UPDATE NUMBER 2:

Kevin commented about a flaw in my script wherein it would keep files open until the PowerShell process was closed - a scenario I should’ve tested but, in all of the excitement (ha), I missed. You won’t believe what the problem was, either. I forgot parentheses on my $inStream.Close() method (it looked like $inStream.Close instead of $inStream.Close() ). The reason the trap { } script block didn’t catch it is because the statement was still valid… it would just display the MethodInfo like below:

MemberType          : Method
OverloadDefinitions : {System.Void Close()}
TypeNameOfValue     : System.Management.Automation.PSMethod
Value               : System.Void Close()
Name                : Close
IsInstance          : True

The [void] statement before it prevented the output from displaying, hence me not catching the bug. The script has now been fixed (hopefully). Thanks for the catch Kevin!

UPDATE:

Ignore my script. Go download the PowerShell Community Extensions instead. It has a great Get-Hash script that does everything that my script does and more. I wish I had downloaded it sooner :-)


Jeffrey Snover posted a suggestion on the PowerShell blog recently to post automation scripts people have written in PowerShell that they use. Well, here is a script I wrote that I also submitted for the PowerShell Scripting Contest a few weeks back. The script is quite basic and is based on other code I found, but I added a little bit to it to handle some of my own needs. It calculates file hashes based on a specified hash algorithm (i.e. SHA1, MD5, etc). I like to use it to determine if a large file I’ve downloaded (like an ISO from MSDN) is a good file or if it was corrupted during the download.

Here is Calc-Hash.ps1:

param (
  [string] $inFile = $(throw "Usage: Calc-Hash.ps1 file.txt [sha1|md5] "),
  [string] $hashType = "sha1"
)

function Main
{
  if ($hashType -eq "")
  {
    throw "Usage: Calc-Hash.ps1 file.txt [sha1|md5] "
  }

  if ($hashType -eq "sha1")
  {
    $provider = New-Object System.Security.Cryptography.SHA1CryptoServiceProvider
  }
  elseif ($hashType -eq "md5")
  {
    $provider = New-Object System.Security.Cryptography.MD5CryptoServiceProvider
  }
  else
  {
    throw "Unsupported hash type $hashType"
  }

  $inFileInfo = New-Object System.IO.FileInfo($inFile)
  if (-not $inFileInfo.Exists)
  {
    # If the file can't be found, try looking for it in the current directory.
    $inFileInfo = New-Object System.IO.FileInfo("$pwd\$inFile")
    if (-not $inFileInfo.Exists)
    {
      throw "Can't find $inFileInfo"
    }
  }

  $inStream = $inFileInfo.OpenRead()
  $hashBytes = $provider.ComputeHash($inStream)
  [void] $inStream.Close()

  trap
  {
    if ($inStream -ne $null)
    {
      [void] $inStream.Close()
    }
    break
  }

  foreach ($byte in $hashBytes)
  {
    Write-Host -NoNewLine $byte.ToString("X2")
  }

  Write-Host
}

. Main