Windows PowerShell高效指南

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"Effective Windows PowerShell" 本教程是针对熟悉DOS命令和.NET Framework架构的管理员而设计的PowerShell学习资料，旨在帮助读者深入理解和充分利用PowerShell的强大功能。作者Keith Hill在2007-2009年间创作了这本书，内容涵盖了PowerShell的基础到高级用法。 1. **四款关键Cmdlet** - **Get-Command**: 这是发现PowerShell中可用命令的基本工具。通过Get-Command，你可以列出所有内置命令、模块中的命令以及自定义脚本或函数。 - **Get-Help**: 提供详细的帮助文档，是学习和掌握PowerShell语法的关键。它不仅包含命令的基本使用，还可能包含示例和参数详细信息。 - **Get-Member**: 显示对象的属性和方法，这对于理解从命令输出中可以访问哪些信息至关重要。 - **Get-PSDrive**: 显示PowerShell的驱动器，包括文件系统驱动和其他特殊驱动，如注册表驱动，这有助于导航PowerShell工作空间。 2. **理解输出** - **输出始终是.NET对象**: PowerShell的输出本质上是.NET对象，这意味着你可以对它们进行更复杂的操作，例如方法调用和属性访问。 - **函数输出**: 函数的输出包含所有未被捕获的数据，这可能包括标准输出和错误输出。 - **无法捕获的其他类型输出**: 有一些输出，如进度条和控制台写入，不能直接存储在变量中，需要特别处理。 3. **识别管道中流动的对象** 理解在命令之间传递的对象是提高效率的关键。通过管道，你可以将一个命令的输出作为另一个命令的输入，实现数据处理的链式操作。 4. **输出基数：标量、集合与空集** - **处理标量**: 标量是单个值，如字符串、整数或布尔值。 - **处理集合**: 集合可以是数组或其他可枚举对象，需要使用特定技巧如重写循环来处理每个元素。 - **处理空集**: 当命令没有输出时，理解如何处理这种情况也非常重要。 5. **使用对象，就像使用原力一样** 强调了利用PowerShell的面向对象特性，通过直接操作对象属性和方法来执行任务。 6. **了解输出格式器** 输出格式器允许你定制命令的显示方式，使其更适合你的需求和报告目的。 7. **理解PowerShell解析模式** PowerShel有不同解析模式，理解这些模式如何影响命令行为是编写有效脚本的关键。 8. **理解按属性名管道绑定参数** 按属性名绑定参数允许你基于对象的属性值传递参数，而不是显式提供值，简化了命令行参数的使用。 9. **理解按值管道绑定参数** 按值绑定参数则将整个对象作为参数传递，而不是其属性值，适用于需要整个对象的情况。 10. **错误处理** - **终止性错误**：导致脚本立即停止的严重错误。 - **非终止性错误**：不会阻止脚本继续执行的错误，但可能会影响结果。 - **错误变量**：如$Error和$LASTEXITCODE，用于存储和检查错误信息。 - **处理非终止性错误**：可以使用try-catch块或其他技术来捕获并处理这些错误。这个教程全面地涵盖了PowerShell的核心概念和最佳实践，对任何想要提升Windows管理效率的人来说都是宝贵的资源。

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Item 2: Understanding Output

In shells that you may have used in the past, everything that appears on the stdout and stderr streams is

considered "the output". In these other shells you can typically redirect stdout to a file using the redirect

operator >. And in some shells like Korn shell, you can capture stdout output to a variable like so:

DIRS=$(find . | sed.exe -e 's/\//\\/g')

If you wanted to capture stderr in addition to stdout then you can use the stream redirect operator like so:

DIRS=$(find . | sed.exe -e 's/\//\\/g' 2>&1)

You can do the same in PowerShell:

PS> $dirs = Get-ChildItem -recurse

PS> $dirs = Get-ChildItem -recurse 2>&1

Looks about the same in PowerShell so what's the big deal? Well there are a number of differences and

subtleties in PowerShell that you need to be aware of.

Output is Always a .NET Object

First, remember that PowerShell output is always a .NET object. That output could be a System.IO.FileInfo

object or a System.Diagnostics.Process object or a System.String object. Basically it could be any .NET object

whose assembly is loaded into PowerShell including your own .NET objects. Be sure not to confuse PowerShell

output with the text you see rendered to the screen. Later on in Item 6: Know Your Output Formatters I cover

the notion that when a .NET object is about to "hit" the host (console) PowerShell uses some fancy formatting

technology to try to determine the best "textual" representation for the object. However, when you capture

output to a variable, you are not capturing the text that was rendered to the host. You are capturing the .NET

object(s). Let's look at an example:

PS> Get-Process PowerShell

Handles NPM(K) PM(K) WS(K) VM(M) CPU(s) Id ProcessName

------- ------ ----- ----- ----- ------ -- -----------

425 9 32660 16052 181 31.63 5128 powershell

Now let's capture that output and examine its type:

PS> $procs = Get-Process PowerShell

PS> $procs.GetType().Fullname

System.Diagnostics.Process

As you can see, a System.Diagnostics.Process object has been stored in $procs and not the text that was

rendered to the screen. But what if we really wanted to capture the rendered text? In this case, we could use

the Out-String cmdlet to render the output as a string which we could then capture in a variable e.g.:

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PS> $procs = Get-Process PowerShell | Out-String

PS> $procs.GetType().Fullname

System.String

PS> $procs

Handles NPM(K) PM(K) WS(K) VM(M) CPU(s) Id ProcessName

------- ------ ----- ----- ----- ------ -- -----------

479 9 32660 16052 181 31.72 5128 powershell

Another nice feature of Out-String is that it has a Width parameter that allows you to specify the maximum

width of the text that is rendered. This is handy when there is wide output that you don't want wrapped or

truncated to the width of your host.

Function Output Consists of Everything That Isn't Captured

I've seen this problem bite folks time and time again on the PowerShell newsgroup. It usually happens to those

of us with programming backgrounds that are familiar with C style functions. What you need to be aware of is

that in PowerShell, a function is a bit different. While a function in PowerShell does provide a separate scope for

variables and a convenient way to invoke the same functionality multiple times without breaking the DRY

principle, the way it deals with output can be confusing at first. Essentially a function handles output in the

same way as any PowerShell script that isn't in a function. What does that mean? Let's look at an example.

PS> function bar {

>> $procs = Get-Process svchost

>> "Returning svchost process objects"

>> return $procs

>> }

That should return an array of System.Diagnostic.Process objects, right? We told PowerShell to "return $procs".

Let's check the output:

PS> $result = bar

PS> $result | foreach {$_.GetType().Fullname}

System.String

System.Diagnostics.Process

...

Whoa! Why is the first object System.String? Well a quick look at its value and you'll see why:

PS> $result[0]

Returning svchost process objects

Notice that the informational message we thought we were displaying to the host actually got returned as part

of the output of the function. There are a couple of subtleties to understand here. First, the return keyword

allows you to exit the function at any particular point. You may also "optionally" specify an argument to the

return statement that will cause the argument to be output just before returning. "return $procs" does not

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mean that the function’s only output is the contents of the $procs variable. In fact this construct is semantically

equivalent to "$procs; return".

The second subtlety to understand is this. The line:

"Returning svchost process objects"

is equivalent to this:

Write-Output "Returning svchost process objects"

That makes it clear that the string is considered part of the function’s output.

Now what if we wanted to make that information available to the end user but not the script consuming the

output of the function? Then we could have used Write-Host like so:

PS> function bar {

>> $Proc = Get-Process svchost

>> Write-Host "Returning svchost process objects"

>> return $Proc

>> }

Write-Host does not contribute to the output of the function. It writes directly and immediately to the host.

This might all seem obvious now but you have to be diligent when you write a PowerShell function to ensure

you get only the output you want. This usually means redirecting unwanted output to $null (or optionally type

casting the expression with the unwanted output to [void]). Here's an example:

PS> function LongNumericString {

>> $strBld = new-object System.Text.StringBuilder

>> for ($i=0; $i -lt 20; $i++) {

>> $strBld.Append($i)

>> }

>> $strBld.ToString()

>> }

Note that we don't need to use the return keyword like we do in C style function. Whatever expressions and

statements that have output will contribute to the output of our function. This is part of a PowerShell function

behaving like ordinary PowerShell script. In the function above, we obviously want the output of

$strBld.ToString() to be the function's only output but we get the following output instead:

PS> LongNumericString

Capacity MaxCapacity Length

-------- ----------- ------

16 2147483647 1

16 2147483647 2

16 2147483647 3

剩余60页未读，继续阅读

allans

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