Archiving and Compression

     unzip

Expanding a Zip archive isn't hard at all. To create a zipped archive, use the zip command; to expand that archive, use the unzip command.

   $ unzip moby.zip
   Archive: moby.zip
   inflating: job.txt
   inflating: moby-dick.txt
   inflating: paradise_lost.txt

The unzip command helpfully tells you what it's doing as it works. To get even more information, add the -v option (which stands, of course, for verbose).

    unzip -v moby.zip
   Archive: moby.zip
   Length   Method  Size   Ratio  CRC-32   Name
   -------  ------  ------ -----  ------   ----
   102519   Defl:X   35747  65%  fabf86c9  job.txt
   1236574  Defl:X  487553  61%  34a8cc3a  moby-dick.txt
   508925   Defl:X  224004  56%  6abe1d0f  paradise_lost.t
   -------          ------  ---            -------
   1848018          747304  60%            3 files

There's quite a bit of useful data here, including the method used to compress the files, the ratio of original to compressed file size, and the cyclic redundancy check (CRC) used for error correction.

     -l

Sometimes you might find yourself looking at a Zip file and not remembering what's in that file. Or perhaps you want to make sure that a file you need is contained within that Zip file. To list the contents of a zip file without unzipping it, use the -l option (which stands for "list").

   $ unzip -l moby.zip
   Archive: moby.zip
   Length     Date    Time   Name
   --------   ----    ----   ----
         0  01-26-06  18:40  bible/
    207254  01-26-06  18:40  bible/genesis.txt
    102519  01-26-06  18:19  bible/job.txt
   1236574  01-26-06  18:19  moby-dick.txt
    508925  01-26-06  18:19  paradise_lost.txt
   --------                  -------
   2055272                   5 files

From these results, you can see that moby.zip contains two files — moby-dick.txt and paradise_lost.txt — and a directory (bible), which itself contains two files, genesis. txt and job.txt. Now you know exactly what will happen when you expand moby.zip. Using the -l command helps prevent inadvertently unzipping a file that spews out 100 files instead of unzipping a directory that contains 100 files. The first leaves you with files strewn pell-mell, while the second is far easier to handle.

-t

Sometimes zipped archives become corrupted. The worst time to discover this is after you've unzipped the archive and deleted it, only to discover that some or even all of the unzipped contents are damaged and won't open. Better to test the archive first before you actually unzip it by using the -t (for test) option.

   $ unzip -t moby.zip
   Archive: moby.zip
   testing: bible/               OK
   testing: bible/genesis.txt    OK
   testing: bible/job.txt        OK
   testing: moby-dick.txt        OK
   testing: paradise_lost.txt    OK
   No errors detected in compressed data of moby.zip.

You really should use -t every time you work with a zipped file. It's the smart thing to do, and although it might take some extra time, it's worth it in the end.

     gzip

Using gzip is a bit easier than zip in some ways. With zip, you need to specify the name of the newly created Zip file or zip won't work; with gzip, though, you can just type the command and the name of the file you want to compress.

   $ ls -l
   -rw-r--r-- scott scott 508925 paradise_lost.txt
   $ gzip paradise_lost.txt
   $ ls -l
   -rw-r--r-- scott scott 224425 paradise_lost.txt.gz

You should be aware of a very big difference between zip and gzip: When you zip a file, zip leaves the original behind so you have both the original and the newly zipped file, but when you gzip a file, you're left with only the new gzipped file. The original is gone.

If you want gzip to leave behind the original file, you need to use the -c (or --stdout or --to-stdout) option, which outputs the results of gzip to the shell, but you need to redirect that output to another file. If you use -c and forget to redirect your output, you get nonsense like this:

Not good. Instead, output to a file.

   $ls -l
   -rw-r--r-- 1 scott scott 508925 paradise_lost.txt
   $ gzip -c paradise_lost.txt > paradise_lost.txt.gz
   $ ls -l
   -rw-r--r-- 1 scott scott 497K paradise_lost.txt
   -rw-r--r-- 1 scott scott 220K paradise_lost.txt.gz

Much better! Now you have both your original file and the zipped version.