PowerShell script to calculate a hash value for specified input file (or directory).

##############################################################################
##
## Get-FileHash
##
## From Windows PowerShell Cookbook (O'Reilly)
## by Lee Holmes (http://www.leeholmes.com/guide)
##
##############################################################################

<#

.SYNOPSIS

    Get the hash of an input file.

.EXAMPLE

    Gets the hash of a specific file:

        PS > Get-FileHash myFile.txt

.EXAMPLE

    Gets the hash of files from the pipeline

        dir | Get-FileHash

.EXAMPLE

    Gets the has of myFile.txt, using the SHA1 hashing algorithm

        Get-FileHash myFile.txt -Hash SHA1

#>

param(
    ## The path of the file to check
    $Path,

    ## The algorithm to use for hash computation
    [ValidateSet("MD5", "SHA1", "SHA256", "SHA384", "SHA512")]
    $HashAlgorithm = "MD5"
)

Set-StrictMode -Version 3

## Create the hash object that calculates the hash of our file.
$hashType = [Type] "System.Security.Cryptography.$HashAlgorithm"
$hasher = $hashType::Create()

## Create an array to hold the list of files
$files = @()

## If they specified the file name as a parameter, add that to the list
## of files to process
if($path)
{
    $files += $path
}
## Otherwise, take the files that they piped in to the script.
## For each input file, put its full name into the file list
else
{
    $files += @($input | Foreach-Object { $_.FullName })
}

## Go through each of the items in the list of input files
foreach($file in $files)
{
    ## Skip the item if it is not a file
    if(-not (Test-Path $file -Type Leaf)) { continue }

    ## Convert it to a fully-qualified path
    $filename = (Resolve-Path $file).Path

    ## Use the ComputeHash method from the hash object to calculate
    ## the hash
    $inputStream = New-Object IO.StreamReader $filename
    $hashBytes = $hasher.ComputeHash($inputStream.BaseStream)
    $inputStream.Close()

    ## Convert the result to hexadecimal
    $builder = New-Object System.Text.StringBuilder
    $hashBytes | Foreach-Object { [void] $builder.Append($_.ToString("X2")) }

    ## Return a custom object with the important details from the
    ## hashing
    $output = New-Object PsObject -Property @{
        Path = ([IO.Path]::GetFileName($file));
        HashAlgorithm = $hashAlgorithm;
        HashValue = $builder.ToString()
    }

    $output
}