about_Scopes

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 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 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 ."
	
   
SEE ALSO
	about_Variables
	about_Environment_Variables
	about_Functions
	about_Script_Blocks