#requires -version 2.0

<#
 -----------------------------------------------------------------------------
 Script: Resolve-EnvVariable.ps1
 Version: 0.9
 Author: Jeffery Hicks
    http://jdhitsolutions.com/blog
    http://twitter.com/JeffHicks
    http://www.ScriptingGeek.com
 Date: 6/29/2012
 Keywords:
 Comments:

 "Those who forget to script are doomed to repeat their work."

  ****************************************************************
  * DO NOT USE IN A PRODUCTION ENVIRONMENT UNTIL YOU HAVE TESTED *
  * THOROUGHLY IN A LAB ENVIRONMENT. USE AT YOUR OWN RISK.  IF   *
  * YOU DO NOT UNDERSTAND WHAT THIS SCRIPT DOES OR HOW IT WORKS, *
  * DO NOT USE IT OUTSIDE OF A SECURE, TEST SETTING.             *
  ****************************************************************
 -----------------------------------------------------------------------------
 #>
 
Function Resolve-EnvVariable {

<#
.Synopsis
Resolve Windows environmental variables
.Description
This command accepts strings that contain Windows environmental variables like
%WINDIR% and resolves them with their actual value. For example, it will take
a string like:

    %SystemRoot%\system32\WindowsPowerShell\v1.0\

and return:    

    C:\Windows\system32\WindowsPowerShell\v1.0\

If the string doesn't contain something that looks like an environmental variable
the original string will be written back to the pipeline.
.Parameter String
The string to examine. It must contain an environmental variable like %WINDIR%.

.Example
PS C:\> Resolve-EnvVariable %SystemRoot%\system32\WindowsPowerShell\v1.0\
C:\Windows\system32\WindowsPowerShell\v1.0\
.Example
PS C:\> $env:path.split(";") | resolve-envvariable | foreach { 
 if (test-path $_ ) {
    $fg='Green'
  } 
 else {
  $fg='Red'
  }
  Write-Host $_ -ForegroundColor $fg
 }
#>

[cmdletbinding()]
Param(
[Parameter(Position=0,ValueFromPipeline=$True,Mandatory=$True,
HelpMessage="Enter a string that contains an environmental variable like %WINDIR%")]
[ValidateNotNullOrEmpty()]
[string]$String
)

Begin {
    Write-Verbose "Starting $($myinvocation.mycommand)"
} #Begin

Process {   
    #if string contains a % then process it
    if ($string -match "%\S+%") {
        Write-Verbose "Resolving environmental variables in $String"
        #split string into an array of values
        $values=$string.split("%") | Where {$_}
        foreach ($text in $values) {
            #find the corresponding value in ENV:
            Write-Verbose "Looking for $text"
            [string]$replace=(Get-Item env:$text -erroraction "SilentlyContinue").Value
            if ($replace) {
                #if found append it to the new string
                Write-Verbose "Found $replace"
                $newstring+=$replace
            }
            else {
                #otherwise append the original text
                $newstring+=$text
            }
            
        } #foreach value

        Write-Verbose "Writing revised string to the pipeline"
        #write the string back to the pipeline
        Write-Output $NewString
    } #if
    else {
        #skip the string and write it back to the pipeline
        Write-Output $String
    }
} #Process

End {
    Write-Verbose "Ending $($myinvocation.mycommand)"
} #End
} #end Resolve-EnvVariable