How to modify parent scope variable using Powershell

The parent scope can actually be modified directly with Set-Variable -Scope 1 without the need for Script or Global scope usage. Example:

$val = 0
function foo {
    Set-Variable -scope 1 -Name "Val" -Value "10"
}
foo
write "The number is: $val"

Returns:

The number is: 10

More information can be found in the Microsoft Docs article About Scopes. The critical excerpt from that doc:

Note: For the cmdlets that use the Scope parameter, you can also refer to scopes by number. The number describes the relative position of one scope to another. Scope 0 represents the current, or local, scope. Scope 1 indicates the immediate parent scope. Scope 2 indicates the parent of the parent scope, and so on. Numbered scopes are useful if you have created many recursive scopes.

Be aware that recursive functions require the scope to be adjusted accordingly:

$val = ,0
function foo {
    $b = $val.Count
    Set-Variable -Name 'val' -Value ($val + ,$b) -Scope $b
    if ($b -lt 10) {
        foo
    }
}

Let me point out a third alternative, even though the answer has already been made. If you want to change a variable, don't be afraid to pass it by reference and work with it that way.

$val=1
function bar ($lcl) 
{
    write "In bar(), `$lcl.Value starts as $($lcl.Value)"
    $lcl.Value += 9
    write "In bar(), `$lcl.Value ends as $($lcl.Value)"
}
$val
bar([REF]$val)
$val

That returns:

1
In bar(), $lcl.Value starts as 1
In bar(), $lcl.Value ends as 10
10

If you want to use this then you could do something like this:

$global:val=0 
function foo()
{
    $global:val=10 
}
foo
write "The number is: $val"

Perhaps the easiest way is to dot source the function:

$val = 0
function foo()
{
    $val = 10
}
. foo
write "The number is: $val"

The difference here being that you call foo via . foo.

Dot sourcing runs the function as normal, but it runs inside the parent scope, so there is no child scope. This basically removes scoping. It's only an issue if you start setting or overwriting variables/definitions in the parent scope unintentionally. For small scripts this isn't usually the case, which makes dot sourcing really easy to work with.

If you only want a single variable, then you can use a return value, e.g.,

$val = 0
function foo()
{
    return 10
}
$val = foo
write "The number is: $val"

(Or without the return, as it's not necessary in a function)

You can also return multiple values to set multiple variables in this manner, e.g., $a, $b, $c = foo if foo returned 1,2,3.

The above approaches are my preferred way to handle cross-scope variables.

As a last alternative, you can also place the write in the function itself, so it's writing the variable in the same scope as it's being defined.

Of course, you can also use the scope namespace solutions, Set-Variable by scope, or pass it via [ref] as demonstrated by others here. There are many solutions in PowerShell.