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about_Automatic_Variables

  • Artikkeli

Short description

Describes variables that store state information for and are created and maintained by PowerShell.

Conceptually, most of these variables are considered to be read-only. Even though they can be written to, for backward compatibility they should not be written to.

Here is a list of the automatic variables in PowerShell:

Long description

$$

Contains the last token in the last line received by the session.

$?

Contains the execution status of the last command. It contains True if the last command succeeded and False if it failed. Parse errors don't result in execution, so they don't affect the value of $?.

For cmdlets and advanced functions that are run at multiple stages in a pipeline, for example in both process and end blocks, calling this.WriteError() or $PSCmdlet.WriteError() respectively at any point sets $? to False, as does this.ThrowTerminatingError() and $PSCmdlet.ThrowTerminatingError().

The Write-Error cmdlet always sets $? to False immediately after it's executed, but won't set $? to False for a function calling it:

function Test-WriteError
{
    Write-Error "Bad"
    "The `$? variable is: $?"
}

Test-WriteError
"Now the `$? variable is: $?"

Test-WriteError:
Line |
   7 |  Test-WriteError
     |  ~~~~~~~~~~~~~~~
     | Bad
The $? variable is: False
Now the $? variable is: True

For the latter purpose, $PSCmdlet.WriteError() should be used instead.

For native commands (executables), $? is set to True when $LASTEXITCODE is 0, and set to False when $LASTEXITCODE is any other value.

Note

Until PowerShell 7, wrapping a statement within parentheses (...), subexpression syntax $(...), or an array expression @(...) always reset $? to True. For example, (Write-Error) shows $? as True. This behavior changed in PowerShell 7, so that $? always reflects the actual success of the last command run in these expressions.

$^

Contains the first token in the last line received by the session.

$_

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

For more information, see about_PSItem.

$args

Contains an array of values for undeclared parameters that are passed to a function, script, or script block. When you create a function, you can declare the parameters with 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's 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 that Get-Event returns.

$ConsoleFileName

Contains the path of the console file (.psc1) that was most recently used in the session. This variable is populated when you start PowerShell with the PSConsoleFile parameter or when you use the Export-Console cmdlet to export snap-in names to a console file.

When you use the Export-Console cmdlet without parameters, it automatically updates the console file that was most recently used in the session. You can use this automatic variable to determine the file to update.

$EnabledExperimentalFeatures

Contains a list of names of the experimental features that are enabled.

$Error

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.

$Event

Contains a PSEventArgs object that represents the event that's being processed. This variable is populated only within the Action block of an event registration command, such as Register-ObjectEvent. The value of this variable is the same object that the Get-Event cmdlet returns. You can use the properties of the Event variable, such as $Event.TimeGenerated, in an Action script block.

$EventArgs

Contains an object that represents the first event argument that derives from EventArgs of the event that's 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 SourceEventArgs property of the PSEventArgs object that Get-Event returns.

$EventSubscriber

Contains a PSEventSubscriber object that represents the event subscriber of the event that's being processed. This variable is populated only within the Action block of an event registration command. The value of this variable is the same object that the Get-EventSubscriber cmdlet returns.

$ExecutionContext

Contains an EngineIntrinsics object that represents the execution context of the PowerShell host. You can use this variable to find the execution objects that are available to cmdlets.

$false

Contains False. You can use this variable to represent False in commands and scripts instead of using the string "false". The string can be interpreted as True if it's converted to a non-empty string or to a non-zero integer.

$foreach

Contains the enumerator (not the resulting values) of a ForEach loop. The $ForEach variable exists only while the ForEach loop is running; it's deleted after the loop is completed.

Enumerators contain properties and methods you can use to retrieve loop values and change the current loop iteration. For more information, see Using Enumerators.

$HOME

Contains the full path of the user's home directory. On Windows, this variable uses the value of the "$env:USERPROFILE" Windows environment variable, typically C:\Users\<UserName>. On Unix, this variable uses the value of the HOME environment variable.

Important

Windows can redirect the location of the user's profile. This means that $HOME may not have the same value as "$env:HOMEDRIVE$env:HOMEPATH".

$Host

Contains an object that represents the current host application for PowerShell. You can use this variable to represent the current host in commands or to display or change the properties of the host, such as $Host.version or $Host.CurrentCulture, or $Host.UI.RawUI.BackGroundColor = "Red".

Note

The color settings in $Host.PrivateData have been replaced by the $PSStyle preference variable. For more information, see about_ANSI_Terminals.

$input

Contains an enumerator that enumerates all input that's passed to a function. The $input variable is available only to functions, script blocks (which are unnamed functions), and script files (which are saved script blocks).

  • In a function without a begin, process, or end block, the $input variable enumerates the collection of all input to the function.

  • In the begin block, the $input variable contains no data.

  • In the process block, the $input variable contains the current object in the pipeline.

  • In the end block, the $input variable enumerates the collection of all input to the function.

    Note

    You can't use the $input variable inside both the process block and the end block in the same function or script block.

Since $input is an enumerator, accessing any of its properties causes $input to no longer be available. You can store $input in another variable to reuse the $input properties.

Enumerators contain properties and methods you can use to retrieve loop values and change the current loop iteration. For more information, see Using Enumerators.

The $input variable is also available to the command specified by the -Command parameter of pwsh when invoked from the command line. The following example is run from the Windows Command shell.

echo Hello | pwsh -Command """$input World!"""

$IsCoreCLR

Contains $true if the current session is running on the .NET Core Runtime (CoreCLR). Otherwise contains $false.

$IsLinux

Contains $true if the current session is running on a Linux operating system. Otherwise contains $false.

$IsMacOS

Contains $true if the current session is running on a MacOS operating system. Otherwise contains $false.

$IsWindows

Contains $true if the current session is running on a Windows operating system. Otherwise contains $false.

$LASTEXITCODE

Contains the exit code of the last native program or PowerShell script that ran.

For PowerShell scripts, the value of $LASTEXITCODE depends on how the script was called and whether the exit keyword was used:

  • When a script uses the exit keyword:

    $LASTEXITCODE is set to value the specified by the exit keyword. For more information, see about_Language_Keywords.

  • When a script is called directly, like ./Test.ps1, or with the call operator (&) like & ./Test.ps1:

    The value of $LASTEXITCODE isn't changed unless:

    • The script calls another script that uses the exit keyword
    • The script calls a native command
    • The script uses the exit keyword

  • When a script is called with pwsh using the File parameter, $LASTEXITCODE is set to:

    • 1 if the script terminated due to an exception
    • The value specified by the exit keyword, if used in the script
    • 0 if the script completed successfully

  • When a script is called with pwsh using the Command parameter, $LASTEXITCODE is set to:

    • 1 if the script terminated due to an exception or if the result of the last command set $? to $false
    • 0 if the script completed successfully and the result of the last command set $? to $true

For more information on the File and Command parameters, see about_Pwsh.

$Matches

The $Matches variable works with the -match and -notmatch operators. When you submit scalar input to the -match or -notmatch operator, and either one detects a match, they return a Boolean value and populate the $Matches automatic variable with a hash table of any string values that were matched. The $Matches hash table can also be populated with captures when you use regular expressions with the -match operator.

For more information about the -match operator, see about_Comparison_Operators. For more information on regular expressions, see about_Regular_Expressions.

The $Matches variable also works in a switch statement with the -Regex parameter. It's populated the same way as the -match and -notmatch operators. For more information about the switch statement, see about_Switch.

Note

When $Matches is populated in a session, it retains the matched value until it's overwritten by another match. If -match is used again and no match is found, it doesn't reset $Matches to $null. The previously matched value is kept in $Matches until another match is found.

$MyInvocation

Contains information about the current command, such as the name, parameters, parameter values, and information about how the command was started, called, or invoked, such as the name of the script that called the current command.

$MyInvocation is populated only for scripts, function, and script blocks. You can use the information in the System.Management.Automation.InvocationInfo object that $MyInvocation returns in the current script, such as the name of a function ($MyInvocation.MyCommand.Name) to identify the current command. This is useful for finding the name of the current script.

Beginning in PowerShell 3.0, MyInvocation has the following new properties.

  • PSScriptRoot - Contains the full path to the script that invoked the current command. The value of this property is populated only when the caller is a script.
  • PSCommandPath - Contains the full path and filename of the script that invoked the current command. The value of this property is populated only when the caller is a script.

Unlike the $PSScriptRoot and $PSCommandPath automatic variables, the PSScriptRoot and PSCommandPath properties of the $MyInvocation automatic variable contain information about the invoker or calling script, not the current script.

$NestedPromptLevel

Contains the current prompt level. A value of 0 indicates the original prompt level. The value is incremented when you enter a nested level and decremented when you exit it.

For example, PowerShell presents a nested command prompt when you use the $Host.EnterNestedPrompt method. PowerShell also presents a nested command prompt when you reach a breakpoint in the PowerShell debugger.

When you enter a nested prompt, PowerShell pauses the current command, saves the execution context, and increments the value of the $NestedPromptLevel variable. To create additional nested command prompts (up to 128 levels) or to return to the original command prompt, complete the command, or type exit.

The $NestedPromptLevel variable helps you track the prompt level. You can create an alternative PowerShell command prompt that includes this value so that it's always visible.

$null

$null is an automatic variable that contains a null or empty value. You can use this variable to represent an absent or undefined value in commands and scripts.

PowerShell treats $null as an object with a value, or a placeholder, so you can use $null to represent an empty value in a collection of values.

For example, when $null is included in a collection, it's counted as one of the objects.

$a = "one", $null, "three"
$a.count

3

If you pipe the $null variable to the ForEach-Object cmdlet, it generates a value for $null, just as it does for the other objects

"one", $null, "three" | ForEach-Object { "Hello " + $_}

Hello one
Hello
Hello three

As a result, you can't use $null to mean no parameter value. A parameter value of $null overrides the default parameter value.

However, because PowerShell treats the $null variable as a placeholder, you can use it in scripts like the following one, which wouldn't work if $null were ignored.

$calendar = @($null, $null, "Meeting", $null, $null, "Team Lunch", $null)
$days = "Sunday","Monday","Tuesday","Wednesday","Thursday",
        "Friday","Saturday"
$currentDay = 0
foreach($day in $calendar)
{
    if($day -ne $null)
    {
        "Appointment on $($days[$currentDay]): $day"
    }

    $currentDay++
}

Appointment on Tuesday: Meeting
Appointment on Friday: Team lunch

$PID

Contains the process identifier (PID) of the process that's hosting the current PowerShell session.

$PROFILE

Contains the full path of the PowerShell profile for the current user and the current host application. You can use this variable to represent the profile in commands. For example, you can use it in a command to determine whether a profile has been created:

Test-Path $PROFILE

Or, you can use it in a command to create a profile:

New-Item -ItemType file -Path $PROFILE -Force

You can use it in a command to open the profile in notepad.exe:

notepad.exe $PROFILE

$PSBoundParameters

Contains a dictionary of the parameters that are passed to a script or function and their current values. This variable has a value only in a scope where parameters are declared, such as a script or function. You can use it to display or change the current values of parameters or to pass parameter values to another script or function.

In this example, the Test2 function passes the $PSBoundParameters to the Test1 function. The $PSBoundParameters are displayed in the format of Key and Value.

function Test1 {
   param($a, $b)

   # Display the parameters in dictionary format.
   $PSBoundParameters
}

function Test2 {
   param($a, $b)

   # Run the Test1 function with $a and $b.
   Test1 @PSBoundParameters
}

Test2 -a Power -b Shell

Key   Value
---   -----
a     Power
b     Shell

$PSCmdlet

Contains an object that represents the cmdlet or advanced function that's being run.

You can use the properties and methods of the object in your cmdlet or function code to respond to the conditions of use. For example, the ParameterSetName property contains the name of the parameter set that's being used, and the ShouldProcess method adds the WhatIf and Confirm parameters to the cmdlet dynamically.

For more information about the $PSCmdlet automatic variable, see about_Functions_CmdletBindingAttribute and about_Functions_Advanced.

$PSCommandPath

Contains the full path and filename of the script that's being run. This variable is valid in all scripts.

$PSCulture

Beginning in PowerShell 7, $PSCulture reflects the culture of the current PowerShell runspace (session). If the culture is changed in a PowerShell runspace, the $PSCulture value for that runspace is updated.

The culture determines the display format of items such as numbers, currency, and dates, and is stored in a System.Globalization.CultureInfo object. Use Get-Culture to display the computer's culture. $PSCulture contains the Name property's value.

$PSDebugContext

While debugging, this variable contains information about the debugging environment. Otherwise, it contains a null value. As a result, you can use it to determine whether the debugger has control. When populated, it contains a PsDebugContext object that has Breakpoints and InvocationInfo properties. The InvocationInfo property has several useful properties, including the Location property. The Location property indicates the path of the script that's being debugged.

$PSEdition

Contains the same value in $PSVersionTable.PSEdition. This variable is available for use in module manifest files, whereas $PSVersionTable isn't.

$PSHOME

Contains the full path of the installation directory for PowerShell, typically, C:\Program Files\PowerShell\7 in Windows systems. You can use this variable in the paths of PowerShell files. For example, the following command searches the conceptual Help topics for the word Help:

Select-String -Pattern Help -Path $PSHOME\en-US\*.txt

$PSItem

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

For more information, see about_PSItem.

$PSScriptRoot

Contains the full path of the executing script's parent directory.

In PowerShell 2.0, this variable is valid only in script modules (.psm1). Beginning in PowerShell 3.0, it's valid in all scripts.

$PSSenderInfo

Contains information about the user who started the PSSession, including the user identity and the time zone of the originating computer. This variable is available only in PSSessions.

The $PSSenderInfo variable includes a user-configurable property, ApplicationArguments, that by default, contains only the $PSVersionTable from the originating session. To add data to the ApplicationArguments property, use the ApplicationArguments parameter of the New-PSSessionOption cmdlet.

$PSUICulture

Contains the name of the user interface (UI) culture that's configured in the operating system. The UI culture determines which text strings are used for user interface elements, such as menus and messages. This is the value of the System.Globalization.CultureInfo.CurrentUICulture.Name property of the system. To get the System.Globalization.CultureInfo object for the system, use the Get-UICulture cmdlet.

$PSVersionTable

Contains a read-only hash table that displays details about the version of PowerShell that's running in the current session. The table includes the following items:

  • PSVersion - The PowerShell version number
  • PSEdition This property has the value of 'Desktop' for PowerShell 4 and below as well as PowerShell 5.1 on full-featured Windows editions. This property has the value of Core for PowerShell 6 and higher as well as Windows PowerShell 5.1 on reduced-footprint editions like Windows Nano Server or Windows IoT.
  • GitCommitId - The commit Id of the source files, in GitHub,
  • OS - Description of the operating system that PowerShell is running on.
  • Platform - Platform that the operating system is running on. The value on Linux and macOS is Unix. See $IsMacOs and $IsLinux.
  • PSCompatibleVersions - Versions of PowerShell that are compatible with the current version
  • PSRemotingProtocolVersion - The version of the PowerShell remote management protocol.
  • SerializationVersion - The version of the serialization method
  • WSManStackVersion - The version number of the WS-Management stack

$PWD

Contains a path object that represents the full path of the current directory location for the current PowerShell runspace.

Note

PowerShell supports multiple runspaces per process. Each runspace has its own current directory. This isn't the same as the current directory of the process: [System.Environment]::CurrentDirectory.

$Sender

Contains the object that generated this event. 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 Sender property of the PSEventArgs object that Get-Event returns.

$ShellId

Contains the identifier of the current shell.

$StackTrace

Contains a stack trace for the most recent error.

$switch

Contains the enumerator not the resulting values of a Switch statement. The $switch variable exists only while the Switch statement is running; it's deleted when the switch statement completes execution. For more information, see about_Switch.

Enumerators contain properties and methods you can use to retrieve loop values and change the current loop iteration. For more information, see Using Enumerators.

$this

The $this variable is used in script blocks that extend classes to refer to the instance of the class itself.

PowerShell's Extensible Type System (ETS) allows you to add properties to classes using script blocks. In a script block that defines a script property or script method, the $this variable refers to an instance of object of the class that's being extended. For example, PowerShell uses ETS to add the BaseName property to the FileInfo class.

PS> Get-ChildItem .\README.md | Get-Member BaseName | Format-List

TypeName   : System.IO.FileInfo
Name       : BaseName
MemberType : ScriptProperty
Definition : System.Object BaseName {get=if ($this.Extension.Length -gt 0)
             {$this.Name.Remove($this.Name.Length - $this.Extension.Length
             )}else{$this.Name};}

For more information, see about_Types.ps1xml.

In a PowerShell class, the $this variable refers to the instance object of the class itself, allowing access to properties and methods defined in the class. For more information, see about_Classes.

The $this variable is also used by .NET event classes that take script blocks as delegates for the event handler. In this scenario, $this represents the object originating the event, known as the event sender.

$true

Contains True. You can use this variable to represent True in commands and scripts.

Using Enumerators

The $input, $foreach, and $switch variables are all enumerators used to iterate through the values processed by their containing code block.

An enumerator contains properties and methods you can use to advance or reset iteration, or retrieve iteration values. Directly manipulating enumerators isn't considered best practice.

  • Within loops, flow control keywords break and continue should be preferred.

  • Within functions that accept pipeline input, it's best practice to use parameters with the ValueFromPipeline or ValueFromPipelineByPropertyName attributes.

    For more information, see about_Functions_Advanced_Parameters.

MoveNext

The MoveNext method advances the enumerator to the next element of the collection. MoveNext returns True if the enumerator was successfully advanced, False if the enumerator has passed the end of the collection.

Note

The Boolean value returned by MoveNext is sent to the output stream. You can suppress the output by typecasting it to [void] or piping it to Out-Null.

$input.MoveNext() | Out-Null

[void]$input.MoveNext()

Reset

The Reset method sets the enumerator to its initial position, which is before the first element in the collection.

Current

The Current property gets the element in the collection, or pipeline, at the current position of the enumerator.

The Current property continues to return the same property until MoveNext is called.

Examples

Example 1: Using the $input variable

In the following example, accessing the $input variable clears the variable until the next time the process block executes. Using the Reset method resets the $input variable to the current pipeline value.

function Test
{
    begin
    {
        $i = 0
    }

    process
    {
        "Iteration: $i"
        $i++
        "`tInput: $input"
        "`tAccess Again: $input"
        $input.Reset()
        "`tAfter Reset: $input"
    }
}

"one","two" | Test

Iteration: 0
    Input: one
    Access Again:
    After Reset: one
Iteration: 1
    Input: two
    Access Again:
    After Reset: two

The process block automatically advances the $input variable even if you don't access it.

$skip = $true
function Skip
{
    begin
    {
        $i = 0
    }

    process
    {
        "Iteration: $i"
        $i++
        if ($skip)
        {
            "`tSkipping"
            $skip = $false
        }
        else
        {
            "`tInput: $input"
        }
    }
}

"one","two" | Skip

Iteration: 0
    Skipping
Iteration: 1
    Input: two

Example 2: Using $input outside the process block

Outside of the process block the $input variable represents all the values piped into the function.

  • Accessing the $input variable clears all values.
  • The Reset method resets the entire collection.
  • The Current property is never populated.
  • The MoveNext method returns false because the collection can't be advanced.
    • Calling MoveNext clears out the $input variable.
Function All
{
    "All Values: $input"
    "Access Again: $input"
    $input.Reset()
    "After Reset: $input"
    $input.MoveNext() | Out-Null
    "After MoveNext: $input"
}

"one","two","three" | All

All Values: one two three
Access Again:
After Reset: one two three
After MoveNext:

Example 3: Using the $input.Current property

With the Current property, the current pipeline value can be accessed multiple times without using the Reset method. The process block doesn't automatically call the MoveNext method.

The Current property is never populated unless you explicitly call MoveNext. The Current property can be accessed multiple times inside the process block without clearing its value.

function Current
{
    begin
    {
        $i = 0
    }

    process
    {
        "Iteration: $i"
        $i++
        "`tBefore MoveNext: $($input.Current)"
        $input.MoveNext() | Out-Null
        "`tAfter MoveNext: $($input.Current)"
        "`tAccess Again: $($input.Current)"
    }
}

"one","two" | Current

Iteration: 0
    Before MoveNext:
    After MoveNext: one
    Access Again: one
Iteration: 1
    Before MoveNext:
    After MoveNext: two
    Access Again: two

Example 4: Using the $foreach variable

Unlike the $input variable, the $foreach variable always represents all items in the collection when accessed directly. Use the Current property to access the current collection element, and the Reset and MoveNext methods to change its value.

Note

Each iteration of the foreach loop automatically calls the MoveNext method.

The following loop only executes twice. In the second iteration, the collection is moved to the third element before the iteration is complete. After the second iteration, there are now no more values to iterate, and the loop terminates.

The MoveNext property doesn't affect the variable chosen to iterate through the collection ($Num).

$i = 0
foreach ($num in ("one","two","three"))
{
    "Iteration: $i"
    $i++
    "`tNum: $num"
    "`tCurrent: $($foreach.Current)"

    if ($foreach.Current -eq "two")
    {
        "Before MoveNext (Current): $($foreach.Current)"
        $foreach.MoveNext() | Out-Null
        "After MoveNext (Current): $($foreach.Current)"
        "Num hasn't changed: $num"
    }
}

Iteration: 0
        Num: one
        Current: one
Iteration: 1
        Num: two
        Current: two
Before MoveNext (Current): two
After MoveNext (Current): three
Num hasn't changed: two

Using the Reset method resets the current element in the collection. The following example loops through the first two elements twice because the Reset method is called. After the first two loops, the if statement fails and the loop iterates through all three elements normally.

Important

This could result in an infinite loop.

$stopLoop = 0
foreach ($num in ("one","two", "three"))
{
    ("`t" * $stopLoop) + "Current: $($foreach.Current)"

    if ($num -eq "two" -and $stopLoop -lt 2)
    {
        $foreach.Reset()
        ("`t" * $stopLoop) + "Reset Loop: $stopLoop"
        $stopLoop++
    }
}

Current: one
Current: two
Reset Loop: 0
        Current: one
        Current: two
        Reset Loop: 1
                Current: one
                Current: two
                Current: three

Example 5: Using the $switch variable

The $switch variable has the exact same rules as the $foreach variable. The following example demonstrates all the enumerator concepts.

Note

Note how the NotEvaluated case is never executed, even though there's no break statement after the MoveNext method.

$values = "Start", "MoveNext", "NotEvaluated", "Reset", "End"
$stopInfinite = $false
switch ($values)
{
    "MoveNext" {
        "`tMoveNext"
        $switch.MoveNext() | Out-Null
        "`tAfter MoveNext: $($switch.Current)"
    }
    # This case is never evaluated.
    "NotEvaluated" {
        "`tAfterMoveNext: $($switch.Current)"
    }

    "Reset" {
        if (!$stopInfinite)
        {
            "`tReset"
            $switch.Reset()
            $stopInfinite = $true
        }
    }

    default {
        "Default (Current): $($switch.Current)"
    }
}

Default (Current): Start
    MoveNext
    After MoveNext: NotEvaluated
    Reset
Default (Current): Start
    MoveNext
    After MoveNext: NotEvaluated
Default (Current): End

See also