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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.

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最佳答案

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.

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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

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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

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There's an even easier way:

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

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Simple:

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

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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'.

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

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

Test-Path variable:\MyVariable

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You can try:

$global:foo -eq $null

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

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

Alternatively