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about_Scopes

Updated: May 8, 2014

Applies To: Windows PowerShell 2.0, Windows PowerShell 3.0, Windows PowerShell 4.0

TOPIC
    about_Scopes

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

    
LONG DESCRIPTION
    Windows PowerShell protects access to variables, aliases, functions, and
    Windows PowerShell drives (PSDrives) by limiting where they can be read and
    changed. By enforcing a few simple rules for scope, Windows PowerShell 
    helps to ensure that you do not inadvertently change an item that should 
    not be changed.


    The following are the basic rules of scope:

        - An item you include in a scope is visible in the scope in which it 
          was created and in any child scope, unless you explicitly make it 
          private. You can place variables, aliases, functions, or Windows
          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.


  Windows PowerShell Scopes

    Scopes in Windows PowerShell have both names and numbers. The named
    scopes specify an absolute scope. The numbers are relative and reflect
    the relationship between scopes.


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

    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.

    Private:
        Items in private scope cannot be seen outside of the current
        scope. You can use private scope to create a private version
        of an item with the same name in another scope.        


    Numbered Scopes:
        You can refer to scopes by name or by a number that
        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.


 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 Windows 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 Windows 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 topic. It includes all the variables that have the
    AllScope option, plus some variables that can be used to customize the 
    scope, such as MaximumFunctionCount.


    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
 
    To specify the scope of a new variable, alias, or function, use a scope 
    modifier. The valid values of a modifier are Global, Local, Private, and
    Script.

    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 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.
 
 
    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 Global scope 
    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 in 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. By default, variables in remote
    commands are assumed to be defined in the remote session.

    The Using scope modifier is introduced in Windows 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 parameters include global 
    parameters, parameters in the parent scope, and parameters 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


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


 Restricting Without Scope
 
    A few Windows 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 Windows PowerShell runs.
        When you create a session on a remote computer, Windows
        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 is 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 Windows PowerShell module to share and deliver
        Windows 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 Windows
        PowerShell scripts, do have their own 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. This occurs even if you use 
        the global scope modifier. 
   

        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, such as a module, in which it 
        was created. 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, Windows 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:

          C:\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.

          C:\PS> $ConfirmPreference
          High

      
      This example shows that changes to the value of a variable in the script
      scope do not affect the value of that variable 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.

          C:\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.

          C:\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 be viewed or changed only 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.

  C:\PS> $ptest
          1
          C:\PS> $ptest = 2
          C:\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."

    
      Because 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

       To indicate that a variable in a remote command was created in the 
       local session, use the Using scope modifier. By default, Windows
       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 Windows PowerShell 3.0.

       In Windows 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



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