Global variables and local variables in PowerShell

Question

I have global variables and want to use them within a function.

I don't use local variables with the same name within the functions!

# Global variables:
$Var1 = @{ .. }
$Var2 = @( .. )
function testing{
    $Var1.keyX = "kjhkjh"
    $Var2[2]   = 6.89768
}

I do it and it works, but is it safe or do I have to use the following?

$Global:Var1.keyX = "kjhkjh"

Keith Hill · Accepted Answer · 2014-06-26 20:44:28Z

23

In your function, you are modifying the contents of the hashtable so there is no need to use $global unless your function (or a function caller between your function and global scope) happens to have local variables $Var1 and $Var2 (BTW aren't you missing $). If this is all your own code then I'd say leave it as is. However, if you code allows other folks' code to call your function, then I would use the $global:Var1 specifier to make sure you're accessing the global variable and not inadvertently accessing a variable of the same name within a function that is calling your function.

Another thing to know about dynamic scoping in PowerShell is that when you assign a value to variable in a function and that variable happens to be a global e.g.:

$someGlobal = 7
function foo { $someGlobal = 42; $someGlobal }
foo
$someGlobal

PowerShell will do a "copy-on-write" operation on the variable $someGlobal within the function. If your intent was to really modify the global then you would use the $global: specifier:

$someGlobal = 7
function foo { $global:someGlobal = 42; $someGlobal }
foo
$someGlobal

answered Jun 26, 2014 at 20:44

Keith Hill

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2 Comments

gooly Over a year ago

Thank you Keith (I changed my typo)! It is exactly what was a bit confusing: arrays and hashes (what else?) can be changed within a function without using $global: but normal variables cannot.

Keith Hill Over a year ago

Basically you can change the contents of a referenced object (properties on an object, elements in an array, elements in a hashtable). What you can't change without use $global:<varname> is the actual object (array/hashtable) the variable references.

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