¿Cómo asigno un valor nulo a una variable en PowerShell?

scripting null

Question

Quiero asignar un valor nulo a una variable llamada $dec , pero me da errores. Aquí está mi código:

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

Answer 1

Estas son variables automáticas, como $null , $true , $false etc.

about_Automatic_Variables , consulte 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++
       } 

       Appointment on Tuesday: Meeting
       Appointment on Friday: Team lunch

Answer 2

Si el objetivo es simplemente listar todos los objetos de la computadora con un atributo de descripción vacío, intente esto

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

Answer 3

Usa $dec = $null

De la documentation :

$ null es una variable automática que contiene un valor NULL o vacío. Puede usar esta variable para representar un valor ausente o indefinido en comandos y scripts.

PowerShell trata $ null como un objeto con un valor, es decir, como un marcador de posición explícito, por lo que puede usar $ null para representar un valor vacío en una serie de valores.