In Jeff Hicks’s Prof. PowerShell series, he continues with his tips on working with functions.  He uses the Check-Service function that was in the previous article and that I provided another option in this post.  Jeff describes how you can make a function act more like a cmdlet.

The problem that you run into in V1 of PoweShell is that it is easy to make your functions work with the pipeline

Function Check-Service {
  Param([string]$service=”spooler” )
  PROCESS
  {
    $wmi=get-wmiobject win32_service -filter “name=’$service’” -computername $_ 
    if ($wmi.state -eq “running”)  {
      write $True
    }
    else {
      write $False
    }
  }
}
 
Get-Content servers.txt | Check-Service

OR work well with parameters being passed in.

Function Check-Service {
  Param([string[]]$server=$env:computername, [string]$service=”spooler” ) 
    $wmi=get-wmiobject win32_service -filter “name=’$service’” -computername $server 
    if ($wmi.state -eq “running”)  {
      write $True
    }
    else {
      write $False
    }
} 
  
Check-Service –Server Server01, Server02, Server03

Advanced functions in V2 of PowerShell can alleviate this problem (a topic for a whole new post), but a workaround I’ve found for V1 is to use the Begin block to take certain command line parameters and pass them back into the function via the pipeline.

Function Check-Service {
Param([string[]]$server=$null,[string]$service=”spooler” )
  BEGIN
  {
    if ($server -ne $null)
    {
      $server | Check-Service –Service $service
    }
  }
  PROCESS
  {
    if ($_ -ne $null)
    {
      $wmi=get-wmiobject win32_service -filter “name=’$service’” -computername $_
      if ($wmi.state -eq “running”) {
        write $True
      }
      else {  write $False  }
    }
  }
}

This enables both of the above scenarios:

Get-Content servers.txt | Check-Service

Check-Service –Server Server01, Server02, Server03

PSMDTAG:FAQ param

PSMDTAG:FAQ pipeline

Updated: (Missed a little recursion bug.. thanks Aleksandar!)