Powershell enters foreach loop with null object

Question

I'm listing all backups in a given directory:

$backups = Get-ChildItem -Path $zipFilepath | 
Where-Object {($_.lastwritetime -lt (Get-Date).addDays(-7)) -and 
(-not $_.PSIsContainer) -and ($_.Name -like "backup*")}

If I set it to deliberately return no files (.addDays(-600) then the following prints "Empty" (as expected):

if (!$backups)
{
    "Empty"
}

If I try to list the names with an empty $backups variable:

foreach ($file in $backups)
{
    $file.FullName;
}

I get nothing (as expected), if I change this to:

"test"+ $file.FullName;

then I get a single "test" under the "Empty". How is this possible if $backups is empty?

jscott · Accepted Answer · 2012-12-11T17:20:40.740

4

This is a "feature" of Powershell. Your $backups may contain multiple $null values, as well as non-null, so Powershell must iterate the object to process the foreach. The fact the we both see $backup as a single-value scalar ($null in this case) doesn't matter, to MS at least... :)

See this Microsoft Connect Bug Report for more details -- It is "fixed" in Powershell v3.

On v2:

PS H:\> foreach ( $i in $null ) { "Hi" }
Hi

On v3:

PS H:\> foreach ( $i in $null ) { "Hi" }

(No output)

edited Dec 11 '12 at 17:20

answered Dec 11 '12 at 17:12

jscott

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I get an output of "Hi" and my $Host.Version gives 3.2.0.2237 – SteB Dec 11 '12 at 17:42
@SteB I cannot find that version of PowerShell -- are you using PowerGui? Can you provide `$Host.Version` from the actual `powershell.exe` binary, or even `$PSVersionTable`? – jscott Dec 11 '12 at 17:51
Yes, I am using PowerGUI. My powershell.exe in WindowsPowerShell/v1.0 is 6.1.7600.16385. $PSVersionTable gives CLRVersion: 4.0.30319.17929 and WSManStackVersion: 2.0 – SteB Dec 11 '12 at 17:57
@SteB Yep, your PowerGUI is v3.2.0.2237, your Powershell is v2. `$PSVersionTable` should have listed `PSVersion` as well, but your `powershell.exe` binary 6.1.7600.16385 confirms v2. – jscott Dec 11 '12 at 17:58
You can see `Get-Help about_Automatic_Variables`, in particular the `$Host` section, to understand how PowerGUI is returning 3.2.0.2237 for `$Host.Version` instead of the "real" `powershell.exe` version. – jscott Dec 11 '12 at 18:10
It appears that I'm only running v2 (I used $psversiontable), I feel an upgrade coming on :) Thanks for your help. – SteB Dec 12 '12 at 09:15

Powershell enters foreach loop with null object

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