How to handle errors for the commands to run in Start-Job?

Question

I am writing an automation script. I had a function which takes either a command or an executable. I had to wait until the command or executable has completed running and return

Answer 1

When in doubt, read the documentation:

$?
Contains the execution status of the last operation. It contains TRUE if the last operation succeeded and FALSE if it failed.

[…]

$LASTEXITCODE
Contains the exit code of the last Windows-based program that was run.

Basically, you need to check both. $? indicates whether the last PowerShell command/cmdlet was run successfully, whereas $LASTEXITCODE contains the exit code of the external program that was last executed.

if (-not $? -or $LASTEXITCODE -ne 0) {
    throw '... whatever ...'
} else {
    return $ret
}

However, Invoke-Expression is not a very good approach to executing commands. Depending on what you actually want to execute there are probably better ways to do it, with better methods for error handling.