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
$null
variable to theForEach-Object
cmdlet, it generates a value for$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