PowerShell: Manage errors with Invoke-Expression - powershell

Question

I try to figure how to determine if a command throw with Invoke-Expression fail.
Even the variable $?, $LASTEXITCODE or the -ErrorVariable don't help me.
For example :
PS C:\> $cmd="cat c:\xxx.txt"
Call $cmd with Invoke-Expression
PS C:\> Invoke-Expression $cmd -ErrorVariable err
Get-Content : Cannot find path 'C:\xxx.txt' because it does not exist.
At line:1 char:4
+ cat <<<< c:\xxx.txt
+ CategoryInfo : ObjectNotFound: (C:\xxx.txt:String) [Get-Content], ItemNotFoundExcep
tion
+ FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Commands.GetContentCommand
The $? is True
PS C:\> $?
True
The $LASTEXITCODE is 0
PS C:\> $LASTEXITCODE
0
And the $err is empty
PS C:\> $err
PS C:\>
The only way I found is to redirect STD_ERR in a file and test if this file is empty
PS C:\> Invoke-Expression $cmd 2>err.txt
PS C:\> cat err.txt
Get-Content : Cannot find path 'C:\xxx.txt' because it does not exist.
At line:1 char:4
+ cat <<<< c:\xxx.txt
+ CategoryInfo : ObjectNotFound: (C:\xxx.txt:String) [Get-Content], ItemNotFoundExcep
tion
+ FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Commands.GetContentCommand
Is it the only and best way to do this ?

Answer 1

I was going crazy trying to make capturing the STDERR stream to a variable work. I finally solved it. There is a quirk in the invoke-expression command that makes the whole 2&>1 redirect fail, but if you omit the 1 it does the right thing.
function runDOScmd($cmd, $cmdargs)
{
# record the current ErrorActionPreference
$ep_restore = $ErrorActionPreference
# set the ErrorActionPreference
$ErrorActionPreference="SilentlyContinue"
# initialize the output vars
$errout = $stdout = ""
# After hours of tweak and run I stumbled on this solution
$null = iex "& $cmd $cmdargs 2>''" -ErrorVariable errout -OutVariable stdout
<# these are two apostrophes after the >
From what I can tell, in order to catch the stderr stream you need to try to redirect it,
the -ErrorVariable param won't get anything unless you do. It seems that powershell
intercepts the redirected stream, but it must be redirected first.
#>
# restore the ErrorActionPreference
$ErrorActionPreference=$ep_restore
# I do this because I am only interested in the message portion
# $errout is actually a full ErrorRecord object
$errrpt = ""
if($errout)
{
$errrpt = $errout[0].Exception
}
# return a 3 member arraylist with the results.
$LASTEXITCODE, $stdout, $errrpt
}

Answer 2

It sounds like you're trying to capture the error output of a native in a variable without also capturing stdout. If capturing stdout was acceptable, you'd use 2>&1.
Redirecting to a file might be the simplest. Using Invoke-Expression for it's -ErrorVariable parameter almost seems like a good idea, but Invoke-Expression has many problems and I usually discourage it.
Another option will look a little cumbersome, but it can be factored into a function. The idea is to merge output streams using 2>&1, but then split them again based on the type of the object. It might look like this:
function Split-Streams
{
param([Parameter(ValueFromPipeline=$true)]$InputObject)
begin
{
$stdOut = #()
$stdErr = #()
}
process
{
if ($InputObject -is [System.Management.Automation.ErrorRecord])
{
# This works well with native commands but maybe not as well
# for other commands that might write non-strings
$stdErr += $InputObject.TargetObject
}
else
{
$stdOut += $InputObject
}
}
end
{
,$stdOut
,$stdErr
}
}
$o, $e = cat.exe c:\xxx.txt 2>&1 | Split-Streams