Dondi: Transcribe brief compression/decompression page.

2017-09-16T22:33:25Z

Transcribe brief compression/decompression page.

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Downloadable software is usually provided in a ''compressed'' format—that is, the files in the software have been processed so that they take up less space, resulting in a faster download. Compression also typically “packs” any software that consists of multiple files into a single file, again making the download process simpler.

== Decompression ==

Compressed files can’t be used “as-is;” they need to be ''decompressed'' by an appropriate application. The key word here is “appropriate”—there are different kinds of decompression, and each kind may require a different application. Each kind of decompression can be thought of as a different file format, the way Microsoft Word files are either ''.doc'' or ''.docx'' files, or how images can be in ''.jpg'', ''.gif'', ''.png'', or other formats.

The [[Week 4]] starter files have been uploaded in the ''.zip'' compression format. As supplementary information, this wiki page discusses this format as well as another common format, ''.tar.gz'' (called a “tarball” in techie circles), that you might also encounter in the future. The two formats serve the same purpose; the only difference is ''how'' the two choices are compressed—a detail that most of us don’t need to know about.

=== .zip ===

The ZIP format is typically more familiar to Windows users, but is widely supported on other operating systems as well.

Typically, you can double-click on the file’s icon to unzip. On Windows, the ability to unzip depends on the version of Windows that you’re using. Some versions of Windows have the capability built-in; other versions require third-party applications such as [http://www.7-zip.org 7-Zip] and [http://www.rarlab.com WinRAR], as well as [http://www.winzip.com WinZip]. The Seaver 120 lab computers have [http://www.7-zip.org 7-Zip].

There is a gotcha that you should be aware of when using the built-in Windows unzip functionality, if it is available to you: Windows defaults to doing “live” decompression; that is, as you double-click a ''.zip'' file and navigate through its contents, the ''.zip'' file may act like a folder, but in reality it’s still a single ''.zip'' file, which Windows opens as-you-go. This works fine when you’re just looking at files, but may cause confusion when you actually want to edit or run them. To be completely sure, right-click on the ''.zip'' and make sure to ''Extract'' the files so that they become actual files on the disk.

Command line tip: in the ''bash'' command line environment, there is a command for unzipping a ''.zip'' file:

unzip filename.zip

Substitute <code>filename.zip</code> with the actual name of the ''.zip'' file that you would like to decompress.

=== .tar.gz ===

The ''.tar.gz'' format is actually two formats: the first one, ''.tar'', is responsible for grouping multiple files and folders into a single file. That single file is then compressed, producing the ''.gz''. This format is generally most readily available on Unix-flavored operating systems like Linux or macOS. In the ''bash'' command line environment, you would “extract” the files in a tarball using this command:

tar xzf filename.tar.gz

The same operation can also be done from the graphical user interface by double-clicking on the file’s icon. On Windows, the open-source [http://www.7-zip.org 7-Zip] and shareware [http://www.rarlab.com WinRAR] applications can handle ''.tar.gz'' and other formats.

If you happen a file that ends in just ''.gz'' and not ''.tar.gz'', then it is compressed only. Command-line decompression then requires a different command:

gunzip filename.gz

Graphical user interface approaches can figure this out on their own and don’t require that you do anything different.

=== Other Formats ===

Other compression formats abound, including ''bunzip2'', ''compress'' (''.Z'' files), ''.rar'', and ''.7z'', to name a few. It’s generally useful to understand how to deal with files such as these, since downloads are frequently compressed in some way.

== Compression ==

Of course, if it is possible to ''de''compress these files, then someone must have ''compressed'' them first. Many of the utilities listed above go both ways: they can create ''and'' extract compressed files. In most operating systems, right-clicking a folder reveals a menu that includes a ''Compress'' command, which uses the ''.zip'' format most of the time. On the command line, there are “mirror” commands for compressing files: '''zip''' instead of '''unzip''' for ''.zip'' files, '''tar czf''' instead of '''tar xzf''' for ''.tar.gz'' files, and '''gzip''' instead of '''gunzip''' for plain ''.gz'' files.

Handling Compressed Files - Revision history

Dondi: Transcribe brief compression/decompression page.