In PowerShell, how do I test whether or not a specific variable exists in global scope?

I'm using PowerShell scripts for some UI automation of a WPF application. Normally, the scripts are run as a group, based on the value of a global variable. It's a little inconvenient to set this variable manually for the times when I want to run just one script, so I'm looking for a way to modify them to check for this variable and set it if not found.

test-path variable:\foo doesn't seem to work, since I still get the following error:

> The variable '$global:foo' cannot be retrieved because it has not been set.

Test-Path can be used with a special syntax:

Test-Path variable:global:foo

This also works for environment variables ($env:foo):

Test-Path env:foo

And for non-global variables (just $foo inline):

Test-Path variable:foo

EDIT: Use stej's answer below. My own (partially incorrect) one is still reproduced here for reference:

---

You can use

Get-Variable foo -Scope Global

and trap the error that is raised when the variable doesn't exist.

Personal preference is to use Ignore over SilentlyContinue here because it's not an error at all. Since we're expecting it to potentially be $false let's prevent it (with Ignore) from being put (albeit silently) in the $Error stack.

You can use:

if (Get-Variable 'foo' -Scope 'Global' -ErrorAction 'Ignore') {
    $true
} else {
    $false
}

More tersely:

[bool](gv foo -s global -ea ig)

Output of either:

False

Alternatively

You can trap the error that is raised when the variable doesn't exist.

try {
    Get-Variable foo -Scope Global -ErrorAction 'Stop'
} catch [System.Management.Automation.ItemNotFoundException] {
    Write-Warning $_
}

Outputs:

WARNING: Cannot find a variable with the name 'foo'.

You can assign a variable to the return value of Get-Variable then check to see if it is null:

$variable = Get-Variable -Name foo -Scope Global -ErrorAction SilentlyContinue

if ($variable -eq $null)
{
    Write-Host "foo does not exist"
}

# else...

Just be aware that the variable has to be assigned to something for it to "exist". For example:

$global:foo = $null

$variable = Get-Variable -Name foo -Scope Global -ErrorAction SilentlyContinue

if ($variable -eq $null)
{
    Write-Host "foo does not exist"
}
else
{
    Write-Host "foo exists"
}

$global:bar

$variable = Get-Variable -Name bar -Scope Global -ErrorAction SilentlyContinue

if ($variable -eq $null)
{
    Write-Host "bar does not exist"
}
else
{
    Write-Host "bar exists"
}

Output:

foo exists
bar does not exist

Simple:

 [boolean](get-variable "Varname" -ErrorAction SilentlyContinue)

So far, it looks like the answer that works is this one.

To break it out further, what worked for me was this:

Get-Variable -Name foo -Scope Global -ea SilentlyContinue | out-null

$? returns either true or false.

$myvar = if ($env:variable) { $env:variable } else { "default_value" } 

You can try:

$global:foo -eq $null

This will return $true if the $global:foo is not set.

Test the existence of variavle MyVariable. Returns boolean true or false.

Test-Path variable:\MyVariable

There's an even easier way:

if ($variable)
{
    Write-Host "bar exist"
}
else
{
    Write-Host "bar does not exists"
}