PowerShell $_ 语法 [英] PowerShell $_ syntax

问题描述

在这个答案中，作者提出了以下片段:

In this answer the author proposed the following snippet:

dir -Path C:\FolderName -Filter *.fileExtension -Recurse | %{$_.FullName}

我可以理解其中的大部分内容，但我无法搜索最后一部分的文档.搜索的输出通过管道传送 | 并在 %{} 和 $_ 中使用.

I can understand the majority of it, but I'm unable to search documentation for the last part. The output of the search is piped | and used in %{} and as $_.

我已经围绕它进行了实验，%{} 是我相信的 for-each 语句，无效.$_ 也有点神奇:它是一个变量，没有名字，因此立即被消耗?我不太关心 .FullName，我整理的那部分.同样，bing 搜索无效，也没有在 PowerShell 文档中搜索这些字符序列.

I have experimented around it, %{} is a for-each statement I believe, bing search was not effective. $_ is also somewhat magic: it is a variable, with no name and thus immediately consumed? I don't care much for the .FullName, that part I sorted out. Again, bing search was not effective, nor searching for those char sequences in PowerShell docs.

谁能给我解释一下?

推荐答案

%{} is not "a thing";- 这是两件事:% 和 {}

% 是 ForEach-Object cmdlet:

%{} is not "a thing" - it's two things: % and {}

% is an alias for the ForEach-Object cmdlet:

PS ~> Get-Alias '%'
CommandType     Name                                               Version    Source
-----------     ----                                               -------    ------
Alias           % -> ForEach-Object

...所以它解析为:

... |ForEach-Object { $_.FullName }

ForEach-Object 基本上是 PowerShell 的 map 函数 - 它通过管道获取输入，并将 {} 块中描述的操作应用于每个输入.

ForEach-Object is basically PowerShell's map function - it takes input via the pipeline and applies the operation described in the {} block to each one of them.

$_ 是对当前正在处理的管道输入项的自动引用

$_ is an automatic reference to the current pipeline input item being processed

你可以把它想象成一个 foreach($thing in $collection){} 循环:

You can think of it a bit like a foreach($thing in $collection){} loop:

1..10 |ForEach-Object { $_ * 10 }
# produces the same output as
foreach($n in 1..10){
    $n * 10
}

除了我们现在可以将循环放在管道中间并让它产生立即消耗的输出:

Except we can now stick our loop in the middle of a pipeline and have it produce output for immediate consumption:

1..10 |ForEach-Object { $_ * 10 } |Do-SomethingElse

---

ForEach-Object 不是唯一在 PowerShell 中使用 $_ 自动变量的东西——它还用于管道绑定表达式:

---

ForEach-Object is not the only thing that makes use of the $_ automatic variable in PowerShell - it's also used for pipeline-binding expressions:

mkdir NewDirectory |cd -Path { $_.FullName }

...以及属性表达式，一种动态属性定义类型，由许多 cmdlet 支持，例如 Sort-Object:

... as well as property expressions, a type of dynamic property definition supported by a number of cmdlets like Sort-Object:

1..10 |Sort-Object { -$_ } # sort in descending order without specifying -Descending

... Group-Object:

1..10 |Group-Object { $_ % 3 } # group terms by modulo congruence

... 和 Select-Object:

1..10 |Select-Object @{Name='TimesTen';Expression={$_ * 10}} # Create synthetic properties based on dynamic value calculation over input 

这篇关于PowerShell $_ 语法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！