Explains the concept of scope in Windows PowerShell and shows how to set (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

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