powershell - how to write a (unix like) function that accepts pipe or file name input? -

e.g. want head either accepts array piped , select-object -first, or, receives list of file names parameters , outputs first lines of each. cat $filename | head should work head $filename

here's i've tried far:

function head( ) {     param (       [switch] $help = $false,       [parameter(mandatory=$false,valuefrompipeline=$true)] [object[]] $inputs,       [parameter(valuefromremainingarguments=$true)] [string[]] $files     )      if( $help )     {         write-host "usage: $($myinvocation.mycommand) [<file>] <numlines>"         return     }       $lines = 0     if ($files -and [int]::tryparse($files[0], [ref]$lines)) {       $null,$files = $files     } else {       $lines = 10     }      $input | select-object -first $lines     if ($files) {get-content -totalcount $lines $files} } 

but causes function ignore first parameter:

c:\users\idror.tlv-wpvaj> head c:\temp\now.ps1 c:\users\idror.tlv-wpvaj> head c:\temp\now.ps1 c:\temp\now.ps1 function head( ) {     param (           [switch] $help = $false,           [parameter(mandatory=$false,valuefrompipeline=$true)] [object[]] $input,           [parameter(valuefromremainingarguments=$true)] [string[]] $files         )          if( $help )         { c:\users\idror.tlv-wpvaj> $a | head 1 2 3 c:\users\idror.tlv-wpvaj> $a | head 1 head : input object cannot bound parameters command either because command not take pipeline input or input , properties not match of parameters take pipeline input. @ line:1 char:6 + $a | head 1 +      ~~~~~~     + categoryinfo          : invalidargument: (1:int32) [head], parameterbindingexception     + fullyqualifiederrorid : inputobjectnotbound,head 

you can achieve using parameter sets:

function head { [cmdletbinding()] param(     [parameter(         parametersetname = 'content',         valuefrompipeline = $true,         mandatory = $true     )]     [string[]]     $content ,      [parameter(         parametersetname = 'file',         valuefromremainingarguments = $true ,         mandatory = $true     )]     [string[]]     $path,      [parameter()]     [uint64]     $count = 5 )      begin {         write-verbose "parameter set name: $($pscmdlet.parametersetname)"     }      process {         write-verbose "content: $content"         write-verbose "path: $path"         write-verbose "count: $count"     } } 

first, take @ output of running get-help head:

name     head  syntax    head -content <string[]> [-count <uint64>]  [<commonparameters>]     head -path <string[]> [-count <uint64>]  [<commonparameters>] 

you can see interprets 2 different sets, , each parameter mandatory in own set.

you can call -verbose see demonstration of how working:

# content pipeline 'one','two','three' | head -verbose  # files array head -verbose 'file1','file2','file3'  # files remaining arguments head -verbose 'file1' 'file2' 'file3'