L in u X ba sics for h acke rs g e t t I n g s t a r t e d w I t h

9
C O M P R E S S I N G A N D A R C H I V I N G
Hackers often need to download and 
install new software, as well as send and 
download multiple scripts and large files. 
These tasks are easier if these files are com-
pressed and combined into a single file. If you come 
from the Windows world, you will probably recognize 
this concept from the .zip format, which combines and compresses files to 
make them smaller for transferring over the internet or removable media. 
There are many ways to do this in Linux, and we look at a few of the most 
common tools for doing so in this chapter. We also look at the 
dd
command, 
which allows you to copy entire drives, including deleted files on those drives.
What Is Compression?
The interesting subject of compression could fill an entire book by itself, 
but for this book we only need a rudimentary understanding of the process. 
Compression, as the name implies, makes data smaller, thereby requiring less 

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Chapter 9
storage capacity and making the data easier to transmit. For your purposes 
as a beginning hacker, it will suffice to categorize compression as either 
lossy or lossless.
Lossy compression is very effective in reducing the size of files, but the 
integrity of the information is lost. In other words, the file after compres-
sion is not exactly the same as the original. This type of compression works 
great for graphics, video, and audio files, where a small difference in the 
file is hardly noticeable—.mp3, .mp4, and .jpg are all lossy compression algo-
rithms. If a pixel in a .jpg file or a single note in an .mp3 file is changed, your 
eye or ear is unlikely to notice the difference—though, of course, music 
aficionados will say that they can definitely tell the difference between an 
.mp3 and an uncompressed .flac file. The strengths of lossy compression are 
its efficiency and effectiveness. The compression ratio is very high, meaning 
that the resulting file is significantly smaller than the original.
However, lossy compression is unacceptable when you’re sending files 
or software and data integrity is crucial. For example, if you are sending a 
script or document, the integrity of the original file must be retained when 
it is decompressed. This chapter focuses on this lossless type of compression, 
which is available from a number of utilities and algorithms. Unfortunately, 
lossless compression is not as efficient as lossy compression, as you might 
imagine, but for the hacker, integrity is often far more important than com-
pression ratio.

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