You probably have used a .zip file before if you have received or sent a large file via e-mail. Once you do a double click on such a file it will expand to show the files which are hidden inside of that archive. The whole idea behind an archive is to make it occupy less space on the hard drive in comparison to the original files so that the content will be easier to send via e-mail.
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If you’re interested in the technology behind file compression, you should know that it looks for repeating data and after that it writes an archive which gets rid of these repetitions in order to save precious space. Some of the compressed text files will occupy only 50% of the space of the original file, while JPEG ones won’t be a lot smaller since this file format already includes compression.

If you haven’t used a Mac before and you don’t know how to make a .zip file, you have to select the item or items in the Finder and after that select File > Compress Item Name. Another way to go would be by doing a right-click on the chosen item/items and after that select Compress Item Name. The process is going to be very fast if you want to compress just one file, while for a larger folder it will take a few minutes. For uncompressing a zip archive all you have to do is perform a double click on it. Once you do this Apple’s Archive Utility kicks in and handles the business. You should know that this Archive Utility will work with a wide range of archives, such as .tar, .gz or .bz. You can figure out if Archive Utility works with a certain archive format if that file is labeled with the regular .zip icon. If you don’t know where Archive Utility is, you can find it in /System/Library/Core Services/Archive Utility. After doing a double click on it, this is going to appear in the Dock. We should mention that a menu will be displayed but this is not going to show any window. In Archive Utility > Preferences you will notice an option where you can configure where the expanded files will be put. The default setting is to place the expanded files in the current directory. For modifying the destination you have to click on the menu called Save Expanded Files and then select Into. A sheet is going to appear and there you’ll have to pick a folder.

If you have old .zip archives spread all over the computer’s HDD, you might want to go to the Archive Utility and access Preferences. Now, you must perform a click on the menu called After Expanding and here you will be able to select Delete Archive, Move Archive To Trash or Move Archive To for which you’ll have to choose a folder. Remember that using the latter option is going to move the archive to the desired location, not just copy it. This means that it will be deleted from its original location. 99.9% of all people that use archives prefer the .zip format so you might want to stick with this one. However, if you are a fan of Unix, you might want to use a .cpgz archive or the .cpio archive format.
Elavil