PowerShell v3 Cmdlet: Get-CourseraVideos

For those who haven't been through any of Coursera's free online courses, I cannot recommend them highly enough. For a while now, I've been wanting to download all of a classes videos to disk. Doing this manually is a huge pain, however. Rather, I wrote a PowerShell v3 script to automate this process. With the introduction of the Invoke-WebRequest cmdlet, PowerShell v3 is perfectly suited for this task.

Please note that you may be violating the Coursera terms of service as of (effective as of 10 April, 2012) by using this script. Otherwise, enjoy and happy learning!

function Get-CourseraVideos
{
<#
.SYNOPSIS

    Downloads all the videos from a Coursera class preview site.

.DESCRIPTION

    The Get-CourseraVideos cmdlet scrapes the video content from a class 'preview' site.

    If you have a Coursera account, do keep in mind that this tool is potentially in violation of the Coursera Terms of Service (Effective as of 10 April, 2012).

.PARAMETER PreviewURL

    Specifies the URL of the Coursera 'preview' site.

.PARAMETER Path

    Specifies the path to which videos will be downloaded.

.EXAMPLE

    C:\PS> Get-CourseraVideos -PreviewURL 'https://class.coursera.org/compilers-2012-selfservice/lecture/preview' -Path 'C:\Videos\Compilers'

.LINK
http://www.exploit-monday.com/2012/08/Get-CourseraVideos.html

.LINK
https://www.coursera.org/about/terms
#>

    [CmdletBinding()] Param (
        [Parameter( Mandatory = $True,
                    Position = 0 )]
        [ValidateNotNullOrEmpty()]
        [String]
        $PreviewURL,

        [Parameter( Position = 1 )]
        [ValidateNotNullOrEmpty()]
        [String]
        $Path = ($pwd) # Download to current working directory by default
    )

    Set-StrictMode -Version 3

    # Throw error if the path does not exist
    if (!(Test-Path $Path))
    {
        Throw [System.Management.Automation.ItemNotFoundException] "Cannot find path '$Path' because it does not exist."
    }

    $SiteContent = Invoke-WebRequest $PreviewURL -ErrorVariable WebError

    if ($WebError) { Return }

    $FileSafeChar = [Char] ' '

    $FileIndex = 1
 
    $Lectures = $SiteContent.Links | Where-Object { $_.class -eq 'lecture-link' } 2> $null

    if ($Lectures -eq $null)
    {
        Throw "An invalid Coursera preview URL was provided."
    }
    else
    {
        $TotalVideos = $Lectures.Length

        $Lectures |
            ForEach-Object {
                # Get video title
                $Title = $_.innerText
                # If the title contains a bad filename character, replace it with ' '
                foreach ($Char in [IO.Path]::GetInvalidFileNameChars()) { $Title = $Title.Replace($Char, $FileSafeChar) }
                # Prepend numerical index to the title
                $Title = "$($FileIndex.ToString('D2')) $($Title)"

                # Get video link
                $SiteContent = Invoke-WebRequest $_.href
                $VideoLink =  ($SiteContent.ParsedHtml.body.getElementsByTagName('SOURCE') |
                    Where-Object {$_.GetAttribute('type', 0) -eq 'video/mp4'}).GetAttribute('src', 0)

                $FilePath = Join-Path $Path "$($Title).mp4"

                Write-Progress -Activity 'Downloading Coursera Videos...' -PercentComplete ((($fileIndex - 1) / $TotalVideos) * 100)`
                    -CurrentOperation $VideoLink
            
                Write-Verbose "Downloading $VideoLink to $FilePath"

                # Download the video
                $WebClient = New-Object System.Net.WebClient
                $WebClient.DownloadFile($VideoLink, $FilePath)

                $FileIndex++
            }
    }
}