Archiving and Compression

     -t

Before gunzipping a file (or files) with gunzip, you might want to verify that they're going to gunzip correctly without any file corruption. To do this, use the -t (or --test) option.

   $ gzip -t paradise_lost.txt.gz
   $

That's right: If nothing is wrong with the archive, gzip reports nothing back to you. If there's a problem, you'll know, but if there's not a problem, gzip is silent. That can be a bit disconcerting, but that's how Unix-based systems work. They're generally only noisy if there's an issue you should know about, not if everything is working as it should.

     bzip2

Working with bzip2 is pretty easy if you're comfortable with gzip, as the creators of bzip2 deliberately made the options and behavior of the new command as similar to its progenitor as possible.

   $ ls -l
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   $ bzip2 moby-dick.txt
   $ ls -l
   -rw-r--r-- scott scott 367248 moby-dick.txt.bz2

Just like gzip, bzip2 leaves you with just the .bz2 file. The original moby-dick.txt is gone. To keep the original file, use the -c (or --stdout) option and pipe the output to a filename that ends with .bz2.

   $ ls -l
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   $ bzip2 -c moby-dick.txt > moby-dick.txt.bz2
   $ ls -l
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   -rw-r--r-- scott scott 367248 moby-dick.txt.bz2

If you look back at "Archive and Compress Files Using gzip," you'll see that gzip and bzip2 are incredibly similar, which is by design.

     -[0-9]

Just as with zip and gzip, it's possible to adjust the level of compression that bzip2 uses when it does its job. The bzip2 command uses a scale from 0 to 9, in which 0 means "no compression at all" (which is like tar, as you'll see later), 1 means "do the job quickly, but don't bother compressing very much," and 9 means "compress the heck out of the files, and I don't mind waiting a bit longer to get the job done." The default is 6, but modern computers are fast enough that it's probably just fine to use 9 all the time.

   $ ls -l
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   $ bzip2 -c -1 moby-dick.txt > moby-dick.txt.bz2
   $ ls -l
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   -rw-r--r-- scott scott 424084 moby-dick.txt.bz2
   $ bzip2 -c -9 moby-dick.txt > moby-dick.txt.bz2
   $ ls -l
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   -rw-r--r-- scott scott 367248 moby-dick.txt.bz2

From 424KB with 1 to 367KB with 9 — that's quite a difference! Also notice the difference in ultimate file size between gzip and bzip2. At -9, gzip compressed moby-dick.txt down to 488KB, while bzip2 mashed it even further to 367KB. The bzip2 command is noticeably slower than the gzip command, but on a fast machine that means that bzip2 takes two or three seconds longer than gzip, which frankly isn't much to worry about.

     bunzip2

In the same way that bzip2 was purposely designed to emulate gzip as closely as possible, the way bunzip2 works is very close to that of gunzip.

   $ ls -l
   -rw-r--r-- scott scott 367248 moby-dick.txt.bz2
   $ bunzip2 moby-dick.txt.bz2
   $ ls -l
   -rw-r--r-- scott scott 1236574 moby-dick.txt

You'll notice that bunzip2 is similar to gunzip in another way: Both commands remove the original compressed file, leaving you with the final uncompressed result. If you want to ensure that you have both the compressed and uncompressed files, you need to use the -c option (or --stdout or --to-stdout) and pipe the results to the file you want to create.

   $ ls -l
   -rw-r--r-- scott scott 367248 moby-dick.txt.bz2
   $ bunzip2 -c moby-dick.txt.bz2 > moby-dick.txt
   $ ls -l
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   -rw-r--r-- scott scott 367248 moby-dick.txt.bz2

It's a good thing when commands copy each other's options and behavior, as it makes them easier to learn. In this, the creators of bzip2 and bunzip2 showed remarkable foresight.