Guy's Scripting Ezine 76 -  Introduction to WMI

Contents for Ezine 76 Introduction to WMI

• This week's secret
• Shell One - A Simple VBScript
• Shell Two - A Template for WMI Commands
• Shell Three - Your working WMI Script
• Selection of Win32 Objects and Properties
• Summary of Getting Started
• See more on WMI

This week’s secret

Guy's third law of computing says, 'The more acronyms on a page, the harder the topic is to understand'. On that score WMI comes out medium difficult.

WMI stands for Windows Management Instrumentation. What WMI delivers is control through scripting.  Another way of looking at WMI is a variation of VBScript, which allows you to measure and automate every aspect of computer behaviour.  The key acronym is CIM, Common Information Model. All that you need to know about CIM is that it is an open standard for defining every conceivable computer object, from the BIOS to Disks, Memory even the Desktop.

My secrets for getting started with WMI are as follows, look for patterns, especially with Win32_Xyz objects where Xyz could be Process, Service, DiskDrive or Memory.  As with all VBScripts, build the script in stages, and then bolt the parts together. You probably need no encouragement to learn by copying and pasting other people’s scripts. Of all the computing tasks, understanding WMI is particularly suited to the attitude: just learn by doing.

If you are looking for handy network utilities, try some of the free downloads at Tools4Ever

Shell One -  A Simple VBScript

Learning WMI is not going to be easy.  The reason that WMI is so difficult is that you have to master not only the core WMI commands, but also the 'wrapper' VBScript commands.  To get you started, I have developed what I call the three shell method.  Traditional WMI scripts can be long and complex, therefore, I am going to strip the code down to the essentials, get success, then gradually build in more layers, or shells.

Remember, that our mission for Shell One is making sure that the VBScript shell or 'wrapper' is ready to receive WMI commands.  Shell One is concerned with only VBScript; there are no WMI commands at this stage.

Instructions for Creating your Shell One VBScript.

1. Copy and paste the example script below into notepad or a VBScript editor.
2. Save the file with a .vbs extension, for example: ShellOne.vbs. 
3. Double click ShellOne.vbs and check the message box.

For a top Script Editor try a free download at OnScript

Shell One - VBScript

Learning Points for Shell One

1) My goal for Shell One is simply to isolate the VBScript commands.

2) If this script works, then it proves that your computer can interpret VBScript and is ready for WMI commands.

3) In Shell One the only real instruction is WScript.Echo.  The purpose of the rest of the script is just to introduce you to my headers, Dim statement and strComputer.

Shell Two - A Template for WMI Commands

Now we are ready to add a simple WMI core to the VBScript in Shell One.  The result will be a template to use for future, more complex WMI scripts.  As far as I know, all the Win32_Objects have these 3 properties, .name, .caption and .description.  Therefore, we could choose any Win32 object on line 16.  As it happens, from all the possible objects, I selected Win32_Bus.

Instructions for Creating your WMI Template

1. Copy and paste the example script below into notepad or a VBScript editor.
2. Begin with strComputer = "."  Once the script works substitute the name of another machine on the network.
3. Save the file with a .vbs extension, for example: ShellTwo.vbs (or Template.vbs)
4. Double click ShellTwo.vbs and check the Bus information in the message box.

Shell Two - The Template for WMI Commands

WMI Tutorial Learning Points

1)  Let us focus on the WMI core statements:

a) Set objWMIService = GetObject("winmgmts:\\" & strComputer & "\root\cimv2").  This command instructs winmgmts to connect to the heart of the namespace, root\cimv2

b) Set colItems = objWMIService.ExecQuery( "Select * from Win32_Bus").  Once the previous line connects to CIM, this line executes a WQL query, Select everything from the Bus.

2)  Next, it's back to VBScript for a For .... Next loop.  Incidentally, all the scripts that we create from this template will use the same loop structure.  This Shell Two script has only three very basic (and boring) properties, future scripts will add more interesting properties.  Every object has a name attribute, hence, objItem.Name.  To my knowledge, all, Win32 objects also have the .Description and .Caption properties.

3) There are three pieces of VBScript syntax that you should master, as a missing ampersand, VbCr or underscore can wreak havoc with more complex WMI scripts.

a) _(Underscore) is the equivalent of word-wrap.  Without this tiny character, VBScript thinks that the next line is a completely different command.  With the underscore, you can string together a dozen or more properties.  Just remember that there is no need for underscore after the last description.  To let you into a secret, this is why I end with a blank line "", to remind me no more underscores.

b) The & (ampersand) is my Achilles heel.  I know & means append or join two pieces of text, but sometimes forget, only for the script to remind me with an 800A0401 - 'Expected end of statement' error message.

c) VbCr means a visual basic carriage return. Perhaps you are thinking, we already covered this with a) Underscore.  No. VbCr is for line breaks in the output, whereas underscore is for the interpretation of the very script.  Put another way, the script will work without any VbCr commands, but it will be exceedingly awkward to read.

Shell Three - Your working WMI Script

Now that we have a template, it's like shelling peas.  Decide on the object's properties that you need, and then just cut and paste them into Shell Two.  The result will be a useful production script.

There are only points to watch.

1) Remember to change the Win32_object to match the properties, for example change,
"Select * from Win32_Bus") to "Select * from Win32_ComputerSystem").  Failure to edit Win32_Xyx results in error message - 800A01B6 Object doesn't support this property.

2) Select the Win32 object's properties from your own research or from my table below.  Here is an example where I have added extra properties on lines 23-31 of Shell Three.

Selection of Win32 Objects and Properties

Employ Shell Two as your template.  Copy and paste different objects, but make sure that choose properties that correspond to the correct object.  Here is the key, change the object in line 16 in Shell Two (Template) : for example,
"Select * from Win32_Bus") amend to read, "Select * from Win32_ComputerSystem")

  For much more on WMI, see my WMI Section here.

Summary of Getting Started with WMI

I love to build on success.  Even if you go back to basics and start with a simple script, it's worth it for that feeling of achievement, that belief that you are on a roll and can tackle anything.  In practical terms, I hope that you will identify the key Win32 commands, and see what I mean by a VBScript shell with a WMI core.

Guy Thomas recommends Computer Training Software. 

Their topics and material are ideal for getting you started with VBScript.  The videos are easy to follow and you can control the pace.  Try their free demo material and then see if you want to buy the full package. See more about VB Script Training CD.

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