PowerShell for Microsoft SharePoint 2010 : Variables in Windows PowerShell (part 4) - Automatic Variables , Preference Variables , Environment Variables

Automatic Variables

The Windows PowerShell automatic variables are fixed variables that store state information. Table 2 describes these variables.

Table 2. Windows PowerShell Automatic Variables

Variable	Contents
$$	The last token in the last line received by the session
$?	True or false, depending on the last performed operation
$^	The first token from the last line received by the session
$_	The current object in the pipeline object
$args	An array of values passed to a function or a script
$ConsoleFileName	The path of the console file that was most recently used in the session
$Error	An array of error objects representing the most recent error that occurred
$Event	A PSEventArgs object that represents the event that is being processed
$EventSubscriber	A PSEventSubscriber object that represents the event subscriber of the event that is being processed
$ExecutionContext	An EngineIntrinsics object that represents the execution context of the Windows PowerShell host
$false	Boolean false
$foreach	The enumerator of a foreach loop
$Home	User’s home directory
$Host	Information regarding the current host
$input	An enumerator that contains the input passed to a function
$LastExitCode	The last exit code of the last program that was run
$Matches	Result of the last successful regular expression match
$MyInvocation	Information regarding the context under which the script, function, or script block was run
$NestedPromptLevel	The current prompt level
$NULL	An empty value
$PID	The process identifier of the process that is hosting the current Windows PowerShell session
$Profile	The full path to the Windows PowerShell profile for the current user
$PSBoundParameters	A dictionary of the active parameters and their current values
$PsCmdlet	The cmdlet that is being run
$PsCulture	The name of the culture currently in use
$PsDebugContext	Information about the debugging environment while debugging
$PsHome	The full path of the installation directory of Windows PowerShell
$PsScriptRoot	The directory from which the script module is being executed
$PsUICulture	The name of the user interface culture that is currently in use in the operating system
$PsVersionTable	A read-only hashtable containing details about the version of Windows PowerShell
$Pwd	The current directory
$ShellID	Identifier of the current shell
$SourceArgs	Objects that represent the event arguments of the current event
$SourceEventArgs	The first object that represents the first event argument that derives from EventArgs of the event that is being processed
$This	Reference to the current object in script methods and properties
$true	Boolean true

You can list the variables in Windows PowerShell by using the Get-ChildItem or Get-Variable cmdlet.

PS > Get-ChildItem Variable:
PS> Get-Variable

These commands return both automatic variables and user-created variables.

To display the value of a specific automatic variable, simply type the automatic variable’s name.

PS > $PSHOME
C:\Windows\System32\WindowsPowerShell\v1.0

It is also possible to use the value of a variable as input to cmdlets in Windows PowerShell. In the next example, we use the value of the $PsHome variable with the Get-ChildItem cmdlet to retrieve all items from the Windows PowerShell home directory.

PS > Get-ChildItem $PSHOME

When working with pipelines and loops in Windows PowerShell, you use the automatic variable $_ to handle the current object in the pipeline. 

You can get more information about automatic variables by using the Get-Help cmdlet.

PS > Get-Help about_Automatic_variables

Preference Variables

Windows PowerShell includes a set of preference variables that let you customize its behavior. The preference variables affect the environment and how commands behave. Table 3 describes the preference variables available in Windows PowerShell.

Table 3. Windows PowerShell Preference Variables

Variable	Description
$ConfirmPreference	Sets the level of impact that operations have before requesting confirmation: none, low, medium, or high
$DebugPreference	Controls how Windows PowerShell handles debugging messages written by a script or a cmdlet
$ErrorActionPreference	Sets the default error-handling action
$ErrorView	Controls how Windows PowerShell should output errors to the shell
$FormatEnumerationLimit	Determines how many enumerated items are included in a display
$LogCommandHealthEvent	Logs errors and exceptions in command
	initialization and processing
$LogCommandLifeCycleEvent	Logs the start and stop of a command in a pipeline
$LogEngineHealthEvent	Logs session error and failures
$LogEngineLifeCycleEvent	Logs the opening and closing of a session
$LogProviderLifeCycleEvent	Logs adding and removing of providers in Windows PowerShell
$LogProviderHealthEvent	Logs provider errors
$MaximumAliasCount	Determines how many aliases are permitted in the current session
$MaximumErrorCount	Determines how many errors are saved in the error history
$MaximumFunctionCount	Determines how many functions are permitted in the current session
$MaximumHistoryCount	Determines how many commands are saved in the command history for the current session
$MaximumVariableCount	Determines how many variables are permitted in the current session
$OFS	Output field separator—specifies the character that an element in an array is separated with when the array is converted to a stringj
$OutputEncoding	Determines the default character encoding used by Windows PowerShell
$ProgressPreference	Determines how Windows PowerShell responds to progress updates generated by a script
$PSEmailServer	Specifies the default e-mail server used by Windows PowerShell cmdlets
$PSSessionApplicationName	Specifies the default application name for a remote command that uses WS-Management technology
$PSSessionConfigurationName	Specifies the default session configuration that is used for PSSessions created in the current session
$PSSessionOption	Establishes the default values for advanced user options in a remote session
$VerbosePreference	Controls how Windows PowerShell handles verbose messages written by a script or a cmdlet
$WarningPreference	Controls how Windows PowerShell handles warning messages written by a script or a cmdlet
$WhatIfPreference	Determines if WhatIf is automatically enabled for every command that supports it

You can modify the preference variables by changing their value. For example, the following example changes the warning preference to silently continue.

PS > $WarningPreference = "SilentlyContinue"
PS > $WarningPreference
SilentlyContinue

When you modify a preference variable in Windows PowerShell, the change affects only the current session. In order to make a persistent change to a preference variable, add the modification to a profile script, as this example shows:

'$WarningPreference = "SilentlyContinue"' | Out-File $PSHOME\profile.ps1 -Append


					  

Environment Variables

Environment variables store information regarding the operating system environment. You can display the environment variables available using the Get-ChildItem cmdlet.

PS > Get-ChildItem env:

Here’s how to retrieve a specific environment variable:

PS > Get-ChildItem env:COMPUTERNAME

Name                               Value

----                           -----
COMPUTERNAME                   SPSERVER01

To display the value of the environment variable, add its name.

PS > $env:COMPUTERNAME
SPSERVER01

As with other variables, you can modify an environment variable by changing its value, as in this example:

PS > $env:COMPUTERNAME = "NewName"
PS > $env:COMPUTERNAME
NewName

As with preference variables, all changes made to the environment variables using Windows PowerShell affect only the current session. To permanently change environment variables, add the modifications to a profile script.

Note

You can also permanently change environment variables through the SetEnvironmentVariable() static method available from the System.Environment class.