About Script Blocks

Short description

Defines what a script block is and explains how to use script blocks in the PowerShell programming language.

Long description

In the PowerShell programming language, a script block is a collection of statements or expressions that can be used as a single unit. A script block can accept arguments and return values.

Syntactically, a script block is a statement list in braces, as shown in the following syntax:

{<statement list>}

A script block returns the output of all the commands in the script block, either as a single object or as an array.

You can also specify a return value using the return keyword. The return keyword does not affect or suppress other output returned from your script block. However, the return keyword exits the script block at that line. For more information, see about_Return.

Like functions, a script block can include parameters. Use the Param keyword to assign named parameters, as shown in the following syntax:

{
Param([type]$Parameter1 [,[type]$Parameter2])
<statement list>
}

Note

In a script block, unlike a function, you cannot specify parameters outside the braces.

Like functions, script blocks can include the DynamicParam, Begin, Process, and End keywords. For more information, see about_Functions and about_Functions_Advanced.

Using Script Blocks

A script block is an instance of a Microsoft .NET Framework type System.Management.Automation.ScriptBlock. Commands can have script block parameter values. For example, the Invoke-Command cmdlet has a ScriptBlock parameter that takes a script block value, as shown in this example:

Invoke-Command -ScriptBlock { Get-Process }
Handles  NPM(K)    PM(K)     WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----     ----- -----   ------     -- -----------
999          28    39100     45020   262    15.88   1844 communicator
721          28    32696     36536   222    20.84   4028 explorer
...

Invoke-Command can also execute script blocks that have parameter blocks. Parameters are assigned by position using the ArgumentList parameter.

Invoke-Command -ScriptBlock { param($p1, $p2)
"p1: $p1"
"p2: $p2"
} -ArgumentList "First", "Second"
p1: First
p2: Second

The script block in the preceding example uses the param keyword to create a parameters $p1 and $p2. The string "First" is bound to the first parameter ($p1) and "Second" is bound to ($p2).

You can use variables to store and execute script blocks. The example below stores a script block in a variable and passes it to Invoke-Command.

$a = { Get-Service BITS }
Invoke-Command -ScriptBlock $a
Status   Name               DisplayName
------   ----               -----------
Running  BITS               Background Intelligent Transfer Ser...

The call operator is another way to execute script blocks stored in a variable. Like Invoke-Command, the call operator executes the script block in a child scope. The call operator can make it easier for you to use parameters with your script blocks.

$a ={ param($p1, $p2)
"p1: $p1"
"p2: $p2"
}
&$a -p2 "First" -p1 "Second"
p1: Second
p2: First

You can store the output from your script blocks in a variable using assignment.

PS>  $a = { 1 + 1}
PS>  $b = &$a
PS>  $b
2
PS>  $a = { 1 + 1}
PS>  $b = Invoke-Command $a
PS>  $b
2

For more information about the call operator, see about_Operators.

Using delay-bind script blocks with parameters

A typed parameter that accepts pipeline input (by Value) or (by PropertyName) enables use of delay-bind script blocks on the parameter. Within the delay-bind script block, you can reference the piped in object using the pipeline variable $_.

# Renames config.log to old_config.log
dir config.log | Rename-Item -NewName {"old_" + $_.Name}

In more complex cmdlets, delay-bind script blocks allow the reuse of one piped in object to populate other parameters.

Notes on delay-bind script blocks as parameters:

  • You must explicitly specify any parameter names you use with delay-bind script blocks.

  • The parameter must not be untyped, and the parameter's type cannot be [scriptblock] or [object].

  • You receive an error if you use a delay-bind script block without providing pipeline input.

    Rename-Item -NewName {$_.Name + ".old"}
    
    Rename-Item : Cannot evaluate parameter 'NewName' because its argument is
    specified as a script block and there is no input. A script block cannot
    be evaluated without input.
    At line:1 char:23
    +  Rename-Item -NewName {$_.Name + ".old"}
    +                       ~~~~~~~~~~~~~~~~~~
        + CategoryInfo          : MetadataError: (:) [Rename-Item],
          ParameterBindingException
        + FullyQualifiedErrorId : ScriptBlockArgumentNoInput,
          Microsoft.PowerShell.Commands.RenameItemCommand
    

See Also

about_Functions

about_Functions_Advanced

about_Operators