Archiving and Compression

Although the differences are sometimes made opaque in casual conversation, there is in fact a complete difference between archiving files and compressing them. Archiving means that you take 10 files and combine them into one file, with no difference in size. If you start with 10 100KB files and archive them, the resulting single file is 1000KB. On the other hand, if you compress those 10 files, you might find that the resulting files range from only a few kilobytes to close to the original size of 100KB, depending upon the original file type.

All of the archive and compression formats in this chapter — zip, gzip, bzip2, and tar — are popular, but zip is probably the world's most widely used format. That's because of its almost universal use on Windows, but zip and unzip are well supported among all major (and most minor) operating systems, so things compressed using zip also work on Linux and Mac OS. If you're sending archives out to users and you don't know which operating systems they're using, zip is a safe choice to make.

gzip was designed as an open-source replacement for an older Unix program, compress. It's found on virtually every Unix-based system in the world, including Linux and Mac OS X, but it is much less common on Windows. If you're sending files back and forth to users of Unix-based machines, gzip is a safe choice.

The bzip2 command is the new kid on the block. Designed to supersede gzip, bzip2 creates smaller files, but at the cost of speed. That said, computers are so fast nowadays that most users won't notice much of a difference between the times it takes gzip or bzip2 to compress a group of files.

zip, gzip, and bzip2 are focused on compression (although zip also archives). The tar command does one thing — archive — and it has been doing it for a long time. It's found almost solely on Unix-based machines. You'll definitely run into tar files (also called tarballs) if you download source code, but almost every Linux user can expect to encounter a tarball some time in his career.

     zip

zip both archives and compresses files, thus making it great for sending multiple files as email attachments, backing up items, or for saving disk space. Using it is simple. Let's say you want to send a TIFF to someone via email. A TIFF image is uncompressed, so it tends to be pretty large. Zipping it up should help make the email attachment a bit smaller.

   $ ls -lh
   -rw-r--r-- scott scott 1006K young_edgar_scott.tif
   $ zip grandpa.zip young_edgar_scott.tif
   adding: young_edgar_scott.tif (deflated 19%)
   $ ls -lh
   -rw-r--r-- scott scott 1006K young_edgar_scott.tif
   -rw-r--r-- scott scott 819K grandpa.zip
   _grandpa.zip

In this case, you shaved off about 200KB on the resulting zip file, or 19%, as zip helpfully informs you. Not bad. You can do the same thing for several images.

   $ ls -l
   -rw-r--r-- scott scott 251980 edgar_intl_shoe.tif
   -rw-r--r-- scott scott 1130922 edgar_baby.tif
   -rw-r--r-- scott scott 1029224 young_edgar_scott.tif
   $ zip grandpa.zip edgar_intl_shoe.tif edgar_baby.tif young_edgar_scott.tif
   adding: edgar_intl_shoe.tif (deflated 4%)
   adding: edgar_baby.tif (deflated 12%)
   adding: young_edgar_scott.tif (deflated 19%)
   $ ls -l
   -rw-r--r-- scott scott 251980 edgar_intl_shoe.tif
   -rw-r--r-- scott scott 1130922 edgar_baby.tif
   -rw-r--r-- scott scott 2074296 grandpa.zip
   -rw-r--r-- scott scott 1029224 young_edgar_scott.tif

It's not too polite, however, to zip up individual files this way. For three files, it's not so bad. The recipient will unzip grandpa.zip and end up with three individual files. If the payload was 50 files, however, the user would end up with files strewn everywhere. Better to zip up a directory containing those 50 files so when the user unzips it, he's left with a tidy directory instead.

   $ ls -lF 
   drwxr-xr-x scott scott edgar_scott/
   $ zip grandpa.zip edgar_scott
   adding: edgar_scott/ (stored 0%)
   adding: edgar_scott/edgar_baby.tif (deflated 12%)
   adding: edgar_scott/young_edgar_scott.tif (deflated 19%)
   adding: edgar_scott/edgar_intl_shoe.tif (deflated 4%)
   $ ls -lF
   drwxr-xr-x scott scott   160 edgar_scott/
   -rw-r--r-- scott scott 2074502 grandpa.zip

Whether you're zipping up a file, several files, or a directory, the pattern is the same: the zip command, followed by the name of the Zip file you're creating, and finished with the item(s) you're adding to the Zip file.