Adding logic and loops in Visual Basic .NET

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Adding logic and loops
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Up to this point, I don t consider anything that we ve done so far to be scripting. It depends on your definition of the word, of course, but to me scripting is a kind of programming, with formal constructs that define logic, repetition, and so forth. You can do a lot in PowerShell without that stuff. But the time will come when you will need to write a script that can make logical decisions, and you ll start to move beyond running commands and moving into simple scripts. The goal in this chapter is to let you experience some of PowerShell s major scripting constructs for logic and repetition, so that you ll be prepared to use these elements when the time comes.
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20.1 Automating complex, multi-step processes
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I typically find a need for these constructs when I m automating more complex, multistep processes. For example, consider a script that provisions a new user: you need to create an Active Directory account, add the user to some groups, create a mailbox, create a home directory on a file server, and so on. Those processes often involve questions, with branching logic: Should the user belong to such-and-such a domain user group Should they have access to certain files Each question leads to a slightly different course of action. In some cases, certain operations may have to be done over and over, such as adding a user to several groups, or perhaps granting them permissions over several folders or files.
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20.2 Now we re scripting
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I admit that we ve been creeping toward actual scripting for a while now. The previous chapter, for example, introduced some structures that would normally only live
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The If construct
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within a .PS1 script file, and that you d probably never type directly on the command line. But I mostly think of things like Param() blocks as window dressing. They don t do anything, but they do provide some structure and definition to our commands. Once we start adding logic and repetition, I ll admit that we ve formally moved into the world of scripting. That s not a bad thing: this isn t going to be the type of programming you d do in Visual Studio (although PowerShell can certainly accommodate pretty intense, complex scripts). We re going to keep it simple, using these scripting constructs primarily to add a little intelligence to a batch of commands.
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Above and beyond
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This chapter will introduce you to all but one of PowerShell s formal scripting constructs. The missing construct is the Do loop, which also uses the keywords While and Until in certain scenarios. You can learn more about them in PowerShell s help: run help about* to get a list of help topics, and look for Do, While, and Until. Why aren t they covered in this chapter Simple: for most administrative scripts, you won t need them. As you start to progress into more advanced scripts, you can familiarize yourself with them on your own, and use them if necessary. They re generally used to repeat some set of commands over and over until a certain condition is either true or false.
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20.3 The If construct
First up is the scripting construct that you ll probably use the most: If. A basic If construct looks like this:
If ($process.pm -gt 10000) { Write-Host "This is a large process" }
There are just a couple of important things to note:
As with most of PowerShell, the If keyword isn t case-sensitive. You can use if or IF or even iF. The parentheses contain an expression of some kind. This has to evaluate to either True or False (or, to use the PowerShell values, $True or $False). After the parentheses, you open the construct by using a curly brace. You complete the construct with a closing brace. Most people indent the commands within the construct, so that it s easier to visually distinguish the commands that are inside the construct.
Here s another way to format this:
If ($process.pm -gt 10000) { Write-Host "This is a large process" }
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