How does -InputObject parameter of the Sort-Object cmdlet work?

Question

PowerShell 4.0

I read about Sort-Object cmdlet here (TechNet page). I don't understand how to use -InputObject parameter. That page hasn't examples for it. Also I didn't find this info in Internet. I would be very grateful for the examples of its using, or for the links to Internet pages with that info.

I have tried to use it how I understand its purpose (according documentation):

$items = ('a','b','c','d')
$result = sort -InputObject $items -Descending

But the result variable has the same value like it has items instead of its descended version.

Thank you.

Answer 1

InputObject is a generic name used for a parameter that takes pipeline input. It's part of internal PowerShell naming convention and there is nothing special about it.

Your example doesn't work as you think it should, because when you pass a collection to the InputObject parameter it's treated as a single item and not unwrapped to individial elements, so it doesn't get sorted. This allows you to sort a collection of collections.

Consider this examples:

This is how Sort-Object works:

function Add-Quotes
{
    Param
    (
        [Parameter(ValueFromPipeline = $true)]
        $InputObject
    )

    Process
    {
        "'$InputObject'"
    }
}

Note that array is automatically unwrapped by the pipeline, then idividial items are assigned the $InputObject variable in each iteration and then processed in Process block:

PS> $items | Add-Quotes
'a'
'b'
'c'
'd'

But when you pass a collection to the InputObject it's not iterated over, because there is no pipeline to unwrap it:

PS> Add-Quotes -InputObject $items
'a b c d'

Sometimes it's a desired behavior, sometimes you need to unwrap collections no matter where they came from. In this case you use internal foreach loop to do so:

function Add-Quotes
{
    Param
    (
        [Parameter(ValueFromPipeline = $true)]
        [string[]]$InputObject
    )

    Process
    {
        foreach($item in $InputObject)
        {
            "'$item'"
        }
    }
}

PS > $items | Add-Quotes
'a'
'b'
'c'
'd'

PS > Add-Quotes -InputObject $items
'a'
'b'
'c'
'd'

Hope this makes it clear for you.

Answer 2

The process block of a function automatically deals with a collection over a pipe, but not if it's passed as a command line argument. Some cmdlets put in an extra foreach loop to process -inputobject collections or lists, but most don't.

However, any parameter that can take input from a pipe, can also take a script block from the command line. So that may come in handy some time. You'll see examples like this in the help.

PS C:\> echo a,b,c,d | sort -InputObject { $_.ToUpper() } -Descending
D
C
B
A