在 PowerShell 脚本中替换 2>&1

Question

We use the below function in one of our PowerShell scripts. Can someone explain what "2>&1" means here? Is there any equivalent full cmdlet to replace this value?

function InvExp {
    param (
        [string]$cmd
    )
        $er = (invoke-expression $cmd) 2>&1
        if($er.Exception){
            throw $er.Exception
        }
} 

Answer 1

As Santiago Squarzon notes, 2>&1 is a redirection ( > ) that redirects output stream number 2 (the error stream) into ( & ) stream number 1 (the success stream) .

• When calling external programs , 2 in redirections refers to such a program's stderr output, and 1 (the default stream) to its stdout output.

What your function in effect does is to promote any error - irrespective of whether it is a non -terminating or a (statement-) terminating error - to a fatal error ( script -terminating aka runspace-terminating error).

However, I propose a simpler implementation , which also avoids the use of Invoke-Expression , which is generally to be avoided , by instead requiring that a script block ( { ... } ) rather than a string be passed as an argument:

function Assert-NoError {
  param(
    [scriptblock] $cmd
  )
  $ErrorActionPreference = 'Stop'
  . $cmd
}

You would then invoke the function as follows, for example:

Assert-NoError { Get-Process test }

Note:

• $ErrorActionPreference = 'Stop' creates a local instance of the the $ErrorActionPreference preference variable , and by setting it to Stop ensures that any error results in a fatal error by default.

• The above implementation has the added advantage that any success output that precedes an error is output before execution is aborted.