PowerShell script to safely zip/compress the contents of a directory.

##
# Zip a folder using PowerShell
# http://www.technologytoolbox.com/blog/jjameson/archive/2012/02/28/zip-a-folder-using-powershell.aspx
#
## Usage:
#     PS C:\NotBackedUp> $directory = Get-Item "C:\NotBackedUp\Fabrikam"
#     PS C:\NotBackedUp> ZipFolder $directory
##

function CountZipItems([__ComObject] $zipFile)
{
    If ($zipFile -eq $null)
    {
        Throw "Value cannot be null: zipFile"
    }

    Write-Host ("Counting items in zip file (" + $zipFile.Self.Path + ")...")

    [int] $count = CountZipItemsRecursive($zipFile)

    Write-Host ($count.ToString() + " items in zip file (" `
        + $zipFile.Self.Path + ").")

    return $count
}

function CountZipItemsRecursive([__ComObject] $parent)
{
    If ($parent -eq $null)
    {
        Throw "Value cannot be null: parent"
    }

    [int] $count = 0

    $parent.Items() |
        ForEach-Object {
            $count += 1

            If ($_.IsFolder -eq $true)
            {
                $count += CountZipItemsRecursive($_.GetFolder)
            }
        }

    return $count
}

function IsFileLocked([string] $path)
{
    If ([string]::IsNullOrEmpty($path) -eq $true)
    {
        Throw "The path must be specified."
    }

    [bool] $fileExists = Test-Path $path

    If ($fileExists -eq $false)
    {
        Throw "File does not exist (" + $path + ")"
    }

    [bool] $isFileLocked = $true

    $file = $null

    Try
    {
        $file = [IO.File]::Open(
            $path,
            [IO.FileMode]::Open,
            [IO.FileAccess]::Read,
            [IO.FileShare]::None)

        $isFileLocked = $false
    }
    Catch [IO.IOException]
    {
        If ($_.Exception.Message.EndsWith(
            "it is being used by another process.") -eq $false)
        {
            Throw $_.Exception
        }
    }
    Finally
    {
        If ($file -ne $null)
        {
            $file.Close()
        }
    }

    return $isFileLocked
}

function GetWaitInterval([int] $waitTime)
{
    If ($waitTime -lt 1000)
    {
        return 100
    }
    ElseIf ($waitTime -lt 5000)
    {
        return 1000
    }
    Else
    {
        return 5000
    }
}

function WaitForZipOperationToFinish([__ComObject] $zipFile, [int] $expectedNumberOfItemsInZipFile)
{
    If ($zipFile -eq $null)
    {
        Throw "Value cannot be null: zipFile"
    }
    ElseIf ($expectedNumberOfItemsInZipFile -lt 1)
    {
        Throw "The expected number of items in the zip file must be specified."
    }

    Write-Host -NoNewLine "Waiting for zip operation to finish..."
    Start-Sleep -Milliseconds 100 # ensure zip operation had time to start

    [int] $waitTime = 0
    [int] $maxWaitTime = 60 * 1000 # [milliseconds]
    while($waitTime -lt $maxWaitTime)
    {
        [int] $waitInterval = GetWaitInterval($waitTime)

        Write-Host -NoNewLine "."
        Start-Sleep -Milliseconds $waitInterval
        $waitTime += $waitInterval

        Write-Debug ("Wait time: " + $waitTime / 1000 + " seconds")

        [bool] $isFileLocked = IsFileLocked($zipFile.Self.Path)

        If ($isFileLocked -eq $true)
        {
            Write-Debug "Zip file is locked by another process."
            Continue
        }
        Else
        {
            Break
        }
    }

    Write-Host

    If ($waitTime -ge $maxWaitTime)
    {
        Throw "Timeout exceeded waiting for zip operation"
    }

    [int] $count = CountZipItems($zipFile)

    If ($count -eq $expectedNumberOfItemsInZipFile)
    {
        Write-Debug "The zip operation completed succesfully."
    }
    ElseIf ($count -eq 0)
    {
        Throw ("Zip file is empty. This can occur if the operation is" `
            + " cancelled by the user.")
    }
    ElseIf ($count -gt $expectedCount)
    {
        Throw "Zip file contains more than the expected number of items."
    }
}

function ZipFolder([IO.DirectoryInfo] $directory)
{
    If ($directory -eq $null)
    {
        Throw "Value cannot be null: directory"
    }

    Write-Host ("Creating zip file for folder (" + $directory.FullName + ")...")

    [IO.DirectoryInfo] $parentDir = $directory.Parent

    [string] $zipFileName

    If ($parentDir.FullName.EndsWith("\") -eq $true)
    {
        # e.g. $parentDir = "C:\"
        $zipFileName = $parentDir.FullName + $directory.Name + ".zip"
    }
    Else
    {
        $zipFileName = $parentDir.FullName + "\" + $directory.Name + ".zip"
    }

    If (Test-Path $zipFileName)
    {
        Throw "Zip file already exists ($zipFileName)."
    }

    Set-Content $zipFileName ("PK" + [char]5 + [char]6 + ("$([char]0)" * 18))

    $shellApp = New-Object -ComObject Shell.Application
    $zipFile = $shellApp.NameSpace($zipFileName)

    If ($zipFile -eq $null)
    {
        Throw "Failed to get zip file object."
    }

    [int] $expectedCount = (Get-ChildItem $directory -Force -Recurse).Count
    $expectedCount += 1 # account for the top-level folder

    $zipFile.CopyHere($directory.FullName)

    # wait for CopyHere operation to complete
    WaitForZipOperationToFinish $zipFile $expectedCount

    Write-Host -Fore Green ("Successfully created zip file for folder (" `
        + $directory.FullName + ").")
}

#
# Example Usage
#

Remove-Item "C:\NotBackedUp\Fabrikam.zip"
[IO.DirectoryInfo] $directory = Get-Item "C:\NotBackedUp\Fabrikam"
ZipFolder $directory