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18. Built-in variables

Note

The below information is extensively based in information taken from the PowerShell® Notes for Professionals book. I plan to extend this information based on my day to day usage of the language.

PowerShell offers a variety of useful "automatic" (built-in) variables. Certain automatic variables are only populated in special circumstances, while others are available globally.

18.1: $PSScriptRoot

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Get-ChildItem -Path $PSScriptRoot

This example retrieves the list of child items (directories and files) from the folder where the script file resides.

The $PSScriptRoot automatic variable is $null if used from outside a PowerShell code file. If used inside a

PowerShell script, it automatically defined the fully-qualified filesystem path to the directory that contains the script file.

In Windows PowerShell 2.0, this variable is valid only in script modules (.psm1). Beginning in Windows PowerShell

3.0, it is valid in all scripts.

18.2: $Args

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$Args

Contains an array of the undeclared parameters and/or parameter values that are passed to a function, script, or script block. When you create a function, you can declare the parameters by using the param keyword or by adding a comma-separated list of parameters in parentheses after the function name.

In an event action, the $Args variable contains objects that represent the event arguments of the event that is being processed. This variable is populated only within the Action block of an event registration command. The value of this variable can also be found in the SourceArgs property of the PSEventArgs object (System.Management.Automation.PSEventArgs) that Get-Event returns.

18.3: $PSItem

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2
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Get-Process | ForEach-Object -Process {
  $PSItem.Name
}

Same as $_. Contains the current object in the pipeline object. You can use this variable in commands that perform an action on every object or on selected objects in a pipeline.

18.4: $?

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Get-Process -Name doesnotexist
Write-Host -Object "Was the last operation successful? $?"

Contains the execution status of the last operation. It contains TRUE if the last operation succeeded and FALSE if it failed.

18.5: $error

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Get-Process -Name doesnotexist
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Write-Host -Object ('The last error that occurred was: {0}' -f $error[ 0 ].Exception.Message)

Contains an array of error objects that represent the most recent errors. The most recent error is the first error object in the array ($Error[0]).

To prevent an error from being added to the $Error array, use the ErrorAction common parameter with a value of Ignore. For more information, see about_CommonParameters (http://go.microsoft.com/fwlink/?LinkID=113216).