Archiving and Compression

     -[0-9]

It's possible to adjust the level of compression that zip uses when it does its job. The zip 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.

Say you're interested in researching Herman Melville's Moby-Dick, so you want to collect key texts to help you understand the book: Moby-Dick itself, Milton's Paradise Lost, and the Bible's book of Job. Let's compare the results of different compression rates.

   $ ls -l
   -rw-r--r-- scott scott 102519 job.txt
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   -rw-r--r-- scott scott 508925 paradise_lost.txt
   $ zip -0 moby.zip *.txt
   adding: job.txt (stored 0%)
   adding: moby-dick.txt (stored 0%)
   adding: paradise_lost.txt (stored 0%)
   $ ls -l
   -rw-r--r-- scott scott 102519 job.txt
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   -rw-r--r-- scott scott 1848444 moby.zip
   -rw-r--r-- scott scott 508925 paradise_lost.txt
   $ zip -1 moby.zip *txt
   updating: job.txt (deflated 58%)
   updating: moby-dick.txt (deflated 54%)
   updating: paradise_lost.txt (deflated 50%)
   $ ls -l
   -rw-r--r-- scott scott 102519 job.txt
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   -rw-r--r-- scott scott 869946 moby.zip
   -rw-r--r-- scott scott 508925 paradise_lost.txt
   $ zip -9 moby.zip *txt
   updating: job.txt (deflated 65%)
   updating: moby-dick.txt (deflated 61%)
   updating: paradise_lost.txt (deflated 56%)
   $ ls -l
   -rw-r--r-- scott scott 102519 job.txt
   -rw-r--r-- scott scott 1236574 moby-dick.txt
   -rw-r--r-- scott scott 747730 moby.zip
   -rw-r--r-- scott scott 508925 paradise_lost.txt

In tabular format, the results look like this:

The results you see here would vary depending on the file types (text files typically compress well) and the sizes of the original files, but this gives you a good idea of what you can expect. Unless you have a really slow machine or you're just naturally impatient, you should just use -9 all the time to get the maximum compression.

     -P

     -e

The Zip program allows you to password-protect your Zip archives using the -P option. You shouldn't use this option. It's completely insecure, as you can see in the following example (the actual password is 12345678):

   $ zip -P 12345678 moby.zip *.txt

Because you had to specify the password on the command line, anyone viewing your shell's history (and you might be surprised how easy it is for other users to do so) can see your password in all its glory. Don't use the -P option!

Instead, just use the -e option, which encrypts the contents of your Zip file and also uses a password. The difference, however, is that you're prompted to type the password in, so it won't be saved in the history of your shell events.

   $ zip -e moby.zip *.txt
   Enter password:
   Verify password:
   adding: job.txt (deflated 65%)
   adding: moby-dick.txt (deflated 61%)
   adding: paradise_lost.txt (deflated 56%)

The only part of this that's saved in the shell is zip -e moby.zip *.txt. The actual password you type disappears into the ether, unavailable to anyone viewing your shell history.