Detecting ZIP files with PowerShell

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Have you heard of magic numbers? Some file formats are designed such that files are always saved with a specific byte sequence in the header. JPEG, PDF, and ZIP are all such formats.

You could look for ZIP files by searching for all files with a .zip extension but a better way would be to look for all files that have 50 4b 03 04 as the first 4 bytes of the file. All ZIP files will start with those bytes. Not all ZIP files have the .zip extension.

Here’s a Test-ZipFile PowerShell cmdlet that will return true or false whether the specified file has this magic header. You may also note that this cmdlet is a good citizen by accepting file path input in an idiomatic way.

This cmdlet is also a great example of accepting -Path and -LiteralPath arguments in an idiomatic way. Including wildcard support for -Path.

View on GitHub →

function Test-ZipFile
{
<#
.SYNOPSIS
	Tests for the magic ZIP file header bytes.

.DESCRIPTION
	Inspired by http://stackoverflow.com/a/1887113/31308
#>
	[CmdletBinding()]
	param(
		[Parameter(
			ParameterSetName  = "Path",
			Mandatory = $true,
			ValueFromPipeline = $true,
			ValueFromPipelineByPropertyName = $true
		)]
		[string[]]$Path,

		[Alias("PSPath")]
		[Parameter(
			ParameterSetName = "LiteralPath",
			Mandatory = $true,
			ValueFromPipeline = $true,
			ValueFromPipelineByPropertyName = $true
		)]
		[string[]]$LiteralPath
	)

	process {
		$provider = $null

		# Only expand wildcards if the -Path parameter was used.
		if ($PSCmdlet.ParameterSetName -eq "Path") {
			$filePaths = $PSCmdlet.GetResolvedProviderPathFromPSPath($Path, [ref]$provider)
		}
		elseif ($PSCmdlet.ParameterSetName -eq "LiteralPath") {
			$filePaths = $PSCmdlet.GetResolvedProviderPathFromPSPath($LiteralPath, [ref]$provider)
		}

		foreach ($filePath in $filePaths) {
			$isZip = $false
			try {
				$stream = New-Object System.IO.StreamReader -ArgumentList @($filePath)
				$reader = New-Object System.IO.BinaryReader -ArgumentList @($stream.BaseStream)
				$bytes = $reader.ReadBytes(4)
				if ($bytes.Length -eq 4) {
					if ($bytes[0] -eq 80 -and
						$bytes[1] -eq 75 -and
						$bytes[2] -eq 3 -and
						$bytes[3] -eq 4) {
						$isZip = $true
					}
				}
			}
			finally {
				if ($reader) {
					$reader.Dispose()
				}
				if ($stream) {
					$stream.Dispose()
				}
			}

			Write-Output $isZip
		}
	}
}

Test-ZipFile is part of Poshato, my personal PowerShell module of miscellaneous goodness.