Operating System Utilities

• Most operating systems have an archive and zip utility built into them. In Mac OS and Windows, you only need to right click on the zipped file and select "Open" or "Unzip" or some similar command depending on the operating system and version. If the file was zipped with a different utility, the operating system may indicate the file is corrupt or just fail to open it.
  
  In UNIX and Linux, the archive utility commands are "tar" and "untar," and are separate from the zip utility, which uses the commands "gzip" and "gunzip." Files usually have the form "filename.tar.gz." You would open and decompress this file using the commands:
  gunzip filename.tar.gz
  untar filename.tar
  
  

External Programs

• Each operating system also has some external programs available for file compression. The programs typically have better compression ratios and provide more security than the built-in utilities. These programs sometimes have their own file extensions. For example, StuffIt Expander, popular with Mac OS, uses the ".sit" file extension. If you are using one of these programs, simply open the program and then open the zipped file in the program. You can run into problems, though, when your zipped file is not compatible with your unzip program.
  
  

Universal Programs

• Some available unzip and unarchive programs are "cross-compatible." This means they work on almost any zipped file regardless of the program used to compress it. The most widely used universal unzip utility is WinRAR from Rarlabs. This utility is available for Windows, Mac, UNIX and Linux. It can open almost every zip file type. In most cases, you can simply double-click the file to open it. Otherwise, you can open the program and then open the zipped file in the program.