如何在 PowerShell 中为变量分配空值? [英] How do I assign a null value to a variable in PowerShell?

问题描述

我想为一个名为 $dec 的变量分配一个空值，但它给了我错误.这是我的代码:

I want to assign a null value to a variable called $dec, but it gives me errors. Here is my code:

import-module activedirectory
$domain = "domain.example.com"
$dec = null
Get-ADComputer -Filter {Description -eq $dec}

推荐答案

这些是自动变量，比如 $null, $true, $false等

These are automatic variables, like $null, $true, $false etc.

about_Automatic_Variables，请参阅 https://technet.microsoft.com/en-us/library/hh847768.aspx?f=255&MSPPError=-2147217396

$NULL
$null 是一个包含 NULL 或空的自动变量价值.您可以使用此变量来表示不存在或未定义的命令和脚本中的值.

$NULL
$null is an automatic variable that contains a NULL or empty value. You can use this variable to represent an absent or undefined value in commands and scripts.

Windows PowerShell 将 $null 视为具有值的对象，即作为显式占位符，因此您可以使用 $null 表示空一系列值中的值.

Windows PowerShell treats $null as an object with a value, that is, as an explicit placeholder, so you can use $null to represent an empty value in a series of values.

例如，当$null被包含在一个集合中时，它被算作对象之一.

For example, when $null is included in a collection, it is counted as one of the objects.

C:\PS> $a = ".dir", $null, ".pdf"
C:\PS> $a.count
3

如果您将 $null 变量通过管道传输到 ForEach-Object cmdlet，它会为 $null 生成一个值，就像它为其他对象一样.

If you pipe the $null variable to the ForEach-Object cmdlet, it generates a value for $null, just as it does for the other objects.

PS C:\ps-test> ".dir", $null, ".pdf" | Foreach {"Hello"}
Hello
Hello
Hello

因此，您不能使用 $null 来表示无参数值".一种$null 的参数值覆盖默认参数值.

As a result, you cannot use $null to mean "no parameter value." A parameter value of $null overrides the default parameter value.

但是，因为 Windows PowerShell 将 $null 变量视为占位符，您可以使用它的脚本如下所示，其中如果 $null 被忽略，将无法工作.

However, because Windows PowerShell treats the $null variable as a placeholder, you can use it scripts like the following one, which would not work if $null were ignored.

$calendar = @($null, $null, "Meeting", $null, $null, "Team Lunch", $null)
$days = Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"
$currentDay = 0

foreach($day in $calendar)
{
    if($day –ne $null)
    {
        "Appointment on $($days[$currentDay]): $day"
    }

    $currentDay++
}

输出:

Appointment on Tuesday: Meeting
Appointment on Friday: Team lunch

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