PowerShell精要：掌握命令行工具的高效使用

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"《精通powershell》是一本深入学习PowerShell的优秀资源，作者Keith Hill在书中逐步介绍了如何更好地利用这种强大的脚本语言。书中涵盖了关键的cmdlet、输出理解、对象管道、输出基数、对象使用、输出格式器、解析模式、参数绑定以及错误处理等多个重要主题，旨在帮助读者全面掌握PowerShell的精髓。" PowerShell是微软开发的一个命令行外壳程序和脚本语言，主要用于系统管理和自动化任务。书中的关键知识点包括： 1. **四款核心cmdlet**： - **Get-Command**: 这个cmdlet允许用户发现并列出所有可用的PowerShell命令，包括内置命令、模块命令和函数。 - **Get-Help**: 提供PowerShell命令的帮助信息，是学习和解决问题的重要工具。 - **Get-Member**: 显示对象的属性和方法，帮助理解对象结构。 - **Get-PSDrive**: 显示或管理PowerShell驱动器，包括文件系统和其他数据源的虚拟驱动器。 2. **理解输出**： - PowerShell的输出本质上都是.NET对象，可以进行深度操作和自定义。 - 函数输出包含未被捕捉的所有信息，理解这一点有助于控制和利用输出。 - 存在无法被捕获的特殊类型输出，需要特别注意。 3. **了解对象在管道中的流动**： - 管道是PowerShell的核心，通过它将一个命令的输出传递给另一个命令处理。 4. **输出基数**： - 包括标量（单个值）、集合（多个值）和空集的处理，理解它们的不同对编写脚本至关重要。 5. **利用对象**： - 书中强调了充分利用PowerShell的对象模型来提高脚本的效率和灵活性。 6. **认识输出格式器**： - PowerShel提供多种输出格式，学习如何定制这些格式能提升输出的可读性。 7. **理解PowerShell解析模式**： - PowerShell有不同解析模式，如命令模式和表达式模式，理解这些模式有助于避免常见错误。 8. **按属性名和按值的管道绑定参数**： - 两种参数绑定方式分别用于处理基于对象属性和基于对象本身值的情况。 9. **错误处理**： - 区分终止性错误和非终止性错误，以及如何使用错误变量和处理非终止性错误的方法。通过这些深入的讲解，读者不仅可以了解PowerShell的基础，还能掌握高级用法，从而在日常工作中更加高效地利用PowerShell进行系统管理和自动化任务。

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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

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