Prof. Powershell

PowerShell File Frontier, Part 6: Adding Files to a Zip Archive

Prof. Powershell's rundown of what can be done with zipped files in PowerShell continues.

• By Jeffery Hicks
• 09/29/2014

More on this topic:

• The File Frontier, Part 1
• The PowerShell File Frontier, Part 2: File Length
• PowerShell File Frontier, Part 3: Right Tool for the Job
• PowerShell File Frontier, Part 4: Zipped Files
• PowerShell File Frontier, Part 5: Analyzing Compressed Flies

Last time we explored opening a zip file with PowerShell and the .NET Framework and seeing what was inside. Today, let's add some files to it. I'll go ahead and open my demo zip file but this time not in read-only mode. I know from last time that there is an Open() method but I don't know how to use it. So I'll look at the OverloadDefinitions again.

PS C:\>  [System.IO.Compression.ZipFile]::open.OverloadDefinitions
static System.IO.Compression.ZipArchive  Open(string archiveFileName, System.IO.Compression.ZipArchiveMode mode)

static System.IO.Compression.ZipArchive  Open(string archiveFileName, System.IO.Compression.ZipArchiveMode mode,  System.Text.Encoding entryNameEncoding)

There are two ways I can invoke the Open() method. Looks like both require the name of the zip archive. Both also seem to require a parameter indicating the ZipArchiveMode. The second parameter set can also take a parameter indicating encoding. The first method should be sufficient for our purposes. But how do we know what to use for a mode?

One technique is to put in a bogus value and hope that the error message indicates the proper values.

PS C:\> $zip =  [System.IO.Compression.ZipFile]::Open("C:\zip\demo.zip","foo")

The result can be seen in Figure 1.

The error message indicates the correct values. Or I could use PowerShell to get the values since I know they are part of something called [System.IO.Compression.ZipArchiveMode]. This class is known as an enumeration which is essentially a list of possible values. The easy way to list the values is to use the builtin [enum] class and invoke the GetNames() static method.

PS C:\>  [enum]::GetNames([System.IO.Compression.ZipArchiveMode])
  Read

  Create

  Update

Since I want to add a file it seems clear I need to use the Update mode.

PS C:\> $zip =  [System.IO.Compression.ZipFile]::Open("C:\zip\demo.zip","Update")

  PS C:\> $zip
Entries                                                              Mode

  -------                                                              ----

  {chi-hvr2-health-full.htm, chi-hvr2-health.htm,  EventLo...        Update

Excellent. Ready to go. Oh, but how do I add a file? Back to Get-Member.

PS C:\> $zip | Get-Member -MemberType Method

From what you see in Figure 2 you might guess that you need to use the CreateEntry() method.

But this is when you need to turn to the documentation. When you read the MSDN docs on this class you'll learn that this method creates an empty entry that you can fill later. That's not what we want. Instead we need to take advantage of the other compression class [System.IO.Compress.ZipExtensions].

PS C:\>  [System.IO.Compression.ZipFileExtensions].getmethods().Name | select -unique

  CreateEntryFromFile

  ExtractToDirectory

  ExtractToFile

  ToString

  Equals

  GetHashCode

  GetType

This looks promising. Let's look closer the CreateEntryFromFile() method.

PS C:\>  [System.IO.Compression.ZipFileExtensions]::CreateEntryFromFile.OverloadDefinitions

  static System.IO.Compression.ZipArchiveEntry  CreateEntryFromFile(System.IO.Compression.ZipArchive destination, string

  sourceFileName, string entryName)

  static System.IO.Compression.ZipArchiveEntry  CreateEntryFromFile(System.IO.Compression.ZipArchive destination, string  sourceFileName, string entryName, System.IO.Compression.CompressionLevel  compressionLevel)

Looks like I need to specify a zip file and the source filename. It also looks like I can optionally specify a compression level. Bet that is another enumeration.

PS C:\>  [enum]::GetNames([System.IO.Compression.CompressionLevel])

  Optimal

  Fastest

  NoCompression

I know what I want to try.

PS C:\>  [System.IO.Compression.ZipFileExtensions]::CreateEntryFromFile($zip,

  "C:\work\csdata.xml","csdata.xml","optimal")
Archive          : System.IO.Compression.ZipArchive

  CompressedLength : 

  FullName         : csdata.xml

  LastWriteTime    : 12/5/2013 5:31:48 PM -05:00

  Length           : 

  Name              : csdata.xml

The entry name is how I want the file to look in the zip file. You'll notice that lengths are empty. But don't worry. Close the file, re-open and look at the entries.

PS C:\> $zip.Dispose()

  PS C:\> $zip =  [System.IO.Compression.ZipFile]::Open("C:\zip\demo.zip","Update")

  PS C:\> $zip.entries.where({$_.name -eq  'csdata.xml'})
Archive          : System.IO.Compression.ZipArchive

  CompressedLength : 313

  FullName          : csdata.xml

  LastWriteTime    : 12/5/2013 5:31:48 PM -05:00

  Length           : 1930

  Name              : csdata.xml

That's all the time we have for now. Next time we'll look at some other techniques and scenarios using these zip files in PowerShell.