How do I assign a null value to a variable in PowerShell?

问题

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}

回答1:

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

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

$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 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.

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

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++
}
output:
Appointment on Tuesday: Meeting
Appointment on Friday: Team lunch

回答2:

Use $dec = $null

From the documentation:

$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.

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.

回答3:

If the goal simply is to list all computer objects with an empty description attribute try this

import-module activedirectory  
$domain = "domain.example.com" 
Get-ADComputer -Filter '*' -Properties Description | where { $_.Description -eq $null }

来源：https://stackoverflow.com/questions/28697349/how-do-i-assign-a-null-value-to-a-variable-in-powershell

标签

powershell

scripting

null