%{} is not "a thing" - it's two things: % and {}
% is an alias for the ForEach-Object cmdlet:
PS ~> Get-Alias '%'
CommandType Name Version Source
----------- ---- ------- ------
Alias % -> ForEach-Object
... so it resolves to:
... |ForEach-Object { $_.FullName }
ForEach-Object is basically PowerShell's map function - it takes input via the pipeline and applies the operation described in the {} block to each one of them.
$_ is an automatic reference to the current pipeline input item being processed
You can think of it a bit like a foreach($thing in $collection){} loop:
1..10 |ForEach-Object { $_ * 10 }
# produces the same output as
foreach($n in 1..10){
$n * 10
}
Except we can now stick our loop in the middle of a pipeline and have it produce output for immediate consumption:
1..10 |ForEach-Object { $_ * 10 } |Do-SomethingElse
ForEach-Object is not the only thing that makes use of the $_ automatic variable in PowerShell - it's also used for pipeline-binding expressions:
mkdir NewDirectory |cd -Path { $_.FullName }
... as well as property expressions, a type of dynamic property definition supported by a number of cmdlets like Sort-Object:
1..10 |Sort-Object { -$_ } # sort in descending order without specifying -Descending
... Group-Object:
1..10 |Group-Object { $_ % 3 } # group terms by modulo congruence
... and Select-Object:
1..10 |Select-Object @{Name='TimesTen';Expression={$_ * 10}} # Create synthetic properties based on dynamic value calculation over input
