 18:20:21Z

6

In Windows PowerShell and PowerShell (Core) up to version 7.3.x, calls to external programs (such as docker) never cause a statement-terminating error^[1] that you can trap with try / catch; similarly, stderr output from external program is (rightfully) not considered error output by PowerShell, and therefore $ErrorActionPreference = 'Stop' has no effect on it (for legacy exceptions in Windows PowerShell, see this answer).
- In PowerShell (Core) v7.4+, opt-in integration with PowerShell's error handling is available, via the $PSNativeCommandUseErrorActionPreference preference variable.
  - If you set this variable to $true, any external-program call that reports a nonzero exit code automatically triggers a PowerShell error in response (obviating the need for explicit $LASTEXITCODE -ne 0 checks), which then integrates with PowerShell's own error handling.^[2]
    That is, with the $ErrorActionPreference preference variable set to 'Stop', any external program reporting a nonzero exit code will then result in a script-terminating (fatal by default) error.
- In general, note that a statement-terminating PowerShell error (which can be caught with try / catch) does occur if PowerShell fails to even launch a command, such as when you supply a non-existent executable name or path; e.g.:
```
try   { nosuchexe foo bar }
catch { Write-Warning "Failed to invoke nosuchexe: $_" }
```
The automatic $LASTEXITCODE variable reflects the process exit code of the most recently executed external program. By convention, exit code 0 signals success, anything else an error condition.

Therefore:

docker push $dockerImage
if ($LASTEXITCODE -ne 0) {
  #Send error details to Slack
}

See also:

This answer has more information on exit-code processing in PowerShell.

^{[1] The only exception is a bug, present in Windows PowerShell and historically in PowerShell (Core) up to the no-longer-supported v7.1.x, where the combination of redirecting stderr output (2> or *>) with $ErrorActionPreference = 'Stop' unexpectedly causes a script-terminating error if there's actual stderr output - see this answer.}

^{[2] Unfortunately, as of PowerShell 7.4.x, the automatically triggered PowerShell error is a non-terminating error rather than a statement-terminating one; the latter would make more sense, as it would allow it to be selectively caught with a try statement - see GitHub issue #18368.}

edited Aug 27, 2024 at 18:20

answered May 28, 2021 at 17:36

mklement0

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2 Comments

not2qubit Over a year ago

I don't think it's a good idea have large random pos/neg numbers as failing result codes from `$LASTEXITCODE.

mklement0 Over a year ago

@not2qubit, indeed, small, positive numbers are typical, and, for cross-platform use you're limited to positive numbers between 0 and 255. Technically, however, process exit codes on Windows can be any signed 32-bit integer value ([int]/ System.Int32, in PowerShell / .NET terms).

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