Archiving and Compression

     -zvtf

Before you take apart a tarball (whether or not it was also compressed using gzip), it's a really good idea to test it. First, you'll know if the tarball is corrupted, saving yourself hair pulling when files don't seem to work. Second, you'll know if the person who created the tarball thoughtfully tarred up a directory containing 100 files, or instead thoughtlessly tarred up 100 individual files, which you're just about to spew all over your desktop.

To test your tarball (once again assuming it was also zipped using gzip), use the -t (or --list) option.

   $ tar -zvtf moby.tar.gz
   scott/scott 0 moby-dick/
   scott/scott 102519 moby-dick/job.txt
   scott/scott 0 moby-dick/bible/
   scott/scott 207254 moby-dick/bible/genesis.txt
   scott/scott 102519 moby-dick/bible/job.txt
   scott/scott 1236574 moby-dick/moby-dick.txt
   scott/scott 508925 moby-dick/paradise_lost.txt

This tells you the permissions, ownership, file size, and time for each file. In addition, because every line begins with moby-dick/, you can see that you're going to end up with a directory that contains within it all the files and subdirectories that accompany the tarball, which is a relief.

Be sure that the -f is the last option because after that you're going to specify the name of the .tar.gz file. If you don't, tar complains:

   $ tar -zvft moby.tar.gz
   tar: You must specify one of the '-Acdtrux' options
   Try 'tar --help' or 'tar --usage' for more information.

Now that you've ensured that your .tar.gz file isn't corrupted, it's time to actually open it up, as you'll see in the following section.

     -zxvf

To create a .tar.gz file, you used a set of options: -zcvf. To untar and uncompress the resulting file, you only make one substitution: -x (or --extract) for -c (or --create).

   $ ls -l
   rsgranne rsgranne 846049 moby.tar.gz
   $ tar -zxvf moby.tar.gz
   moby-dick/
   moby-dick/job.txt
   moby-dick/bible/
   moby-dick/bible/genesis.txt
   moby-dick/bible/job.txt
   moby-dick/moby-dick.txt
   moby-dick/paradise_lost.txt
   $ ls -l
   rsgranne rsgranne  168 moby-dick
   rsgranne rsgranne 846049 moby.tar.gz

Make sure you always test the file before you open it, as covered in the previous section, "Test Files That Will Be Untarred and Uncompressed." That means the order of commands you should run will look like this:

   $ tar -zvtf moby.tar.gz
   $ tar -zxvf moby.tar.gz

Back in the days of slow modems and tiny hard drives, archiving and compression was a necessity. These days, it's more of a convenience, but it's still something you'll find yourself using all the time. For instance, if you ever download source code to compile it, more than likely you'll find yourself face-to-face with a file such as sourcecode.tar.gz. In the future, you'll probably see more and more of those files ending with .tar.bz2. And if you exchange files with Windows users, you're going to run into files that end with .zip. Learn how to use your archival and compression tools because you're going to be using them far more than you think.

About the Author:

Scott Granneman is a monthly columnist for SecurityFocus and Linux Magazine, as well as a professional blogger on The Open Source Weblog. He is an adjunct Professor at Washington University, St. Louis and at Webster University, teaching a variety of courses about technology and the Internet.