Power Tips

Type Accelerators

ps1 — Thu, 02 Sep 2010 13:00:00 GMT

PowerShell has a few shortcuts for popular .NET types like [WMI], [ADSI] or [Int]. You should read the FullName property if you'd like to know the underlying full type name:

[WMI].FullName

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Comparing Versions

ps1 — Wed, 01 Sep 2010 13:00:00 GMT

When you compare version strings, PowerShell will use alphanumeric algorithms, which may lead to confusing results:

'3.4.22.12' -gt '22.1.4.34'
True

You should convert the strings to a System.Version type to compare version strings right. In fact, converting the left hand value is enough because PowerShell automatically converts the right hand value to the same type:

[System.Version] '3.4.22.12' -gt '22.1.4.34'
False

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View Object Inheritance

ps1 — Tue, 31 Aug 2010 13:00:00 GMT

A hidden object property called "PSTypeNames" will tell you the object type as well as the inherited chain:

(Get-WMIObject Win32_BIOS).PSTypeNames

In contrast to GetType(), this property will work for all objects, including COM objects. The most specific type is always found at the beginning of that array:

(Get-WMIObject Win32_BIOS).PSTypeNames[0]

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Using Scripts to Validate Input

ps1 — Mon, 30 Aug 2010 13:00:00 GMT

For tricky validation checks, you should use arbitrary PowerShell code to validate. The function Copy-OldFiles will only accept files (no folders) and those that are older (in days) than specified in -Days:

function Copy-OldFiles {
param(
$Days=30,
[Parameter(ValueFromPipeline=$true)]
[System.IO.FileInfo]
[ValidateScript({ (New-TimeSpan $_.LastWriteTime).Days -gt $Days })]
$FileObject
)

process {
"Archiving file {0} (Age {1})..." -f $FileObject.FullName, (New-TimeSpan $_.LastWriteTime).Days
}
}

The validation script will then check whether the file submitted via the pipeline is older than specified in -Days. If not, the file is rejected. And this is how you could archive all log files in your Windows folder that are older than 40 days:

dir $env:windir -Filter *.log | Copy-OldFiles -Days 40 -ErrorAction SilentlyContinue

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Restrict Input to Numeric Ranges

ps1 — Fri, 27 Aug 2010 13:00:00 GMT

Let's say you'd like to set the PowerShell console cursor size. This size must be a number between 0 and 100. The following template will validate that the user cannot specify an argument outside the allowed range:

function Set-CursorSize {
param(
[ValidateRange(1,100)]
[Int]
$Percent
)

$Host.UI.RawUI.CursorSize = $Percent
}

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Converting Object Types

ps1 — Thu, 26 Aug 2010 13:00:00 GMT

Once you know the name of an object type, you can use that type for conversion. The next line converts a string into a date-time type:

[DateTime] '4.5.2010'

You should note that conversion uses the culture-neutral date format which happens to be the U.S. format. If you need to use your local date format, you should use the Parse() method found inside the DateTime type:

[DateTime]::Parse('4.5.2010')

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Finding Object Types with Powershell

ps1 — Wed, 25 Aug 2010 13:00:00 GMT

Anything in PowerShell is an object. You can use GetType() to get the object type:

'Hallo'.GetType().FullName
(4).GetType().FullName
(2.6).GetType().FullName
(Get-Date).GetType().FullName

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Validate Input Using Regular Expressions

ps1 — Tue, 24 Aug 2010 13:00:00 GMT

Function parameters can be validated using standard regular expressions. For example, the next template function accepts only valid Knowledge Base article numbers beginning with "KB" and a six- digit number:

function Get-KnowledgeBaseArticle {
param(
[ValidatePattern('^KB\d{6}$')]
[String]
$KB
)

"You entered Knowledgebase ID $KB"
}

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Limiting String Input Length

ps1 — Mon, 23 Aug 2010 13:00:00 GMT

If a function parameter should receive a string of a given length only, you should use the following validation attribute. In the example, it limits filenames to eight characters:

function Get-FileName {
param(
[ValidateLength(1,8)]
[String]
$FileName
)

"Your filename {0} is {1} chars long" -f $FileName, $FileName.Length
}

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Limiting Number of Arguments

ps1 — Fri, 20 Aug 2010 13:00:00 GMT

Function parameters can receive multiple values when they accept arrays. You should use this template to limit the number of values acceptable:

function Add-Users {
param(
[ValidateCount(1,3)]
[String[]]
$UserName
)

$UserName -ForEach-Object { "Adding $_" }

}

Here, Add-Users will accept one to three user names as a comma-separated list. If you specify more or less, you will get an error message.

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Defining Alias Properties

ps1 — Thu, 19 Aug 2010 13:00:00 GMT

Your functions can have properties with built-in alias names. The user can then either use the descriptive "long" name or its short and convenient alias name:

function Get-BIOSInfo {
param(
[Alias("CN")]
$ComputerName = "."
)

Get-WmiObject Win32_BIOS -ComputerName $computername
}

You should either use -ComputerName or its alias -CN to retrieve BIOS information from a remote system..

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Validate Set of Inputs

ps1 — Wed, 18 Aug 2010 13:00:00 GMT

If you need to limit a function parameter to only a set of allowed choices, you should use the next example:

function Get-24hEventLogEntries {
param(
[String]
[ValidateSet('System','Application')]
$LogName='System',

[String]
[ValidateSet('Error','Warning','Information')]
$EntryType='Error'
)

Get-EventLog -LogName $LogName -EntryType $EntryType -After ((Get-Date).AddDays(-1))

}

You can use Get-24hEventLogEntries to retrieve events written in the past 24 hours. The user can choose to see the 'system' or the 'application' log. EntryType is limited to 'Error,' 'Warning,' or 'Information.'

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Finding Cmdlets With a Given Parameter

ps1 — Tue, 17 Aug 2010 14:09:00 GMT

Finding cmdlets by name is easy:

Get-Command *service* -commandType Cmdlet

But how can you list all cmdlets that support a given parameter? If you’d like to see all cmdlets with a -List parameter?

The easiest way is to use Get-Help with the parameter -parameter:

Get-Help * -parameter list

Or, you can use Get-Command and filter the result by parameter. You’ll need a custom filter to accomplish this:

filter Contains-Parameter {
param($name)
$number = @($_ | % { $_.ParameterSets | % { $_.Parameters | ? { $_.Name -eq $name}}}).Count
if ($number -gt 0) {
$_
}
}

This filter allows only those to pass the pipeline that supports the given parameter. Here is how you use the filter:

Get-Command | Contains-Parameter 'list'

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Removing All Internet Explorer Cookies

ps1 — Mon, 16 Aug 2010 13:00:00 GMT

You'd probably like to regularly clean your machine and remove all cookies set by Web sites you may have visited? Here is how:

Dir ([Environment]::GetFolderPath("Cookies")) | del -whatif

You will want to make sure you remove the -whatif to actually delete the cookie files.

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Listing Internet Explorer Cookies

ps1 — Fri, 13 Aug 2010 13:00:00 GMT

Have you ever wanted to find out who stored your information while you were surfing the Web? Check out your cookies folder:

Dir ([Environment]::GetFolderPath("Cookies"))

You should try this to open that folder in your Explorer:

Explorer ([Environment]::GetFolderPath("Cookies"))

And if you'd like, you could also select cookies based on content. The next line lists all cookies that contain the word "prefbin:”

Dir ([Environment]::GetFolderPath("Cookies")) | Select-String prefbin -List

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