PowerShell Predefined Variables

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PowerShell Predefined Variables

By Xah Lee. Date: , 

PowerShell has several predefined variables, called automatic variables. Some of them are used for scripting, some give info about the environment. For example, $_ is a variable that contains the current argument (similar to Perl's “$_”), and is a most frequently used variable in scripting. The $Host is a variable that contains info about current PowerShell environment.

This page gives explanation and sample use, of the most frequently used automatic variables. The page is divided into 2 sections. One section on environment related variables, and the other section is about scripting related variables.

For a complete list of automatic variables, type get-help about_automatic_variable.

Environment Related Automatic Variables

Home Dir, Init File

$Home

Evaluates to the home dir env var, equivalent to %homedrive%%homepath% env var. Sample output:

C:\Users\xah

$Profile

Eval to the full path of your PowerShell profile file. The profile is a init file. Sample output:

C:\Users\xah\Documents\WindowsPowerShell\Microsoft.PowerShell_profile.ps1

Note that even if this file does not exist, it still returns the path. To check if the file exists, use: test-path $profile.

PowerShell Info

$PsHome

Evaluates to the full path of the installation directory for Windows PowerShell. Sample outputs:

C:\Windows\system32\WindowsPowerShell\v1.0\
C:\Windows\SysWOW64\WindowsPowerShell\v1.0\

Note that even if you have PowerShell version 2, the dir path is still “v1.0”, because PowerShell 2 is meant to be compatible with version 1.

$Host

Eval to a object that represents the current host application. Sample output:

Name             : Windows PowerShell ISE Host
Version          : 2.0
InstanceId       : b18c3b59-d4c3-4373-9ce3-3f54768bb3e4
UI               : System.Management.Automation.Internal.Host.InternalHostUserInterface
CurrentCulture   : en-US
CurrentUICulture : en-US
PrivateData      :
IsRunspacePushed : False
Runspace         : System.Management.Automation.Runspaces.LocalRunspace

If you are running PowerShell in Windows Console or emacs, the name line may be:

Name             : ConsoleHost

$PsVersionTable

Eval to a hash table object containing info about your PowerShell version.

Sample output:

Name                           Value
----                           -----
CLRVersion                     2.0.50727.4016
BuildVersion                   6.1.6949.0
PSVersion                      2.0
PSCompatibleVersions           {1.0, 2.0}

Scripting Related Automatic Variables

Standard Values

$True

Eval to .NET “True”.

Whenever you need boolean, use $true or $false.

If you simply type “true” in the command line, it's a error, because PowerShell tries to interprete it as a command or expression. By design, “true” or “false” are not recognized as expressions.

PowerShell True/False Interpretation
Value Value in Boolean Context
$true True
Nonzero number True
Nonempty string True
Nonempty array True
Hashtable (empty or not) True
$false False
$null False
Zero False
Empty string False
Empty array False

$False

Evaluates to the .NET “False”.

$NULL

Eval to the .NET “Null”.

Arguments and Script Name

$_

The current object in a pipeline.

Note that a pipe can pass multiple objects, and each one is processed in turn. The “$_” is the current one.

Example use:

# get all aliases of get-childitem
get-alias | where-object {$_.Definition -eq "get-childitem"}

In the above, the “get-alias” returns many objects. We use the syntax “xyz.Definition” to refer to the “Definition” property of the object xyz. In this case, we use “$_” to refer to the current object, instead of a object name such as “xyz”.

$Args

A array of the argument received by your function or script or block of code.

Note, PowerShell provide a few ways to define how a function receives its arguments. A function can be defined with explicit parameters, or it can be defined without explicit parameters. When a function does not have explicit parameters, it can still receive arguments, and these arguments can be accessed by $Args.

get-help about_functions, get-help about_parameter

$MyInvocation

Returns the object that contains info of your script, function. Example usage:

# get the script path
$myinvocation.mycommand.path

# get the script name
$myinvocation.mycommand.name

# look at what members this object has
$myinvocation | get-member

$Input

Returns a enumerator object that contains the input that is passed to a function. The items in the enumerator are the objects in the current pipeline.

$Pwd

Returns a path object that represents the full path of the current dir.

Iterators; Misc

$ForEach

Returns a enumerator object of the current ForEach-Object loop. This var exists only when a “for loop” is running.

get-help about_foreach get-help Foreach-Object

$Matches

Returns a hash table that represents matched text, from using the “-match” operator.

about_comparison_operators

Error

$?

Returns True if last operation succeeded, else False.

$LastExitCode

Returns the exit code of the last program.

$Error

Returns a array of objects, representing the recent errors. $Error[0] is the most recent, $Error[1] is the error before that, etc.

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