PowerShell $_ syntax - powershell

Question

In this answer the author proposed the following snippet:
dir -Path C:\FolderName -Filter *.fileExtension -Recurse | %{$_.FullName}
I can understand the majority of it, but I'm unable to search documentation for the last part. The output of the search is piped | and used in %{} and as $_.
I have experimented around it, %{} is a for-each statement I believe, bing search was not effective. $_ is also somewhat magic: it is a variable, with no name and thus immediately consumed? I don't care much for the .FullName, that part I sorted out. Again, bing search was not effective, nor searching for those char sequences in PowerShell docs.
Can anybody explain it to me?

Answer 1

%{} 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

Answer 2

To complement Mathias' answer, which explains the specific constructs well, with how you could / couldn't have discovered this information yourself, using PowerShell's own help system:
Relevant help topics and use of the help system:
Note: To get an overview of all aspects of PowerShell's help system, simply run help.
% is a built-in alias for the ForEach-Object cmdlet:
Use Get-Help ForEach-Object to view the help topic in the terminal.
If no local topics are found, you must download them via the Update-Help cmdlet.
Tips:
Add the -Online switch to open the (potentially more current) online version of the topic in your browser.
You can bootstrap your use of Get-Help with Get-Help Get-Help (or even help help):
Cmdlet-specific help comes in detail levels: terse (default, shows the syntax and overview description only), -Detailed (includes parameter descriptions and example commands) and -Full (additionally includes technical parameter information and extended notes).
-Examples can be used to show example commands only.
With keyword-based search (see below), you can limit results to topics of a certain category with the -Category parameter.
For convenience, you can also use the built-in help function, which wraps Get-Help calls with display paging (simply put: by piping the output to the more utility) and defaults to detail level -Full.
{...} is a script block literal, a block of arbitrary PowerShell code that can be invoked on demand:
help about_Script_Blocks shows the topic locally; the about_ prefix indicates that the topic is a conceptual help topic (rather than one covering a specific command); when you use Get-Help to search for a keyword (see below), you can (somewhat obscurely) limit the results to conceptual topics with -Category HelpFile.
Note: As of this writing, about_ topics can not yet be directly viewed online by adding -Online - see GitHub issue #13550 - but it's easy to google them by name.
$_ is a variable, as the $ sigil followed by an identifier implies, and is more specifically an automatic (built-in) variable:
help about_Variables covers variables in general.
help about_Automatic_Variables covers the automatic ones.
How the above can / cannot be discovered based on symbols and aliases alone:
Doing a web search for symbols is notoriously unhelpful.
As an aside: Running distinct syntax constructs such as % and { ... } together without whitespace between them (e.g. %{$_.FullName}) constitutes an additional barrier, and should therefore be avoided.
Narrowing your search by using only PowerShell's help system helps, but only to a limited degree:
%
Because Get-Help is aware of aliases, help % actually works fine and directly shows ForEach-Object's help topic.
help % -Examples shows example commands that include the use of script blocks and the automatic $_ variable.
Even though Get-Help supports keyword-based search, searching for symbol-based terms {} and $_ directly isn't helpful, because even when limiting the search to conceptual (about_-prefixed topics) with -Category HelpFile, there are either too many hits (help '$_' -Category HelpFile) or the relevant topic doesn't show at all (help '{}' -Category HelpFile)
$_ can be discovered indirectly, IF you already know that it is an instance of a variable:
help variables -Category HelpFile happens to take you directly to the relevant (local) about_Automatic_Variables topic,
whereas help variable -Category HelpFile lists the following matching topics about_Variable_Provider, ``, about_Automatic_Variables, about_Preference_Variables, about_Remote_Variables, about_Variables, and about_Environment_Variables
Note: Thanks to PowerShell's pervasive support for wildcard expressions, you could have performed the search also as follows: help about*variable* - be sure to enclose both sides of the search term in *.
$_ can be discovered indirectly, IF you already know that it is an instance of a script (code) block:
help about_*block* takes you directly to the relevant (local) about_Script_Blocks topic.
Potential future improvements:
It would be a great improvement if PowerShell's help system supported focused symbol-based searches.
Similarly, the ability to directly look up operators, such as -match, the regular-expression matching operator, would be helpful:
GitHub issue #11339 proposes just that.
On a related note, GitHub issue #11338 proposes adding the ability to look up documentation for .NET types (online).
This answer contains custom functions Show-OperatorHelp and Show-TypeHelp, which fill that gap for now (also available as Gists).