About Scopes

Short description

Explains the concept of scope in PowerShell and shows how to set and change the scope of elements.

Long description

PowerShell protects access to variables, aliases, functions, and PowerShell drives (PSDrives) by limiting where they can be read and changed. PowerShell uses scope rules to ensure that you do not inadvertently change an item that should not be changed.

The following are the basic rules of scope:

• Scopes may nest. An outer scope is referred to as a parent scope. Any nested scopes are child scopes of that parent.

• An item is visible in the scope in which it was created and in any child scopes, unless you explicitly make it private. You can place variables, aliases, functions, or PowerShell drives in one or more scopes.

• An item that you created within a scope can be changed only in the scope in which it was created, unless you explicitly specify a different scope.

If you create an item in a scope, and the item shares its name with an item in a different scope, the original item might be hidden under the new item, but it is not overridden or changed.

PowerShell Scopes

PowerShell supports the following scopes:

• Global: The scope that is in effect when PowerShell starts. Variables and functions that are present when PowerShell starts have been created in the global scope, such as automatic variables and preference variables. The variables, aliases, and functions in your PowerShell profiles are also created in the global scope.

• Local: The current scope. The local scope can be the global scope or any other scope.

• Script: The scope that is created while a script file runs. Only the commands in the script run in the script scope. To the commands in a script, the script scope is the local scope.

Parent and Child Scopes

You can create a new scope by running a script or function, by creating a session, or by starting a new instance of PowerShell. When you create a new scope, the result is a parent scope (the original scope) and a child scope (the scope that you created).

In PowerShell, all scopes are child scopes of the global scope, but you can create many scopes and many recursive scopes.

Unless you explicitly make the items private, the items in the parent scope are available to the child scope. However, items that you create and change in the child scope do not affect the parent scope, unless you explicitly specify the scope when you create the items.

Inheritance

A child scope does not inherit the variables, aliases, and functions from the parent scope. Unless an item is private, the child scope can view the items in the parent scope. And, it can change the items by explicitly specifying the parent scope, but the items are not part of the child scope.

However, a child scope is created with a set of items. Typically, it includes all the aliases that have the AllScope option. This option is discussed later in this article. It includes all the variables that have the AllScope option, plus some automatic variables.

To find the items in a particular scope, use the Scope parameter of Get-Variable or Get-Alias.

For example, to get all the variables in the local scope, type:

Get-Variable -Scope local

To get all the variables in the global scope, type:

Get-Variable -Scope global

Scope Modifiers

A variable, alias, or function name can include any one of the following optional scope modifiers:

• global: - Specifies that the name exists in the Global scope.

• local: - Specifies that the name exists in the Local scope. The current scope is alway the Local scope.

• private: - Specifies that the name is Private and only visible to the current scope.

• script: - Specifies that the name exists in the Script scope. Script scope is the nearest ancestor script file's scope or Global if there is no nearest ancestor script file.

• using: - Used to access variables defined in another scope while running scripts via cmdlets like Start-Job and Invoke-Command.

• workflow: - Specifies that the name exists within a workflow. Note: Workflows are not supported in PowerShell Core.

• <variable-namespace> - A modifier created by a PowerShell PSDrive provider. For example:

  Namespace	Description
  Alias:	Aliases defined in the current scope
  Env:	Environment variables defined in the current scope
  Function:	Functions defined in the current scope
  Variable:	Variables defined in the current scope

The default scope for scripts is the script scope. The default scope for functions and aliases is the local scope, even if they are defined in a script.

Using scope modifiers

To specify the scope of a new variable, alias, or function, use a scope modifier.

The syntax for a scope modifier in a variable is:

$[<scope-modifier>:]<name> = <value>

The syntax for a scope modifier in a function is:

function [<scope-modifier>:]<name> {<function-body>}

The following command, which does not use a scope modifier, creates a variable in the current or local scope:

$a = "one"

To create the same variable in the global scope, use the scope global: modifier:

$global:a = "one"

To create the same variable in the script scope, use the script: scope modifier:

$script:a = "one"

You can also use a scope modifier with functions. The following function definition creates a function in the global scope:

function global:Hello {
  Write-Host "Hello, World"
}

You can also use scope modifiers to refer to a variable in a different scope. The following command refers to the $test variable, first in the local scope and then in the global scope:

$test
$global:test

The Using: scope modifier

Using is a special scope modifier that identifies a local variable in a remote command. Without a modifier, PowerShell expects variables in remote commands to be defined in the remote session.

The Using scope modifier is introduced in PowerShell 3.0.

For more information, see about_Remote_Variables.

The AllScope Option

Variables and aliases have an Option property that can take a value of AllScope. Items that have the AllScope property become part of any child scopes that you create, although they are not retroactively inherited by parent scopes.

An item that has the AllScope property is visible in the child scope, and it is part of that scope. Changes to the item in any scope affect all the scopes in which the variable is defined.

Managing Scope

Several cmdlets have a Scope parameter that lets you get or set (create and change) items in a particular scope. Use the following command to find all the cmdlets in your session that have a Scope parameter:

Get-Help * -Parameter scope

To find the variables that are visible in a particular scope, use the Scope parameter of Get-Variable. The visible variables include global variables, variables in the parent scope, and variables in the current scope.

For example, the following command gets the variables that are visible in the local scope:

Get-Variable -Scope local

To create a variable in a particular scope, use a scope modifier or the Scope parameter of Set-Variable. The following command creates a variable in the global scope:

New-Variable -Scope global -Name a -Value "One"

You can also use the Scope parameter of the New-Alias, Set-Alias, or Get-Alias cmdlets to specify the scope. The following command creates an alias in the global scope:

New-Alias -Scope global -Name np -Value Notepad.exe

To get the functions in a particular scope, use the Get-Item cmdlet when you are in the scope. The Get-Item cmdlet does not have a Scope parameter.

Using Dot Source Notation with Scope

Scripts and functions follow all the rules of scope. You create them in a particular scope, and they affect only that scope unless you use a cmdlet parameter or a scope modifier to change that scope.

But, you can add a script or function to the current scope by using dot source notation. Then, when a script runs in the current scope, any functions, aliases, and variables that the script creates are available in the current scope.

To add a function to the current scope, type a dot (.) and a space before the path and name of the function in the function call.

For example, to run the Sample.ps1 script from the C:\Scripts directory in the script scope (the default for scripts), use the following command:

c:\scripts\sample.ps1

To run the Sample.ps1 script in the local scope, use the following command:

. c:\scripts.sample.ps1

When you use the call operator (&) to run a function or script, it is not added to the current scope. The following example uses the call operator:

& c:\scripts.sample.ps1

You can read more about the call operator in about_operators.

Any aliases, functions, or variables that the Sample.ps1 script creates are not available in the current scope.

Restricting Without Scope

A few PowerShell concepts are similar to scope or interact with scope. These concepts may be confused with scope or the behavior of scope.

Sessions, modules, and nested prompts are self-contained environments, but they are not child scopes of the global scope in the session.

Sessions

A session is an environment in which PowerShell runs. When you create a session on a remote computer, PowerShell establishes a persistent connection to the remote computer. The persistent connection lets you use the session for multiple related commands.

Because a session is a contained environment, it has its own scope, but a session is not a child scope of the session in which it was created. The session starts with its own global scope. This scope is independent of the global scope of the session. You can create child scopes in the session. For example, you can run a script to create a child scope in a session.

Modules

You can use a PowerShell module to share and deliver PowerShell tools. A module is a unit that can contain cmdlets, scripts, functions, variables, aliases, and other useful items. Unless explicitly defined, the items in a module are not accessible outside the module. Therefore, you can add the module to your session and use the public items without worrying that the other items might override the cmdlets, scripts, functions, and other items in your session.

The privacy of a module behaves like a scope, but adding a module to a session does not change the scope. And, the module does not have its own scope, although the scripts in the module, like all PowerShell scripts, do have their own scope.

By default, modules are loaded into the top-level of the current session state not the current scope. This could be a module session state or the global session state. If you are in the global scope, then modules are loaded into the global session state. Any exports are placed into the global tables. If you load module2 from within module1, module2 is loaded into the module1's session state, not the global session state. Any exports from module2 are placed at the top of the module1's session state. If you use Import-Module -Scope local, then the exports are placed into the current scope object rather than at the top level. If you are in a module and use Import-Module -Scope global (or Import-Module -Global) to load another module, that module and it's exports are loaded into the global session state instead of the module's local session state. This feature was designed for writing module that manipulate modules. The WindowsCompatibility module does this to import proxy modules into the global scope.

Nested Prompts

Similarly, nested prompts do not have their own scope. When you enter a nested prompt, the nested prompt is a subset of the environment. But, you remain within the local scope.

Scripts do have their own scope. If you are debugging a script, and you reach a breakpoint in the script, you enter the script scope.

Private Option

Aliases and variables have an Option property that can take a value of Private. Items that have the Private option can be viewed and changed in the scope in which they are created, but they cannot be viewed or changed outside that scope.

For example, if you create a variable that has a private option in the global scope and then run a script, Get-Variable commands in the script do not display the private variable. Using the global scope modifier in this instance does not display the private variable.

You can use the Option parameter of the New-Variable, Set-Variable, New-Alias, and Set-Alias cmdlets to set the value of the Option property to Private.

Visibility

The Visibility property of a variable or alias determines whether you can see the item outside the container, in which it was created. A container could be a module, script, or snap-in. Visibility is designed for containers in the same way that the Private value of the Option property is designed for scopes.

The Visibility property takes the Public and Private values. Items that have private visibility can be viewed and changed only in the container in which they were created. If the container is added or imported, the items that have private visibility cannot be viewed or changed.

Because visibility is designed for containers, it works differently in a scope.

• If you create an item that has private visibility in the global scope, you cannot view or change the item in any scope.
• If you try to view or change the value of a variable that has private visibility, PowerShell returns an error message.

You can use the New-Variable and Set-Variable cmdlets to create a variable that has private visibility.

Examples

Example 1: Change a Variable Value Only in a Script

The following command changes the value of the $ConfirmPreference variable in a script. The change does not affect the global scope.

First, to display the value of the $ConfirmPreference variable in the local scope, use the following command:

PS>  $ConfirmPreference
High

Create a Scope.ps1 script that contains the following commands:

$ConfirmPreference = "Low"
"The value of `$ConfirmPreference is $ConfirmPreference."

Run the script. The script changes the value of the $ConfirmPreference variable and then reports its value in the script scope. The output should resemble the following output:

The value of $ConfirmPreference is Low.

Next, test the current value of the $ConfirmPreference variable in the current scope.

PS>  $ConfirmPreference
High

This example shows that changes to the value of a variable in the script scope does not affect the variable`s value in the parent scope.

Example 2: View a Variable Value in Different Scopes

You can use scope modifiers to view the value of a variable in the local scope and in a parent scope.

First, define a $test variable in the global scope.

$test = "Global"

Next, create a Sample.ps1 script that defines the $test variable. In the script, use a scope modifier to refer to either the global or local versions of the $test variable.

In Sample.ps1:

$test = "Local"
"The local value of `$test is $test."
"The global value of `$test is $global:test."

When you run Sample.ps1, the output should resemble the following output:

The local value of $test is Local.
The global value of $test is Global.

When the script is complete, only the global value of $test is defined in the session.

PS>  $test
Global

Example 3: Change the Value of a Variable in a Parent Scope

Unless you protect an item by using the Private option or another method, you can view and change the value of a variable in a parent scope.

First, define a $test variable in the global scope.

$test = "Global"

Next, create a Sample.ps1 script that defines the $test variable. In the script, use a scope modifier to refer to either the global or local versions of the $test variable.

In Sample.ps1:

$global:test = "Local"
"The global value of `$test is $global:test."

When the script is complete, the global value of $test is changed.

PS>  $test
Local

Example 4: Creating a Private Variable

A private variable is a variable that has an Option property that has a value of Private. Private variables are inherited by the child scope, but they can only be viewed or changed in the scope in which they were created.

The following command creates a private variable called $ptest in the local scope.

New-Variable -Name ptest -Value 1 -Option private

You can display and change the value of $ptest in the local scope.

PS>  $ptest
1

PS>  $ptest = 2
PS>  $ptest
2

Next, create a Sample.ps1 script that contains the following commands. The command tries to display and change the value of $ptest.

In Sample.ps1:

"The value of `$Ptest is $Ptest."
"The value of `$Ptest is $global:Ptest."

The $ptest variable is not visible in the script scope, the output is empty.

"The value of $Ptest is ."
"The value of $Ptest is ."

Example 5: Using a Local Variable in a Remote Command

For variables in a remote command created in the local session, use the Using scope modifier. PowerShell assumes that the variables in remote commands were created in the remote session.

The syntax is:

$Using:<VariableName>

For example, the following commands create a $Cred variable in the local session and then use the $Cred variable in a remote command:

$Cred = Get-Credential
Invoke-Command $s {Remove-Item .\Test*.ps1 -Credential $Using:Cred}

The Using scope was introduced in PowerShell 3.0. In PowerShell 2.0, to indicate that a variable was created in the local session, use the following command format.

$Cred = Get-Credential
Invoke-Command $s {
  param($c)
  Remove-Item .\Test*.ps1 -Credential $c
} -ArgumentList $Cred

See also

about_Variables

about_Environment_Variables

about_Functions

about_Script_Blocks