$ Files中的PowerShell ForEach $ file [英] PowerShell ForEach $file in $Files

问题描述

我有一个PowerShell脚本，该脚本获取文件列表并移动符合特定条件的文件.为什么foreach循环即使对象为空也要运行?

I have a PowerShell script that gets a file listing and moves the files that meet a specific criteria. Why does the foreach loop run even if the object is empty?

我假设如果$ i不存在，它将不会运行.但是，如果$ filePath中没有结果，为什么forEach循环运行一次?我已经解决了这个问题，但是很好奇，我尝试搜索，但是找不到答案.

I would assume if $i did not exist it would not run. But if there are no results in $filePath why does the forEach loop run once? I have worked around my issue, but I am curious and I tried searching, but I could not find the answer.

要使其正常工作，我只需检查以确保$ filePath在forEach循环之前存在.

To get this to work I just check to make sure $filePath exists before the forEach loop.

例如，如果($ filePath){...

For example, if ($filePath){...

 $filePath = Get-ChildItem -Path $sourceDir | Where-Object {! $_.PSIsContainer -AND ($_.Name -Match "^XXX_XXX*" -OR $_.Name -Match "^YYY_XX*")}

 ForEach($i in $filePath){
     $sfName =  $i.Name
     $sfDir = $i.Directory
     $tFileName = testFile $destDir $sfName
     $sFile = $sourceDir + $sfName
     $tFile = $destDir + $tFileName

     moveFile $sFile $tFile

推荐答案

这是PowerShell V3 IIRC中已解决的经典问题. PowerShell的foreach循环将允许您遍历标量值，例如:

This is a classic issue that has been fixed in PowerShell V3 IIRC. PowerShell's foreach loop will allow you to iterate over a scalar value e.g.:

foreach ($i in 1) {$i}

这很方便，因为您迭代的集合很多时候可以包含0、1或N个项目，例如:

That is handy because many times the collection you iterate can contain 0, 1 or N items e.g.:

$files = Get-ChildItem c:\temp\*.txt
foreach ($file in $files) {$file}

在这种情况下，Get-ChildItem可以返回0、1或N个文件. N个文件很棒，我们可以对其进行迭代.但是，如果它返回了1个文件，然后您必须将该文件粘贴到数组中，以便可以使用foreach-那样会很烂.因此，foreach允许您迭代标量对象.当Get-ChildItem返回0对象时，就会出现此问题.在这种情况下，未将空数组分配给$ files.而是分配了$ null. PowerShell将$ null视为标量，因此foreach循环将在迭代值为$ null的情况下执行一次.真烂我和其他许多人都早早告诉了团队，因此在V3中，他们更改了foreach，以使值不为$ null时，不对标量进行迭代.

In this case, Get-ChildItem could return 0, 1 or N files. N files is great, we can iterate over that. However if it returned 1 file and then you had to stick that one file in an array so that you could use foreach - well that would kind of suck. So foreach allows you to iterate over a scalar object. The problem occurs when Get-ChildItem returns 0 objects. In that case, an empty array is not assigned to $files. Instead $null is assigned. PowerShell considers $null a scalar and hence the foreach loop will execute once where the iterated value is $null. That kind of sucks. And I and many other folks told the team this early on, so in V3 they changed foreach to not iterate a scalar if the value is $null.

在V1/V2中解决此问题的另一种方法是确保空大小写生成一个空数组而不是$ null.您可以使用@()数组表达式来做到这一点，例如

Another way to work around this in V1/V2 is to ensure the empty case generates an empty array instead of $null. You can do that using an @() array expression e.g.

$files = @(Get-ChildItem c:\temp\*.txt)
foreach ($file in $files) {$file}

这将适用于0、1和N个文件.它适用于0个文件，因为@()将导致将空数组分配给$ files.循环空数组时，foreach不会执行其主体.

This will work for 0, 1, and N files. It's works for 0 files because the @() will result in an empty array being assigned to $files. foreach will not execute its body when looping over an empty array.

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