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Windows PowerShell Variables

Introduction to Windows PowerShell Variables

All scripting languages use placeholders or variables to hold data.  Furthermore, each language has its own rules and symbols.  I have found that using any PowerShell variable is straightforward, just remember to  introduce your PowerShell variable with a dollar sign, for example: $memory.

Topics for PowerShell's Variables

• $Dollar Variables
• Set-Variable, Scope and Option
• PowerShell's Dot Properties
• Special Pipeline Variable:  $_.
• More PowerShell Variables
• Summary of PowerShell Variables

 ♣

PowerShell's $Dollar Variables

Variables are handy for storing data which can be used later in the script. For example, storing a file path. Creating a PowerShell variable could not be more straightforward; just put the dollar sign in front of the name you wish to call the variable.  Let us create, then set, a variable called $Mem:

$Mem= WmiObject Win32_ComputerSystem

Once we have created $Mem, then we can put the variable to work and calculate the RAM memory in Mega bytes.

PowerShell has no built-in mechanism for enforcing variable types, for example, string variables can begin with letters or numbers. Yet numeric variables can also begin with letters (or numbers). However, you can restrict the values a PowerShell variable accepts by preceding it with [int] or [string], for example:

Example Declaring the PowerShell Variable as an Integer

Note 1: I cannot resist pointing out the significance of [Square brackets].  The reason is that PowerShell only ever uses square brackets for optional items, and declaring the type of a variable is just that - optional.

Example Without Declaring the Variable Type.

No error here because $b was not declared as number or a string.

Do you think that PowerShell variables are case sensitive or insensitive?  The answer is insensitive, just as with most other commands, upper or lower case work equally.

When PowerShell evaluates a potential variable name, it carries on from the $Dollar until it meets a word breaking character such as a space or punctuation mark.  This does not give me a problem because I only use snappy OneWord names, but if you use variables with strange characters - watch out!  If you insist on using variables with names such as a*?,v**, then you could enclose them in braces - thus {a*?,v**}.  Clever stuff, but best to keep it simple and don't ask for trouble I say.

Reserved Words - Not To Be Used For Variables

Avoid using these reserved keywords for your variables: Break, continue, do, else, elseif, for, foreach, function, filter, in, if, return, switch, until, where and while.

Incidentally, you can join string variables simply by using a plus (+) sign. The reason that I mention this is because I spent ages searching fruitlessly for a special text concatenator, only to discover that the plain plus sign was all I needed.

Declaring Multiple Variables

In scripting we are always seeking ways of writing tighter code.  In the case of variables you can make multiple declarations on the same line. For example:

PowerShell Variables - Text Examples

What you find when researching WMI objects is that there are so many.  The purpose of this script is filter, or home in on, a keyword such as Win32, or network.  To aid the research and to recycle the code I introduce a variable $Type.

To begin with I set the value of $Type to "Win32", later it would be a trivial task to amend to "Network".  Once again, note how this script uses the built in $_. variable, and see how it uses .name in this pipeline.

Set-Variable, Scope and Option

You can control, or restrict, PowerShell variables with the set-Variable command.  These extra properties of 'Option' and 'Scope' are not really necessary for beginners, nevertheless as you grow in ambition, you may like to revisit these additional features.

Option can set the variable to be read-only or constant.  Constant variables sound strange, but their killer feature is they cannot be deleted.

Note 2: That when initializing with set-variable, $Thermometer is wrong, plain Thermometer is what you need here.  Once the value is set to 32 it cannot be changed.

Scope can be local, global or script.  The default value for the scope of a variable is local. 

Note 3: For more information about Set-Variable, try Help Set-Variable.

Note 4: The value would be 99, again you don't add the $dollar sign when you execute set-variable.  Actually, there is an alternative method for setting and creating Scope:

PowerShell's Dot Properties

Before we look at the pipeline variable $_.  let us have a general look at PowerShell .dot properties. PowerShell variables support the dot (.) properties.  For example:

Not only is $variable.property a neat technique, but also note that Get-Service alerter.status does not work, you get an error saying: 'Cannot find any service called alerter.status'.

Special Pipeline Variable:  $_.

$_ or $_. takes the dot notation one stage further.  It acts a placeholder for the current object.  The official definition for $_. is the current pipeline object; used in script blocks, filters, the process clause of functions, where-object, foreach-object and switches.  However, I believe that special PowerShell variable $_. is best explained by examples.

PowerShell Example to find all services that are Running (not stopped)

Remember that this Get-Service command lists all those services on the machine.  Status is a property of the service.  One of the possible values for status is "Running", another value is "Stopped".  Should you wish to employ a 'where' clause, then you need the $_. variable to introduce or link to the property 'status', hence $_.status.

Example to find all wmiobject containing 'CIM'

Note 5: The point is that the list is too long when you try:
Get-wmiobject -list

By pipelining $_.name, we can filter just names containing "CIM".  Incidentally it does not work without the wildcard * -like "CIM*".  However you could experiment with -match, or -contains.

See more on PowerShell's $_.  meaning In the Current Pipeline

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More Built-in PowerShell Variables

You can enumerate PowerShell's variables with this command:

Another example showing more information and more control:

PowerShell Variables in Action

Example 1)
To discover which version of PowerShell you are running, go to the PowerShell command line and type:
$host

Example 2)
You could modify the Environment Path value thus:

Note 6: The plus (+) means that you keep the existing path locations and append C:\Wizzo\Stuff

Note 7: Talking of Env variables, you can list them with gci thus:

PowerShell Variable Theory

PowerShell variables are mapped to classes in the Microsoft .NET Framework. One benefit is that variables are objects and thus can be manipulated in many ways.  There is also a family of variable cmdlets which you can see with this command:

Get-Command -noun variable 

Although there is cmdlet called New-Variable, I have never seen anyone use it for real because you can just wade in with a declaration.

Declaring Variables with New-Variable

String and Int32 Variables

PowerShell takes care of strings and numbers automatically, thus you can produce numerous excellent scripts without worrying about this PowerShell theory.

Result (At the very top of the list)
TypeName System.String

PowerShell takes care of numbers automatically, but assigns the variable to a different class.

Result (At the very top of the list)
TypeName System.Int32

Summary of PowerShell Variables

In PowerShell, variables are easy to create, just precede the name with a dollar sign, for example $Disk.  For more ambitious scripting you can restrict their type for example [int]$Memory, you can also prescribe the variable's scope, local or global.

One variable worth mastering is the special pipeline variable controlled by $_.

What impresses programmers is the ability to assign not just text to the variable, but also to assign an object to a variable.  While most proper scripting languages are able to handle objects through variables, CMD lacks this ability.

See more PowerShell examples

• PowerShell Home   • Foreach loops   • PowerShell Foreach   • Foreach-Object cmdlet

• Syntax   • Variables   • -whatIf   • -ErrorAction   • Windows PowerShell   • PowerShell 2.0

• PowerShell Functions   • [System.Math]   • Get-Credential   • Windows 7 PowerShell 2.0

Please email me if you have a better example script.  Also please report any factual mistakes, grammatical errors or broken links, I will be happy to  correct the fault.

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