You want to store your commands in a script so that you can share them or reuse them later.

To write a PowerShell script, create a plain-text file with your editor of choice. Add your PowerShell commands to that script (the same PowerShell commands you use from the interactive shell), and then save it with a .ps1 extension.

One of the most important things to remember about PowerShell is that running scripts and working at the command line are essentially equivalent operations. If you see it in a script, you can type it or paste it at the command line. If you typed it on the command line, you can paste it into a text file and call it a script.

Once you write your script, PowerShell lets you call it in the same way that you call other programs and existing tools. Running a script does the same thing as running all the commands in that script.

The first time you try to run a script in PowerShell, you’ll likely see the following error message:

File c:	oolsmyFirstScript.ps1 cannot be loaded because the execution of 
scripts is disabled on this system. Please see "get-help about_signing" for 
more details.
At line:1 char:12
+ myFirstScript <<<<

Since relatively few computer users write scripts, PowerShell’s default security policies prevent scripts from running. Once you begin writing scripts, though, you should configure this policy to something less restrictive. For information on how to configure your execution policy, see Enable Scripting Through an Execution Policy.

When it comes to the filename of your script, picking a descriptive name is the best way to guarantee that you will always remember what that script does—or at least have a good idea. This is an issue that PowerShell tackles elegantly, by naming every cmdlet in the Verb-Noun pattern: a command that performs an action (verb) on an item (noun). As an example of the usefulness of this philosophy, consider the names of typical Windows commands given in Example 11-1.

PS > dir $env:WINDIRSystem32*.exe | Select-Object Name

Name
----
accwiz.exe
actmovie.exe
ahui.exe
alg.exe
append.exe
arp.exe
asr_fmt.exe
asr_ldm.exe
asr_pfu.exe
at.exe
atmadm.exe
attrib.exe
(...)

Compare this to the names of some standard Windows PowerShell cmdlets, given in Example 11-2.

PS > Get-Command | Select-Object Name

Name
----
Add-Content
Add-History
Add-Member
Add-PSSnapin
Clear-Content
Clear-Item
Clear-ItemProperty
Clear-Variable
Compare-Object
ConvertFrom-SecureString
Convert-Path
ConvertTo-Html
(...)

As an additional way to improve discovery, PowerShell takes this even further with the philosophy (and explicit goal) that “you can manage 80 percent of your system with less than 50 verbs.” As you learn the standard verbs for a concept, such as Get (which represents the standard concepts of Read, Open, and so on), you can often guess the verb of a command as the first step in discovering it.

When you name your script (especially if you intend to share it), make every effort to pick a name that follows these conventions. Find a Verb Appropriate for a Command Name shows a useful cmdlet to help you find a verb to name your scripts properly. As evidence of its utility for scripts, consider some of the scripts included in this book:

PS > dir | select Name

Name
----
Compare-Property.ps1
Connect-WebService.ps1
Convert-TextObject.ps1
Get-AliasSuggestion.ps1
Get-Answer.ps1
Get-Characteristics.ps1
Get-OwnerReport.ps1
Get-PageUrls.ps1
Invoke-CmdScript.ps1
New-GenericObject.ps1
Select-FilteredObject.ps1
(...)

Like the PowerShell cmdlets, the names of these scripts are clear, are easy to understand, and use verbs from PowerShell’s standard verb list.

Run Programs, Scripts, and Existing Tools

Find a Verb Appropriate for a Command Name

Appendix J

You have commands in your script that you want to call multiple times or a section of your script that you consider to be a “helper” for the main purpose of your script.

Place this common code in a function, and then call that function instead. For example, this Celsius conversion code in a script:

param([double] $fahrenheit)

## Convert it to Celsius
$celsius= $fahrenheit - 32
$celsius = $celsius / 1.8

## Output the answer
"$fahrenheit degrees Fahrenheit is $celsius degrees Celsius."

could be placed in a function (itself placed in a script):

param([double] $fahrenheit)

## Convert Fahrenheit to Celsius
function ConvertFahrenheitToCelsius([double] $fahrenheit)
{
    $celsius = $fahrenheit - 32
    $celsius = $celsius / 1.8 
    $celsius
}

$celsius = ConvertFahrenheitToCelsius $fahrenheit

## Output the answer
"$fahrenheit degrees Fahrenheit is $celsius degrees Celsius."

Although using a function arguably makes this specific script longer and more difficult to understand, the technique is extremely valuable (and used) in almost all nontrivial scripts.

Once you define a function, any command after that definition can use it. This means that you must define your function before any part of your script that uses it. You might find this unwieldy if your script defines many functions, as the function definitions obscure the main logic portion of your script. If this is the case, you can put your main logic in a “Main” function, as described in Organize Scripts for Improved Readability.

In PowerShell, calling a function is designed to feel just like calling a cmdlet or a script. As a user, you should not have to know whether a little helper routine was written as a cmdlet, script, or function. When you call a function, simply add the parameters after the function name, with spaces separating each one (as shown in the solution). This is in contrast to the way that you call functions in many programming languages (such as C#), where you use parentheses after the function name and commas between each parameter.

## Correct
ConvertFahrenheitToCelsius $fahrenheit

## Incorrect
ConvertFahrenheitToCelsius($fahrenheit)

Also, notice that the return value from a function is anything that the function writes to the output pipeline (such as $celsius in the solution). You can write return $celsius if you want, but it is unnecessary.

For more information about writing functions, see Writing Scripts, Reusing Functionality. For more information about PowerShell’s looping statements, see Repeat Operations with Loops.

Repeat Operations with Loops

Writing Scripts, Reusing Functionality

You are writing a new script or function and want to select an appropriate verb for that command.

Review the output of the Get-Verb command to find a verb appropriate for your command:

PS > Get-Verb In* | Format-Table -Auto

Verb       Group
----       -----
Initialize Data
Install    Lifecycle
Invoke     Lifecycle

Consistency of command names is one of PowerShell’s most beneficial features, largely due to its standard set of verbs. While descriptive command names (such as Stop-Process) make it clear what a command does, standard verbs make commands easier to discover.

For example, many technologies have their own words for creating something: New, Create, Instantiate, Build, and more. When a user looks for a command (without the benefit of standard verbs), the user has to know the domain-specific terminology for that action. If the user doesn’t know the domain-specific verb, the user is forced to page through long lists of commands in the hope that something rings a bell.

When commands use PowerShell’s standard verbs, however, discovery becomes much easier. Once users learn the standard verb for an action, they don’t need to search for its domain-specific alternatives. Most importantly, the time they invest (actively or otherwise) learning the standard PowerShell verbs improves their efficiency with all commands, not just commands from a specific domain.

To make it easier to select a standard verb while writing a script or function, PowerShell provides a Get-Verb function. You can review the output of that function to find a verb suitable for your command. For an even more detailed description of the standard verbs, see Appendix J.

Selectively Export Commands from a Module

Appendix J

You have a section of your script that works nearly the same for all input, aside from a minor change in logic.

As shown in Example 11-3, place the minor logic differences in a script block, and then pass that script block as a parameter to the code that requires it. Use the invoke operator (&) to execute the script block.

##############################################################################
##
## Invoke-ScriptBlock
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

<#

.SYNOPSIS

Apply the given mapping command to each element of the input. (Note that
PowerShell includes this command natively, and calls it Foreach-Object)

.EXAMPLE

1,2,3 | Invoke-ScriptBlock { $_ * 2 }

#>

param(
    ## The scriptblock to apply to each incoming element
    [ScriptBlock] $MapCommand
)

begin
{
    Set-StrictMode -Version Latest
}
process
{
    & $mapCommand
}

      

Imagine a script that needs to multiply all the elements in a list by two:

function MultiplyInputByTwo
{ 
   process
   {
       $_ * 2 
   }
}

but it also needs to perform a more complex calculation:

function MultiplyInputComplex
{ 
   process
   {
       ($_ + 2) * 3
   }
}

These two functions are strikingly similar, except for the single line that actually performs the calculation. As we add more calculations, this quickly becomes more evident. Adding each new seven-line function gives us only one unique line of value!

PS > 1,2,3 | MultiplyInputByTwo
2
4
6
PS > 1,2,3 | MultiplyInputComplex
9
12
15

If we instead use a script block to hold this “unknown” calculation, we don’t need to keep on adding new functions:

PS > 1,2,3 | Invoke-ScriptBlock { $_ * 2 }
2
4
6
PS > 1,2,3 | Invoke-ScriptBlock { ($_ + 2) * 3 }
9
12
15
PS > 1,2,3 | Invoke-ScriptBlock { ($_ + 3) * $_ }
4
10
18

In fact, the functionality provided by Invoke-ScriptBlock is so helpful that it is a standard PowerShell cmdlet—called Foreach-Object. For more information about script blocks, see Writing Scripts, Reusing Functionality. For more information about running scripts, see Run Programs, Scripts, and Existing Tools.

Run Programs, Scripts, and Existing Tools

Writing Scripts, Reusing Functionality

You want your script or function to return data to whatever called it.

To return data from a script or function, write that data to the output pipeline:

##############################################################################
##
## Get-Tomorrow
##
## Get the date that represents tomorrow
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

Set-StrictMode -Version Latest

function GetDate
{
    Get-Date
}

$tomorrow = (GetDate).AddDays(1)
$tomorrow

In PowerShell, any data that your function or script generates gets sent to the output pipeline, unless something captures that output. The GetDate function generates data (a date) and does not capture it, so that becomes the output of the function. The portion of the script that calls the GetDate function captures that output and then manipulates it.

Finally, the script writes the $tomorrow variable to the pipeline without capturing it, so that becomes the return value of the script itself.

PS > $collection = New-Object System.Collections.ArrayList
PS > $collection.Add("Hello")
0
PS > [void] $collection.Add("Hello")

Even with this “pipeline output becomes the return value” philosophy, PowerShell continues to support the traditional return keyword as a way to return from a function or script. If you specify anything after the keyword (such as return "Hello"), PowerShell treats that as a "Hello" statement followed by a return statement.

If you write a collection (such as an array or ArrayList) to the output pipeline, PowerShell in fact writes each element of that collection to the pipeline. To keep the collection intact as it travels down the pipeline, prefix it with a comma when you return it. This returns a collection (that will be unraveled) with one element: the collection you wanted to keep intact.

function WritesObjects
{
    $arrayList = New-Object System.Collections.ArrayList
    [void] $arrayList.Add("Hello")
    [void] $arrayList.Add("World")

    $arrayList
}

function WritesArrayList
{
    $arrayList = New-Object System.Collections.ArrayList
    [void] $arrayList.Add("Hello")
    [void] $arrayList.Add("World")

    ,$arrayList
}

$objectOutput = WritesObjects

# The following command would generate an error
# $objectOutput.Add("Extra")

$arrayListOutput = WritesArrayList
$arrayListOutput.Add("Extra")

Although relatively uncommon in PowerShell’s world of fully structured data, you may sometimes want to use an exit code to indicate the success or failure of your script. For this, PowerShell offers the exit keyword.

For more information about the return and exit statements, see Writing Scripts, Reusing Functionality and Determine the Status of the Last Command.

Determine the Status of the Last Command

Writing Scripts, Reusing Functionality

You’ve developed a useful set of commands or functions. You want to offer them to the user or share them between multiple scripts.

First, place these common function definitions by themselves in a file with the extension .psm1, as shown in Example 11-4.

##############################################################################
##
## Temperature.psm1
## Commands that manipulate and convert temperatures
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

## Convert Fahrenheit to Celsius
function Convert-FahrenheitToCelsius([double] $fahrenheit)
{
    $celsius = $fahrenheit - 32
    $celsius = $celsius / 1.8
    $celsius
}

## Convert Celsius to Fahrenheit
function Convert-CelsiusToFahrenheit([double] $celsius)
{
    $fahrenheit = $celsius * 1.8
    $fahrenheit = $fahrenheit + 32
    $fahrenheit
}
      

Next, place that file in your Modules directory (as defined in the PSModulePath environment variable), in a subdirectory with the same name. For example, place Temperature.psm1 in <My Documents>WindowsPowerShellModulesTemperature. Call the Import-Module command to import the module (and its commands) into your session, as shown by Example 11-5.

PS > Import-Module Temperature
PS > Convert-FahrenheitToCelsius 81
27.2222222222222

PowerShell modules give you an easy way to package related commands and functionality. As the solution demonstrates, writing a module is as simple as adding functions to a file.

As with the naming of core commands, the naming of commands packaged in a module plays a critical role in giving users a consistent and discoverable PowerShell experience. When you name the commands in your module, ensure that they follow a Verb-Noun syntax and that you select verbs from PowerShell’s standard set of verbs. If your module does not follow these standards, your users will receive a warning message when they load your module. For information about how make your module commands discoverable (and as domain-specific as required), see Selectively Export Commands from a Module.

In addition to creating the .psm1 file that contains your module’s commands, you should also create a module manifest to describe its contents and system requirements. Module manifests let you define the module’s author, company, copyright information, and more. For more information, see the New-ModuleManifest cmdlet.

After writing a module, the last step is making it available to the system. When you call Import-Module <module name> to load a module, PowerShell looks through each directory listed in the PSModulePath environment variable.

If PowerShell finds a directory named <module name>, it looks in that directory for a psm1 file with that name as well. Once it finds the psm1 file, it loads that module into your session. In addition to psm1 files, PowerShell also supports module manifest (psd1) files that let you define a great deal of information about the module: its author, description, nested modules, version requirements, and much more. For more information, type Get-Help New-ModuleManifest.

If you want to make your module available to just yourself (or the “current user” if installing your module as part of a setup process), place it in the per-user modules folder: <My Documents>WindowsPowerShellModules<module name>. If you want to make the module available to all users of the system, place your module in its own directory under the Program Files directory, and then add that directory to the system-wide PSModulePath environment variable.

If you don’t want to permanently install your module, you can instead specify the complete path to the psm1 file when you load the module. For example:

Import-Module c:	oolsTemperature.psm1

If you want to load a module from the same directory that your script is in, see Find Your Script’s Location.

When you load a module from a script, PowerShell makes the commands from that module available to the entire session. If your script loads the Temperature module, for example, the functions in that module will still be available after your script exits. To ensure that your script doesn’t accidentally influence the user’s session after it exits, you should remove any modules that you load:

$moduleToRemove = $null
if(-not (Get-Module <Module Name>))
{
    $moduleToRemove = Import-Module <Module Name> -Passthru
}

######################
##
## script goes here
##
######################

if($moduleToRemove)
{
    $moduleToRemove | Remove-Module
}

If you have a module that loads a helper module (as opposed to a script that loads a helper module), this step is not required. Modules loaded by a module impact only the module that loads them.

If you want to let users configure your module when they load it, you can define a parameter block at the beginning of your module. These parameters then get filled through the -ArgumentList parameter of the Import-Module command. For example, a module that takes a “retry count” and website as parameters:

param(
    [int] $RetryCount,
    [URI] $Website
)

function Get-Page
{
    ....

The user would load the module with the following command line:

Import-Module <module name> -ArgumentList 10,"http://www.example.com"
Get-Page "/index.html"

One important point when it comes to the -ArgumentList parameter is that its support for user input is much more limited than support offered for most scripts, functions, and script blocks. PowerShell lets you access the parameters in most param() statements by name, by alias, and in or out of order. Arguments supplied to the Import-Module command, on the other hand, must be supplied as values only, and in the exact order the module defines them.

For more information about accessing arguments of a command, see Access Arguments of a Script, Function, or Script Block. For more information about importing a module (and the different types of modules available), see Extend Your Shell with Additional Commands. For more information about modules, type Get-Help about_Modules.

Extend Your Shell with Additional Commands

Access Arguments of a Script, Function, or Script Block

Get-Help about_Modules

You have a function or script that needs to maintain state between invocations.

Place those commands in a module. Store any information you want to retain in a variable, and give that variable a SCRIPT scope. See Example 11-6.

##############################################################################
##
## PersistentState.psm1
## Demonstrates persistent state through module-scoped variables
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

$SCRIPT:memory = $null

function Set-Memory
{
    param(
        [Parameter(ValueFromPipeline = $true)]
        $item
    )

    begin { $SCRIPT:memory = New-Object System.Collections.ArrayList }
    process { $null = $memory.Add($item) }
}

function Get-Memory
{
    $memory.ToArray()
}

Set-Alias remember Set-Memory
Set-Alias recall Get-Memory

Export-ModuleMember -Function Set-Memory,Get-Memory
Export-ModuleMember -Alias remember,recall
      

When writing scripts or commands, you’ll frequently need to maintain state between the invocation of those commands. For example, your commands might remember user preferences, cache configuration data, or store other types of module state. See Example 11-7.

PS > Import-Module PersistentState
PS > Get-Process -Name PowerShell | remember
PS > recall

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
    527       6    32704      44140   172     2.13   2644 powershell
    517       7    23080      33328   154     1.81   2812 powershell
    357       6    31848      33760   165     1.42   3576 powershell      

In PowerShell version one, the only way to accomplish these goals was to store the information in a global variable. This introduces two problems, though.

The first problem is that global variables impact much more than just the script that defines them. Once your script stores information in a global variable, it pollutes the user’s session. If the user has a variable with the same name, your script overwrites its contents. The second problem is the natural counterpart to this pollution. When your script stores information in a global variable, both the user and other scripts have access to it. Due to accident or curiosity, it is quite easy for these “internal” global variables to be damaged or corrupted.

PowerShell version two resolves this issue through the introduction of modules. By placing your commands in a module, PowerShell makes variables with a script scope available to all commands in that module. In addition to making script-scoped variables available to all of your commands, PowerShell maintains their value between invocations of those commands.

New-PSDrive -Name Temp FileSystem -Root C:Temp -Scope Global

For more information about variables and their scopes, see Control Access and Scope of Variables and Other Items. For more information about defining a module, see Package Common Commands in a Module.

Control Access and Scope of Variables and Other Items

Package Common Commands in a Module

You have a module and want to export only certain commands from that module.

Use the Export-ModuleMember cmdlet to declare the specific commands you want exported. All other commands then remain internal to your module. See Example 11-8.

##############################################################################
##
## SelectiveCommands.psm1
## Demonstrates the selective export of module commands
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

## An internal helper function
function MyInternalHelperFunction
{
    "Result from my internal helper function"
}

## A command exported from the module
function Get-SelectiveCommandInfo
{
    "Getting information from the SelectiveCommands module"
    MyInternalHelperFunction
}

## Alternate names for our standard command
Set-Alias gsci Get-SelectiveCommandInfo
Set-Alias DomainSpecificVerb-Info Get-SelectiveCommandInfo

## Export specific commands
Export-ModuleMember -Function Get-SelectiveCommandInfo
Export-ModuleMember -Alias gsci,DomainSpecificVerb-Info
      

When PowerShell imports a module, it imports all functions defined in that module by default. This makes it incredibly simple (as module authors) to create a library of related commands.

Once your module commands get more complex, you’ll often write helper functions and support routines. Since these commands aren’t intended to be exposed directly to users, you’ll instead need to selectively export commands from your module. The Export-ModuleMember command allows exactly that.

Once your module includes a call to Export-ModuleMember, PowerShell no longer exports all functions in your module. Instead, it exports only the commands that you define. The first call to Export-ModuleMember in Example 11-8 demonstrates how to selectively export a function from a module.

Since consistency of command names is one of PowerShell’s most beneficial features, PowerShell generates a warning message if your module exports functions (either explicitly or by default) that use nonstandard verbs. For example, imagine that you have a technology that uses regenerate configuration as a highly specific phrase for a task. In addition, it already has a regen command to accomplish this task.

You might naturally consider Regenerate-Configuration and regen as function names to export from your module, but doing that would alienate users who don’t have a strong background in your technology. Without your same technical expertise, they wouldn’t know the name of the command, and instead would instinctively look for Reset-Configuration, Restore-Configuration, or Initialize-Configuration based on their existing PowerShell knowledge. In this situation, the solution is to name your functions with a standard verb and also use command aliases to support your domain-specific experts.

The Export-ModuleMember cmdlet supports this situation as well. In addition to letting you selectively export commands from your module, it also lets you export alternative names (aliases) for your module commands. The second call to Export-ModuleMember in Example 11-8 (along with the alias definitions that precede it) demonstrates how to export aliases from a module.

For more information about command naming, see Find a Verb Appropriate for a Command Name. For more information about writing a module, see Package Common Commands in a Module.

Control Access and Scope of Variables and Other Items

Find a Verb Appropriate for a Command Name

Package Common Commands in a Module

You have a module and want to examine its internal variables and functions.

Use the Enter-Module script (Example 11-9) to temporarily enter the module and invoke commands within its scope.

##############################################################################
##
## Enter-Module
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

<#

.SYNOPSIS

Lets you examine internal module state and functions by executing user
input in the scope of the supplied module.

.EXAMPLE

PS >Import-Module PersistentState
PS >Get-Module PersistentState

ModuleType Name                      ExportedCommands
---------- ----                      ----------------
Script     PersistentState           {Set-Memory, Get-Memory}


PS >"Hello World" | Set-Memory
PS >$m = Get-Module PersistentState
PS >Enter-Module $m
PersistentState: dir variable:mem*

Name                           Value
----                           -----
memory                         {Hello World}

PersistentState: exit
PS >

#>

param(
    ## The module to examine
    [System.Management.Automation.PSModuleInfo] $Module
)

Set-StrictMode -Version Latest

$userInput = Read-Host $($module.Name)
while($userInput -ne "exit")
{
    $scriptblock = [ScriptBlock]::Create($userInput)
    & $module $scriptblock

    $userInput = Read-Host $($module.Name)
}

      

PowerShell modules are an effective way to create sets of related commands that share private state. While commands in a module can share private state between themselves, PowerShell prevents that state from accidentally impacting the rest of your PowerShell session.

When you are developing a module, though, you might sometimes need to interact with this internal state for diagnostic purposes. To support this, PowerShell lets you target a specific module with the invocation (&) operator:

PS > $m = Get-Module PersistentState
PS > & $m { dir variable:mem* }

Name                           Value
----                           -----
memory                         {Hello World}

This syntax gets cumbersome for more detailed investigation tasks, so Enter-Module automates the prompting and invocation for you.

For more information about writing a module, see Package Common Commands in a Module.

Package Common Commands in a Module

You have a module and want to perform some action (such as cleanup tasks) when that module is removed.

Assign a script block to the $MyInvocation.MyCommand.ScriptBlock.Module.OnRemove event. Place any cleanup commands in that script block. See Example 11-10.

##############################################################################
##
## TidyModule.psm1
## Demonstrates how to handle cleanup tasks when a module is removed
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

<#

.EXAMPLE

PS >Import-Module TidyModule
PS >$TidyModuleStatus
Initialized
PS >Remove-Module TidyModule
PS >$TidyModuleStatus
Cleaned Up

#>

## Perform some initialization tasks
$GLOBAL:TidyModuleStatus = "Initialized"

## Register for cleanup
$MyInvocation.MyCommand.ScriptBlock.Module.OnRemove = {
    $GLOBAL:TidyModuleStatus = "Cleaned Up"
}
      

PowerShell modules have a natural way to define initialization requirements (any script written in the body of the module), but cleanup requirements are not as simple.

During module creation, you can access your module through the $MyInvocation.MyCommand.ScriptBlock.Module property. Each module has an OnRemove event, which you can then subscribe to by assigning it a script block. When PowerShell unloads your module, it invokes that script block.

Beware of using this technique for extremely sensitive cleanup requirements. If the user simply exits the PowerShell window, the OnRemove event is not processed. If this is a concern, register for the PowerShell.Exiting engine event and remove your module from there:

Register-EngineEvent PowerShell.Exiting { Remove-Module TidyModule }

For PowerShell to handle this event, the user must use the exit keyword to close the session, rather than the X button at the top right of the console window. In the Integrated Scripting Environment, the close button generates this event as well. This saves the user from having to remember to call Remove-Module.

For more information about writing a module, see Package Common Commands in a Module. For more information about PowerShell events, see Create and Respond to Custom Events.

Package Common Commands in a Module

Create and Respond to Custom Events

You want to access the arguments provided to a script, function, or script block.

To access arguments by name, use a param statement:

param($firstNamedArgument, [int] $secondNamedArgument = 0)

"First named argument is: $firstNamedArgument"
"Second named argument is: $secondNamedArgument"

To access unnamed arguments by position, use the $args array:

"First positional argument is: " + $args[0]
"Second positional argument is: " + $args[1]

You can use these techniques in exactly the same way with scripts, functions, and script blocks, as illustrated by Example 11-11.

##############################################################################
##
## Get-Arguments
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

<#

.SYNOPSIS

Uses command-line arguments

#>

param(
    ## The first named argument
    $FirstNamedArgument,

    ## The second named argument
    [int] $SecondNamedArgument = 0
)

Set-StrictMode -Version Latest

## Display the arguments by name
"First named argument is: $firstNamedArgument"
"Second named argument is: $secondNamedArgument"

function GetArgumentsFunction
{
    ## We could use a param statement here, as well
    ## param($firstNamedArgument, [int] $secondNamedArgument = 0)

    ## Display the arguments by position
    "First positional function argument is: " + $args[0]
    "Second positional function argument is: " + $args[1]
}

GetArgumentsFunction One Two

$scriptBlock =
{
    param($firstNamedArgument, [int] $secondNamedArgument = 0)

    ## We could use $args here, as well
    "First named scriptblock argument is: $firstNamedArgument"
    "Second named scriptblock argument is: $secondNamedArgument"
}

& $scriptBlock -First One -Second 4.5
      

Example 11-11 produces the following output:

PS > Get-Arguments First 2
First named argument is: First
Second named argument is: 2
First positional function argument is: One
Second positional function argument is: Two
First named scriptblock argument is: One
Second named scriptblock argument is: 4

Although PowerShell supports both the param keyword and the $args array, you will most commonly want to use the param keyword to define and access script, function, and script block parameters.

When you use the param keyword to define your parameters, PowerShell provides your script or function with many useful features that allow users to work with your script much as they work with cmdlets:

function Invoke-MyAdvancedFunction
{
    [CmdletBinding()]
    param()
    
    Write-Verbose "Verbose Message"
}

PS > Invoke-MyAdvancedFunction
PS > Invoke-MyAdvancedFunction -Verbose
VERBOSE: Verbose Message

function Reverse
{
    $argsEnd = $args.Length - 1
    $args[$argsEnd..0]
}

PS > Reverse 1 2 3 4
4
3
2
1

Run Programs, Scripts, and Existing Tools

Add Validation to Parameters

Provide -WhatIf, -Confirm, and Other Cmdlet Features

Access Information About Your Command’s Invocation

Writing Scripts, Reusing Functionality

You want to ensure that user input to a parameter satisfies certain restrictions or constraints.

Use the [Parameter()] attribute to declare the parameter as mandatory, positional, part of a mutually exclusive set of parameters, or able to receive its input from the pipeline.

param(
    [Parameter(
        Mandatory = $true,
        Position = 0,
        ValueFromPipeline = $true,
        ValueFromPipelineByPropertyName = $true)]
    [string[]] $Name
)

Use additional validation attributes to define aliases, support for null or empty values, count restrictions (for collections), length restrictions (for strings), regular expression requirements, range requirements (for numbers), permissible value requirements, or even arbitrary script requirements.

param(
    [ValidateLength(5,10)]
    [string] $Name
)
        
"Hello $Name"

Traditional shells require extensions (scripts and commands) to write their parameter support by hand, resulting in a wide range of behavior. Some implement a bare, confusing minimum of support. Others implement more complex features, but differently than any other command. The bare, confusing minimum is by far the most common, as writing fully featured parameter support is a complex endeavor.

Luckily, the PowerShell engine already wrote all of the complex parameter handling support and manages all of this detail for you. Rather than write the code to enforce it, you can simply mark parameters as mandatory or positional or state their validation requirements. This built-in support for parameter behavior and validation forms a centerpiece of PowerShell’s unique consistency.

Parameter validation is one of the main distinctions between scripts that are well behaved and those that are not. When running a new script (or one you wrote distantly in the past), reviewing the parameter definitions and validation requirements is one of the quickest ways to familiarize yourself with how that script behaves.

From the script author’s perspective, validation requirements save you from writing verification code that you’ll need to write anyway.

Program: Learn Aliases for Common Parameters

Access Pipeline Input

Providing Input to Commands

Get-Help about_functions_advanced_parameters

Your command takes a script block as a parameter. When you invoke that script block, you want variables to refer to variables from the user’s session, not your script.

Call the GetNewClosure() method on the supplied script block before either defining any of your own variables or invoking the script block. See Example 11-13.

##############################################################################
##
## Invoke-ScriptBlockClosure
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

<#

.SYNOPSIS

Demonstrates the GetNewClosure() method on a script block that pulls variables
in from the user's session (if they are defined).

.EXAMPLE

PS >$name = "Hello There"
PS >Invoke-ScriptBlockClosure { $name }
Hello There
Hello World
Hello There

#>

param(
    ## The script block to invoke
    [ScriptBlock] $ScriptBlock
)

Set-StrictMode -Version Latest

## Create a new script block that pulls variables
## from the user's scope (if defined).
$closedScriptBlock = $scriptBlock.GetNewClosure()

## Invoke the script block normally. The contents of
## the $name variable will be from the user's session.
& $scriptBlock

## Define a new variable
$name = "Hello World"

## Invoke the script block normally. The contents of
## the $name variable will be "Hello World", now from
## our scope.
& $scriptBlock

## Invoke the "closed" script block. The contents of
## the $name variable will still be whatever was in the user's session
## (if it was defined).
& $closedScriptBlock

Whenever you invoke a script block (for example, one passed by the user as a parameter value), PowerShell treats variables in that script block as though you had typed them yourself. For example, if a variable referenced by the script block is defined in your script or module, PowerShell will use that value when it evaluates the variable.

This is often desirable behavior, although its use ultimately depends on your script. For example, Write a Script Block accepts a script block parameter that is intended to refer to variables defined within the script: $_, specifically.

Alternatively, this might not always be what you want. Sometimes, you might prefer that variable names refer to variables from the user’s session, rather than potentially from your script.

The solution, in this case, is to call the GetNewClosure() method. This method makes the script block self-contained, or closed. Variables maintain the value they had when the GetNewClosure() method was called, even if a new variable with that name is created.

Control Access and Scope of Variables and Other Items

Write a Script Block

You want to specify the parameters of a command you are about to invoke but don’t know beforehand what those parameters will be.

Define the parameters and their values as elements of a hashtable, and then use the @ character to pass that hashtable to a command:

PS > $parameters = @{
    Name = "PowerShell";
    WhatIf = $true
}

PS > Stop-Process @parameters
What if: Performing operation "Stop-Process" on Target "powershell (2380)".
What if: Performing operation "Stop-Process" on Target "powershell (2792)".

When writing commands that call other commands, a common problem is not knowing the exact parameter values that you’ll pass to a target command. The solution to this is simple, and comes by storing the parameter values in variables:

PS > function Stop-ProcessWhatIf($name)
{
    Stop-Process -Name $name -Whatif
}

PS > Stop-ProcessWhatIf PowerShell
What if: Performing operation "Stop-Process" on Target "powershell (2380)".
What if: Performing operation "Stop-Process" on Target "powershell (2792)".

In version one of PowerShell, things were unreasonably more difficult if you didn’t know beforehand which parameter names you wanted to pass along. Version two of PowerShell significantly improves the situation through a technique called splatting that lets you pass along parameter values and names.

The first step is to define a variable, for example, parameters. In that variable, store a hashtable of parameter names and their values. When you call a command, you can pass the hashtable of parameter names and values with the @ character and the variable name that stores them. Note that you use the @ character to represent the variable, instead of the usual $ character:

Stop-Process @parameters

This is a common need when writing commands that are designed to enhance or extend existing commands. In that situation, you simply want to pass all of the user’s input (parameter values and names) on to the existing command, even though you don’t know exactly what they supplied.

To simplify this situation even further, advanced functions have access to an automatic variable called PSBoundParameters. This automatic variable is a hashtable that stores all parameters passed to the current command, and it is suitable for both tweaking and splatting. For an example of this approach, see Program: Enhance or Extend an Existing Cmdlet. For more information about advanced functions, see Access Arguments of a Script, Function, or Script Block.

Access Arguments of a Script, Function, or Script Block

Program: Enhance or Extend an Existing Cmdlet

You want to support the standard -WhatIf and -Confirm parameters, and access cmdlet-centric support in the PowerShell engine.

Ensure your script or function declares the [CmdletBinding()] attribute, and then access engine features through the $psCmdlet automatic variable.

function Invoke-MyAdvancedFunction
{
    [CmdletBinding(SupportsShouldProcess = $true)]
    param()
    
    if($psCmdlet.ShouldProcess("test.txt", "Remove Item"))
    {
        "Removing test.txt"
    }
    
    Write-Verbose "Verbose Message"
}

When a script or function progresses to an advanced function, PowerShell defines an additional $psCmdlet automatic variable. This automatic variable exposes support for the -ShouldProcess and -Confirm automatic parameters. If your command defined parameter sets, it also exposes the parameter set name that PowerShell selected based on the user’s choice of parameters. For more information about advanced functions, see Access Arguments of a Script, Function, or Script Block.

To support the -WhatIf and -Confirm parameters, add the [CmdletBinding(SupportsShouldProcess = $true)] attribute inside of your script or function. You should support this on any scripts or functions that modify system state, as they let your users investigate what your script will do before actually doing it. Then, you simply surround the portion of your script that changes the system with an if($psCmdlet.ShouldProcess(...) ) { } block. Example 11-14 demonstrates this approach.

function Invoke-MyAdvancedFunction
{
    [CmdletBinding(SupportsShouldProcess = $true)]
    param()
    
    if($psCmdlet.ShouldProcess("test.txt", "Remove Item"))
    {
        "Removing test.txt"
    }
    
    Write-Verbose "Verbose Message"
}

Now your advanced function is as well-behaved as built-in PowerShell cmdlets!

PS > Invoke-MyAdvancedFunction -WhatIf
What if: Performing operation "Remove Item" on Target "test.txt".

If your command causes a high-impact result that should be evaluated with caution, call the $psCmdlet.ShouldContinue() method. This generates a warning for users—but be sure to support a -Force parameter that lets them bypass this message.

function Invoke-MyDangerousFunction
{
    [CmdletBinding()]
    param(
        [Switch] $Force
    )
    
    if($Force -or $psCmdlet.ShouldContinue(
        "Do you wish to invoke this dangerous operation? Changes can not be undone.",
        "Invoke dangerous action?"))
    {
        "Invoking dangerous action"
    }
}

This generates a standard PowerShell confirmation message:

PS > Invoke-MyDangerousFunction

Invoke dangerous action?
Do you wish to invoke this dangerous operation? Changes can not be undone.
[Y] Yes  [N] No  [S] Suspend  [?] Help (default is "Y"):
Invoking dangerous action

PS > Invoke-MyDangerousFunction -Force
Invoking dangerous action

To explore the $psCmdlet automatic variable further, you can use Example 11-15. This command creates the bare minimum of advanced function, and then invokes whatever script block you supply within it.

param(
    [Parameter(Mandatory = $true)]
    [ScriptBlock] $Scriptblock
    )

## Invoke the script block supplied by the user.
& $scriptblock
      

For open-ended exploration, use $host.EnterNestedPrompt() as the script block:

PS > Invoke-AdvancedFunction { $host.EnterNestedPrompt() }
PS > $psCmdlet | Get-Member


   TypeName: System.Management.Automation.PSScriptCmdlet

Name                                MemberType Definition
----                                ---------- ----------
(...)
WriteDebug                          Method     System.Void WriteDebug(s...
WriteError                          Method     System.Void WriteError(S...
WriteObject                         Method     System.Void WriteObject(...
WriteProgress                       Method     System.Void WriteProgres...
WriteVerbose                        Method     System.Void WriteVerbose...
WriteWarning                        Method     System.Void WriteWarning...
(...)
ParameterSetName                    Property   System.String ParameterS...

PS > >exit
PS > 

For more about cmdlet support in the PowerShell engine, see the developer’s reference at http://msdn.microsoft.com/en-us/library/dd878294%28VS.85%29.aspx.

Access Arguments of a Script, Function, or Script Block

You want to make your command and usage information available to the Get-Help command.

Add descriptive help comments at the beginning of your script for its synopsis, description, examples, notes, and more. Add descriptive help comments before parameters to describe their meaning and behavior.

##############################################################################
##
##   Measure-CommandPerformance
##
##   From Windows PowerShell Cookbook (O'Reilly)
##   by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

<#

.SYNOPSIS
Measures the average time of a command, accounting for natural variability by
automatically ignoring the top and bottom ten percent.

.EXAMPLE
PS > .Measure-CommandPerformance.ps1 { Start-Sleep -m 300 }

Count    : 30
Average  : 312.10155
(...)

#>

param(
    ## The command to measure
    [Scriptblock] $command,

    ## The number of times to measure the command's performance
    [int] $iterations = 30)

(...)

Like parameter validation, discussed in Add Validation to Parameters, rich help is something traditionally supported in only the most high-end commands. For most commands, you’re lucky if you can figure out how to get some form of usage message.

As with PowerShell’s easy-to-define support for advanced parameter validation, adding help to commands and functions is extremely simple. Despite its simplicity, comment-based help provides all the power you’ve come to expect of fully featured PowerShell commands: overview, description, examples, parameter-specific details, and more.

PowerShell creates help for your script or function by looking at its comments. If the comments include any supported help tags, PowerShell adds those to the help for your command.

You can place your help tags in either single-line comments or multiline (block) comments. You may find multiline comments easier to work with, as you can write them in editors that support spelling and grammar checks and then simply paste them into your script. Also, adjusting the word-wrapping of your comment is easier when you don’t have to repair comment markers at the beginning of the line. From the user’s perspective, multiline comments offer a significant benefit for the .EXAMPLES section because they require much less modification before being tried.

Comment-based help supports the following tags, which are all case-insensitive.

Although these are all of the supported help tags you are likely to use, comment-based help also supports tags for some of Get-Help’s more obscure features: .COMPONENT, .ROLE, .FUNCTIONALITY, .FORWARDHELPTARGETNAME, .FORWARDHELPCATEGORY, .REMOTEHELPRUNSPACE, and .EXTERNALHELP. For more information about these, type Get-Help about_Comment_Based_Help.

Add Validation to Parameters

Get-Help about_Comment_Based_Help

You want to tag or add your own custom information to a function or script block.

If you want the custom information to always be associated with the function or script block, declare a System.ComponentModel.Description attribute inside that function:

function TestFunction
{
    [System.ComponentModel.Description("Information I care about")]
    param()

    "Some function with metadata"
}

If you don’t control the source code of the function, create a new System.ComponentModel.Description attribute, and add it to the script block’s Attributes collection manually:

$testFunction = Get-Command TestFunction
$newAttribute = 
    New-Object ComponentModel.DescriptionAttribute "More information I care about"
$testFunction.ScriptBlock.Attributes.Add($newAttribute)

To retrieve any attributes associated with a function or script block, access the ScriptBlock.Attributes property:

PS > $testFunction = Get-Command TestFunction
PS > $testFunction.ScriptBlock.Attributes

Description                           TypeId
-----------                           ------
Information I care about              System.ComponentModel.Description...

Although a specialized need for sure, it is sometimes helpful to add your own custom information to functions or script blocks. For example, once you’ve built up a large set of functions, many are really useful only in a specific context. Some functions might apply to only one of your clients, whereas others are written for a custom website you’re developing. If you forget the name of a function, you might have difficulty going through all of your functions to find the ones that apply to your current context.

You might find it helpful to write a new function, Get-CommandForContext, that takes a context (for example, website) and returns only commands that apply to that context.

function Get-CommandForContext($context)
{
    Get-Command -CommandType Function |
        Where-Object { $_.ScriptBlock.Attributes |
            Where-Object { $_.Description -eq "Context=$context" } }
}

Then write some functions that apply to specific contexts:

function WebsiteFunction
{
    [System.ComponentModel.Description("Context=Website")]
    param()

    "Some function I use with my website"
}

function ExchangeFunction
{
    [System.ComponentModel.Description("Context=Exchange")]
    param()

    "Some function I use with Exchange"
}

Then, by building on these two, we have a context-sensitive equivalent to Get-Command:

PS > Get-CommandForContext Website

CommandType     Name                          Definition
-----------     ----                          ----------
Function        WebsiteFunction               ...


PS > Get-CommandForContext Exchange

CommandType     Name                          Definition
-----------     ----                          ----------
Function        ExchangeFunction              ...

While the System.ComponentModel.Description attribute is the most generically useful, PowerShell lets you place any attribute in a function. You can define your own (by deriving from the System.Attribute class in the .NET Framework) or use any of the other attributes included in the .NET Framework. Example 11-16 shows the PowerShell commands to find all attributes that have a constructor that takes a single string as its argument. These attributes are likely to be generally useful.

$types = [Appdomain]::CurrentDomain.GetAssemblies() |
    Foreach-Object { $_.GetTypes() }

foreach($type in $types)
{
    if($type.BaseType -eq [System.Attribute])
    {
        foreach($constructor in $type.GetConstructors())
        {
            if($constructor.ToString() -match "(System.String)")
            {
                $type
            }
        }
    }
}

For more information about working with .NET objects, see Work with .NET Objects.

Work with .NET Objects

You want to interact with input that a user sends to your function, script, or script block via the pipeline.

To access pipeline input, use the $input variable, as shown in Example 11-17.

function InputCounter 
{
    $count = 0

    ## Go through each element in the pipeline, and add up
    ## how many elements there were.
    foreach($element in $input)
    {
        $count++
    }

    $count 
}

This function produces the following (or similar) output when run against your Windows system directory:

PS > dir $env:WINDIR | InputCounter
295

In your scripts, functions, and script blocks, the $input variable represents an enumerator (as opposed to a simple array) for the pipeline input the user provides. An enumerator lets you use a foreach statement to efficiently scan over the elements of the input (as shown in Example 11-17) but does not let you directly access specific items (such as the fifth element in the input, for example).

If you need to access specific elements in the input (or access items multiple times), the best approach is to store the input in an array. This prevents your script from taking advantage of the $input enumerator’s streaming behavior, but is sometimes the only alternative. To store the input in an array, use PowerShell’s list evaluation syntax ( @() ) to force PowerShell to interpret it as an array.

function ReverseInput
{
    $inputArray = @($input)
    $inputEnd = $inputArray.Count - 1

    $inputArray[$inputEnd..0]
}

This produces:

PS > 1,2,3,4 | ReverseInput
4
3
2
1

If dealing with pipeline input plays a major role in your script, function, or script block, PowerShell provides an alternative means of dealing with pipeline input that may make your script easier to write and understand. For more information, see Write Pipeline-Oriented Scripts with Cmdlet Keywords.

Write Pipeline-Oriented Scripts with Cmdlet Keywords

Your script, function, or script block primarily takes input from the pipeline, and you want to write it in a way that makes this intention both easy to implement and easy to read.

To cleanly separate your script into regions that deal with the initialization, per-record processing, and cleanup portions, use the begin, process, and end keywords, respectively. For example, a pipeline-oriented conversion of the solution in Access Pipeline Input looks like Example 11-18.

function InputCounter
{
    begin
    {
        $count = 0
    {
    
    ## Go through each element in the pipeline, and add up
    ## how many elements there were.
    process
    {
        Write-Debug "Processing element $_"
        $count++
    }

    end
    {
        $count
    }
}

This produces the following output:

PS > $debugPreference = "Continue"
PS > dir | InputCounter
DEBUG: Processing element Compare-Property.ps1
DEBUG: Processing element Connect-WebService.ps1
DEBUG: Processing element Convert-TextObject.ps1
DEBUG: Processing element ConvertFrom-FahrenheitWithFunction.ps1
DEBUG: Processing element ConvertFrom-FahrenheitWithoutFunction.ps1
DEBUG: Processing element Get-AliasSuggestion.ps1
(...)
DEBUG: Processing element Select-FilteredObject.ps1
DEBUG: Processing element Set-ConsoleProperties.ps1
20

If your script, function, or script block deals primarily with input from the pipeline, the begin, process, and end keywords let you express your solution most clearly. Readers of your script (including you!) can easily see which portions of your script deal with initialization, per-record processing, and cleanup. In addition, separating your code into these blocks lets your script consume elements from the pipeline as soon as the previous script produces them.

Take, for example, the Get-InputWithForeach and Get-InputWithKeyword functions shown in Example 11-19. The first function visits each element in the pipeline with a foreach statement over its input, whereas the second uses the begin, process, and end keywords.

## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)

Set-StrictMode -Version Latest

## Process each element in the pipeline, using a
## foreach statement to visit each element in $input
function Get-InputWithForeach($identifier)
{
    Write-Host "Beginning InputWithForeach (ID: $identifier)"

    foreach($element in $input)
    {
        Write-Host "Processing element $element (ID: $identifier)"
        $element
    }

    Write-Host "Ending InputWithForeach (ID: $identifier)"
}

## Process each element in the pipeline, using the
## cmdlet-style keywords to visit each element in $input
function Get-InputWithKeyword($identifier)
{
    begin
    {
        Write-Host "Beginning InputWithKeyword (ID: $identifier)"
    }

    process
    {
        Write-Host "Processing element $_ (ID: $identifier)"
        $_
    }

    end
    {
        Write-Host "Ending InputWithKeyword (ID: $identifier)"
    }
}

Both of these functions act the same when run individually, but the difference becomes clear when we combine them with other scripts or functions that take pipeline input. When a script uses the $input variable, it must wait until the previous script finishes producing output before it can start. If the previous script takes a long time to produce all its records (for example, a large directory listing), then your user must wait until the entire directory listing completes to see any results, rather than seeing results for each item as the script generates it.

When we chain together two scripts that process their input with the begin, process, and end keywords, the second script gets to process input as soon as the first script produces it.

PS > 1,2,3 | Get-InputWithKeyword 1 | Get-InputWithKeyword 2
Starting InputWithKeyword (ID: 1)
Starting InputWithKeyword (ID: 2)
Processing element 1 (ID: 1)
Processing element 1 (ID: 2)
1
Processing element 2 (ID: 1)
Processing element 2 (ID: 2)
2
Processing element 3 (ID: 1)
Processing element 3 (ID: 2)
3
Stopping InputWithKeyword (ID: 1)
Stopping InputWithKeyword (ID: 2)

When we chain together two scripts that process their input with the $input variable, the second script can’t start until the first completes.

PS > 1,2,3 | Get-InputWithForeach 1 | Get-InputWithForeach 2
Starting InputWithForeach (ID: 1)
Processing element 1 (ID: 1)
Processing element 2 (ID: 1)
Processing element 3 (ID: 1)
Stopping InputWithForeach (ID: 1)
Starting InputWithForeach (ID: 2)
Processing element 1 (ID: 2)
1
Processing element 2 (ID: 2)
2
Processing element 3 (ID: 2)
3
Stopping InputWithForeach (ID: 2)

When the first script uses the cmdlet-style keywords, and the second script uses the $input variable, the second script can’t start until the first completes.

PS > 1,2,3 | Get-InputWithKeyword 1 | Get-InputWithForeach 2
Starting InputWithKeyword (ID: 1)
Processing element 1 (ID: 1)
Processing element 2 (ID: 1)
Processing element 3 (ID: 1)
Stopping InputWithKeyword (ID: 1)
Starting InputWithForeach (ID: 2)
Processing element 1 (ID: 2)
1
Processing element 2 (ID: 2)
2
Processing element 3 (ID: 2)
3
Stopping InputWithForeach (ID: 2)

When the first script uses the $input variable and the second script uses the cmdlet-style keywords, the second script gets to process input as soon as the first script produces it. Notice, however, that InputWithKeyword starts before InputWithForeach. This is because functions with no explicit begin, process, or end blocks have all of their code placed in an end block by default.

PS > 1,2,3 | Get-InputWithForeach 1 | Get-InputWithKeyword 2
Starting InputWithKeyword (ID: 2)
Starting InputWithForeach (ID: 1)
Processing element 1 (ID: 1)
Processing element 1 (ID: 2)
1
Processing element 2 (ID: 1)
Processing element 2 (ID: 2)
2
Processing element 3 (ID: 1)
Processing element 3 (ID: 2)
3
Stopping InputWithForeach (ID: 1)
Stopping InputWithKeyword (ID: 2)

For more information about dealing with pipeline input, see Writing Scripts, Reusing Functionality.

Access Pipeline Input

Writing Scripts, Reusing Functionality

Your function primarily takes its input from the pipeline, and you want it to perform the same steps for each element of that input.

To write a pipeline-oriented function, define your function using the filter keyword, rather than the function keyword. PowerShell makes the current pipeline object available as the $_ variable.

filter Get-PropertyValue($property)
{
    $_.$property
}

A filter is the equivalent of a function that uses the cmdlet-style keywords and has all its code inside the process section.

The solution demonstrates an extremely useful filter: one that returns the value of a property for each item in a pipeline.

PS > Get-Process | Get-PropertyValue Name
audiodg
avgamsvr
avgemc
avgrssvc
avgrssvc
avgupsvc
(...)

For a more complete example of this approach, see Program: Simplify Most Foreach-Object Pipelines. For more information about the cmdlet-style keywords, see Write Pipeline-Oriented Scripts with Cmdlet Keywords.

Program: Simplify Most Foreach-Object Pipelines

Write Pipeline-Oriented Scripts with Cmdlet Keywords

You have a long script that includes helper functions, but those helper functions obscure the main intent of the script.

Place the main logic of your script in a function called Main, and place that function at the top of your script. At the bottom of your script (after all the helper functions have also been defined), dot source the Main function.

## LongScript.ps1

function Main
{
    "Invoking the main logic of the script"
    CallHelperFunction1
    CallHelperFunction2
}

function CallHelperFunction1
{
    "Calling the first helper function"
}

function CallHelperFunction2
{
    "Calling the second helper function"
}

. Main

When PowerShell invokes a script, it executes it in order from the beginning to the end. Just as when you type commands in the console, PowerShell generates an error if you try to call a function that you haven’t yet defined.

When writing a long script with lots of helper functions, this usually results in those helper functions migrating to the top of the script so that they are all defined by the time your main logic finally executes them. When reading the script, then, you are forced to wade through pages of seemingly unrelated helper functions just to reach the main logic of the script.

To resolve this problem, you can place the main script logic in a function of its own. The name doesn’t matter, but Main is a traditional name. If you place this function at the top of the script, your main logic is visible immediately.

Functions aren’t automatically executed, so the final step is to invoke the Main function. Place this call at the end of your script, and you can be sure that all the required helper functions have been defined. Dot sourcing this function ensures that it is processed in the script scope, rather than the isolated function scope that would normally be created for it.

For more information about dot sourcing and script scopes, see Control Access and Scope of Variables and Other Items.

Control Access and Scope of Variables and Other Items

You want to take an action based on the pattern of a command name, as opposed to the name of the command itself.

Add a command wrapper for the Out-Default cmdlet that intercepts CommandNotFound errors and takes action based on the TargetObject of that error.

Example 11-20 illustrates this technique by supporting relative path navigation without an explicit call to Set-Location.

##############################################################################
##
## Add-RelativePathCapture
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

<#

.SYNOPSIS

Adds a new Out-Default command wrapper that captures relative path
navigation without having to explicitly call 'Set-Location'

.EXAMPLE

PS C:UsersLeeDocuments>..
PS C:UsersLee>...
PS C:>

.NOTES

This commands builds on New-CommandWrapper, also included in the Windows
PowerShell Cookbook.

#>

Set-StrictMode -Version Latest

New-CommandWrapper Out-Default `
    -Process {
        if(($_ -is [System.Management.Automation.ErrorRecord]) -and
            ($_.FullyQualifiedErrorId -eq "CommandNotFoundException"))
        {
            ## Intercept all CommandNotFound exceptions, where the actual
            ## command consisted solely of dots.
            $command = $_.TargetObject
            if($command -match '^(.)+$')
            {
                ## Count the number of dots, and go that many levels (minus
                ## one) up the directory hierarchy.
                $newLocation = ".." * ($command.Length - 1)
                if($newLocation) { Set-Location $newLocation }

                ## Handle the error
                $error.RemoveAt(0)
                $_ = $null
            }
        }
    }

        

PowerShell supports several useful forms of named commands (cmdlets, functions, and aliases), but you may find yourself wanting to write extensions that alter their behavior based on the form of the name, rather than the arguments passed to it. For example, you might want to automatically launch URLs just by typing them or navigate around providers just by typing relative path locations.

While this is not a built-in feature of PowerShell, it is possible to get a very reasonable alternative by intercepting the errors that PowerShell generates when it can’t find a command. The example in the Solution does just this, by building a command wrapper over the Out-Default command to intercept and act on commands that consist solely of dots.

Intercept Stages of the Pipeline

Program: Enhance or Extend an Existing Cmdlet

Run Programs, Scripts, and Existing Tools