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about_Variables

Updated: April 21, 2010

Applies To: Windows PowerShell 2.0

TOPIC
    about_Variables

SHORT DESCRIPTION
    Describes how variables store values that can be used in Windows 
    PowerShell.    

LONG DESCRIPTION
    You can store all types of values in Windows PowerShell variables. 
    They are typically used to store the results of commands and to 
    store elements that are used in commands and expressions, such as
    names, paths, settings, and values.

    A variable is a unit of memory in which values are stored. In Windows
    PowerShell, variables are represented by text strings that begin 
    with a dollar sign ($), such as $a, $process, or $my_var. 

    Variable names are not case-sensitive. Variable names can include
    spaces and special characters, but these are difficult to use and
    should be avoided.

    There are several different types of variables in Windows PowerShell.

    -- User-created variables: User-created variables are created and 
       maintained by the user. By default, the variables that you create at
       the Windows PowerShell command line exist only while the Windows 
       PowerShell window is open, and they are lost when you close the window.
       To save a variable, add it to your Windows PowerShell profile. You can 
       also create variables in scripts with global, script, or local scope.    

    -- Automatic variables: Automatic variables store the state of 
       Windows PowerShell. These variables are created by Windows PowerShell, 
       and Windows PowerShell changes their values as required to maintain 
       their accuracy. Users cannot change the value of these variables.
       For example, the $PSHome variable stores the path to the Windows 
       PowerShell installation directory. 

       For more information, a list, and a description of the automatic
       variables, see about_Automatic_Variables.

    -- Preference variables: Preference variables store user preferences for
       Windows PowerShell. These variables are created by Windows PowerShell
       and are populated with default values. Users can change the values of 
       these variables. For example, the $MaximumHistoryCount variable 
       determines the maximum number of entries in the session history. 
 
       For more information, a list, and a description of the preference
       variables, see about_Preference_Variables.


  WORKING WITH VARIABLES

    To create a new variable, use an assignment statement to assign
    a value to the variable. You do not have to declare the variable
    before using it. The default value of all variables is $null.
    
    For example:

        PS> $MyVariable = 1, 2, 3

        PS> $path = "C:\Windows\System32"

        
    Variables are very useful for storing the results of commands. 

    For example:
          
        PS> $processes = Get-Process         
     
        PS> $Today = (Get-Date).date


    To display the value of a variable, type the variable name, preceded
    by a dollar sign ($). 

    For example:

        PS> $MyVariable
        1
        2
        3 

        PS> $Today
        Thursday, September 03, 2009 12:00:00 AM


    To change the value of a variable, assign a new value to the variable.

    The following examples displays the value of the $MyVariable variable,
    changes the value of the variable, and then displays the new value.


        PS> $MyVariable
        1
        2
        3

        PS> $MyVariable = "The green cat."

        PS> $MyVariable
        The green cat.


    To delete the value of a variable, use the Clear-Variable cmdlet or 
    change the value to $null.

        PS> Clear-Variable -name MyVariable  

        -or-

        PS> $MyVariable = $null


    
    To delete the variable, use the Remove-Variable or Remove-Item
    cmdlets. (These cmdlets are discussed later in this topic.)

       PS> remove-variable -name MyVariable

       PS> remove-item -path variable:\myvariable


    To get a list of all of the variables in your Windows PowerShell
    session, type:

       get-variable


  TYPES OF VARIABLES
    You can store any type of object in a variable, including integers, 
    strings, arrays, hash tables, and objects that represent processes, 
    services, event logs, and computers. 

    Windows PowerShell variables are "loosely typed," which means that
    they are not limited to a particular type of object. A single variable
    can even contain a collection (an "array") of different types of objects
    at the same time.

    The data type of a variable, which is a .NET Framework type, is
    determined by the .NET types of the values of the variable. 

    For example:

        PS> $a = 12     (System.Int32)
        
        PS> $a = "Word" (System.String)

        PS> $a = 12, "Word" (System.Int32, System.String)

        PS> $a = dir C:\Windows\System32  (Files and folders)


    You can use a type attribute and cast notation to ensure that a 
    variable can contain only objects of the specified type or objects 
    that can be converted to that type. If you try to assign a value
    of another type, Windows PowerShell tries to convert the value to
    its type. If it cannot, the assignment statement fails.

    To use cast notation, enter a type name, enclosed in brackets, before
    the variable name (on the left side of the assignment statement).
    The following example creates an $number variable that can contain
    only integers, a $words variable that can contain only strings, and
    a $dates variable that can contain only DateTime objects.
  
        PS> [int]$number = 8

        PS> $a = "12345" (The string is converted to an integer.)

        PS> $a = "Hello"
        Cannot convert value "Hello" to type "System.Int32". Error: "Input string was not in a correct format."
        At line:1 char:3
        + $a <<<<  = "Hello"
            + CategoryInfo          : MetadataError: (:) [], ArgumentTransformationMetadataException
            + FullyQualifiedErrorId : RuntimeException


        PS> [string]$words = "Hello"

        PS> $words = 2   (The integer is converted to a string.)

        PS> $words + 10  (The strings are concatenated.)
        210              


        PS> [datetime] $dates = "09/12/91" (The string is converted to a DateTime object.)

        PS> $dates
        Thursday, September 12, 1991 12:00:00 AM

        PS> $dates = 10  (The integer is converted to a DateTime object.)
        PS> $dates
        Monday, January 01, 0001 12:00:00 AM
                
       
  USING VARIABLES IN COMMANDS AND EXPRESSIONS 

    To use a variable in a command or expression, type the variable name, 
    preceded by the dollar sign ($). 

    If the variable name (and dollar sign) are not enclosed in quotation marks,
    or if they are enclosed in double quotation marks ("), the value of the
    variable is used in the command or expression. 

    If the variable name (and dollar sign) are enclosed in single quotation
    marks, ('), the variable name is used in the expression. 

    For example, the first command gets the value of the $profile variable,
    which is the path to the Windows PowerShell user profile file in the Windows
    PowerShell console. The second command opens the file in Notepad, and the
    third and fourth commands use the name of the variable in an expression.


        PS> $profile
        C:\Documents and Settings\User01\My Documents\WindowsPowerShell\Microsoft.PowerShell_profile.ps1

        PS> noteapd $profile 
        - or -
        PS> notepad "$profile"
        C:\Documents and Settings\User01\My Documents\WindowsPowerShell\Microsoft.PowerShell_profile.ps1

        PS> '$profile'
        $profile

        PS> 'Use the $profile variable.'
        Use the $profile variable. 

    For more information about using quotation marks in Windows PowerShell, see
    about_Quoting_Rules.


  VARIABLE NAMES THAT INCLUDE SPECIAL CHARACTERS

      Variable names begin with a dollar sign. They can include 
      alphanumeric characters and special characters. The length
      of the variable name is limited only by available memory.

      Whenever possible, variable names should include only
      alphanumeric characters and the underscore character (_). 
      Variable names that include spaces and other special characters,
      are difficult to use and should be avoided.
 
      To create or display a variable name that includes spaces or
      special characters, enclose the variable name in braces. This directs
      Windows PowerShell to interpret the characters in the variable name
      literally.

      For example, the following command creates and then displays a variable
      named "save-items".

          C:\PS> ${save-items} = "a", "b", "c"
          C:\PS> ${save-items}
          a
          b
          c

      The following command gets the child items in the directory that is 
      represented by the "ProgramFiles(x86)" environment variable.

          C:\PS> Get-childitem ${env:ProgramFiles(x86)}


      To refer to a variable name that includes braces, enclose the
      variable name in braces, and use the backtick (escape) character
      to escape the braces. For example, to create a variable
      named "this{value}is" with a value of 1, type: 

          C:\PS> ${this`{value`}is} = 1
          C:\PS> ${this`{value`}is}
          1


  VARIABLES AND SCOPE
      By default, variables are available only in the scope in which
      they are created.  

      For example, a variable that you create in a function is 
      available only within the function. A variable that you
      create in a script is available only within the script (unless
      you dot-source the script, which adds it to the current scope).

      You can use a scope modifier to change the default scope of the
      variable. The following expression creates a variable named 
      "Computers". The variable has a global scope, even when it is
      created in a script or function.

            $global:computers = "Server01"

      For more information, see about_Scopes.       
  

  SAVING VARIABLES
      Variables that you create are available only in the session in which
      you create them. They are lost when you close your session.
     
      To create the in every Windows PowerShell session that you start, 
      add the variable to your Windows PowerShell profile. 

      For example, to change the value of the $VerbosePreference variable in
      every Windows PowerShell session, add the following command to your Windows
      PowerShell profile.

	  $VerbosePreference = "Continue"

      You can add this command to your profile by opening the profile file in a
      text editor, such as Notepad. For more information about Windows PowerShell
      profiles, see about_profiles.



  THE VARIABLE: DRIVE

     Windows PowerShell Variable provider creates a Variable: drive that
     looks and acts like a file system drive, but it contains the variables
     in your session and their values.

     To change to the variable: drive, type:

        set-location variable:
        
         (or "cd variable:")

     
     To list the items (variables) in the Variable: drive, use the
     Get-Item or Get-ChildItem cmdlets. For example:

         get-childitem variable:
         (or "dir" or "ls")


     To get the value of a particular variable, use file system notation
     to specify the name of the drive and the name of the variable. For
     example, to get the $PSCulture automatic variable, use the following
     command.
     
         get-item variable:\PSCulture      

         Name                           Value
         ----                           -----
         PSCulture                      en-US        


     For more information about the Variable: drive and the Windows
     PowerShell Variable provider, type "get-help variable".


  THE VARIABLE CMDLETS

     Windows PowerShell includes a set of cmdlets that are designed to 
     manage variables.

         Cmdlet Name           Description
         -----------           -----------

         Clear-Variable        Deletes the value of a variable.
         Get-Variable          Gets the variables in the current console.
         New-Variable          Creates a new variable.
         Remove-Variable       Deletes a variable and its value.
         Set-Variable          Changes the value of a variable.

     To get help for these cmdlets, type: "Get-Help <cmdlet-hame>".


SEE ALSO
    about_Automatic_Variables
    about_Environment_Variables
    about_Preference_Variables
    about_Profiles
    about_Quoting_Rules
    about_Scopes
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