Powershell array scope; why is my array empty?

Question

I want to have an array and add elements to it from different functions in my script. My example below illustrates where I might be misunderstanding something regarding scope. My understanding currently is that if an array is defined outside the function, and then elements are added inside the function, those elements should be available outside the function.

Function ListRunningServices
{
    $services+= Get-Service | ?{$_.Status -eq "Running"} | sort Name | select Name;
}

$services = @();
ListRunningServices;
$services;

What am I missing here? Perhaps my style is completely wrong.

mjolinor · Accepted Answer · 2013-12-11 14:26:31Z

7

You can solve this with global variables, but using globals is genereally considered bad practice. If you use a generic collection type, like arraylist, instead of an array then you have an add() method that will update the collection in a parent scope without needing to explicitly scope it in the function:

Function ListRunningServices
{
    Get-Service | ?{$_.Status -eq "Running"} | sort Name | select Name |
     ForEach-Object {$services.add($_)}
}

$services = New-Object collections.arraylist
ListRunningServices
$services

answered Dec 11, 2013 at 14:26

mjolinor

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W0lfw00ds · Accepted Answer · 2018-05-21 07:14:55Z

6

Powershell requires specifying the scope with prefixes on array variables. Seems that "normal String" variables don't. Use them like this:

# Visible everywhere inside the script file (f.ex: myNameScript.ps1)
$script:names = @()

Function AddStringToArray ([string]$i_name) {
    $script:names += $i_name
}

AddStringToArray -i_name "Markie"
AddStringToArray -i_name "Harry"

I know this is late, but I hope this helps at least someone.

answered May 21, 2018 at 7:14

W0lfw00ds

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jscott · Accepted Answer · 2013-12-11 14:03:22Z

5

The $services within the function block is scoped to the function. You can do something like the following instead:

Function ListRunningServices {
    Get-Service | ?{$_.Status -eq "Running"} | sort Name | select Name
}

$services = ListRunningServices
$services

Else, you may explicitly use global: to alter the scope:

Function ListRunningServices {
    $global:services = Get-Service | ?{$_.Status -eq "Running"} | sort Name | select Name
}

$services = @()
ListRunningServices
$services

answered Dec 11, 2013 at 14:03

jscott

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9 Comments

Mark Allison Over a year ago

I want to add to the services array from many different functions in my script. Which is the best practise way to do that? Use $global?

jscott Over a year ago

I would avoid using $global: if possible. Is there a reason you could not supply $services as a parameter to the other functions? It may help to see how you intend to interact with $services using the other functions.

Mark Allison Over a year ago

Just testing the $global solution you posted and it doesn't produce any output.

jscott Over a year ago

I've just copy/pasted the second example into a new PS session and it produces identical output to the first example.

Mark Allison Over a year ago

Interesting, if I paste into a Powershell window it works, but if I save into a file and then call the file as a script with .\file.ps1 I get no output.

|

JPBlanc · Accepted Answer · 2013-12-11 14:07:00Z

4

$service in the scope of the function has nothing to do with the one outside the function.

Try :

Function ListRunningServices
{
    $services = @()
    $services+= Get-Service | ?{$_.Status -eq "Running"} | sort Name | select Name;
    return $services
}


$services = ListRunningServices
$services

Keeping most of your code you can use :

Function ListRunningServices
{
    $global:services+= Get-Service | ?{$_.Status -eq "Running"} | sort Name | select Name;
}

$services = @();
ListRunningServices;
$services;

You can read the full explanation in About_scope.

edited Dec 11, 2013 at 14:07

answered Dec 11, 2013 at 14:00

JPBlanc

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