Error Handling

There can be levels of severity to the errors which determine if script should be stopped etc and totally depends your approach.

Errors

You can get different types of errors.
For example, if you are trying to write to a file, you might get errors like. 1. File does not exist 2. You are not allowed to write to the file

In PowerShell, there are 2 types of errors.

Terminating Errors

This error can be handled by using Try / Catch blocks and if these are not handled, the script will terminate at the statement which caused the error.

Non Terminating Errors

These errors don't terminate the script.

Managing Errors by Using Try/Catch/Finally

Try

You can encapsulate the potential error prone statements into a try block

Try {
   # Statement. For example, call a command.
   # Another statement. For example, assign a variable.
}

Catch

The try block may raise an error.
In this case, while trying to write to a file, we can get a System.IO.IOException.
The Catch block catches this and tries to recover.
The Catch block in the end catches all exceptions other than System.IO.IOException.

Try {
   # Do something with a file.
} Catch [System.IO.IOException] {
   Write-Host "Something went wrong"
}  Catch {
   # Catch all. It's not an IOException but something else.
}

Finally

This is used in the end for clean up purposes.

Try {
   # Do something with a file.
} Catch [System.IO.IOException] {
   Write-Host "Something went wrong"
}  Catch {
   # Catch all. It's not an IOException but something else.
} Finally {
   # Clean up resources.
}

Inspecting Errors

There is a built-in variable (or object) that is $_

Try {
     # Do something with a file.
   } Catch [System.IO.IOException] {
     Write-Host "Something IO went wrong: $($_.exception.message)"
   }  Catch {
     Write-Host "Something else went wrong: $($_.exception.message)"
   }

Raising Errors

Try {
   Get-Content './file.txt' -ErrorAction Stop
} Catch {
   Write-Error "File can't be found"
}

We used ErrorAction with value Stop so that the try / catch block can deal with the error.

Try {
   If ($Path -eq './forbidden') 
   {
     Throw "Path not allowed"
   }
   # Carry on.

} Catch {
   Write-Error "$($_.exception.message)" # Path not allowed.
}

We can also used a Throw block to raise an error.