Nov 192009
 

Data compression has been extremely useful to us over the years. Whether its a zip file containing images to be sent in a mail or a compressed data backup stored on a server, we use data compression to save valuable hard drive space or to make the downloading of files easier. There are compression formats out there which allow us to sometimes compress our data by 60% or more. I’ll run you through using some of these formats to compress and decompress files and directories on a Linux machine. We’ll cover the basic usage of zip, tar, tar.gz and the tar.bz2 formats. These are some of the most popular formats for compression used on Linux machines.

Before we delve into the usage of the formats I’d like to share some of my experience using the various formats of archiving. I’m talking about only a few data compression formats here, and there are many more out there. I’ve realized that I need two or three formats of compression that I’m comfortable using, and stick to them. The zip format is definitely one of them. This is because zip has become the de-facto standard choice for data compression, and it works on Windows as well. I use the zip format for files that I might need to share with Windows users. I like to use the tar.gz format for files that I would only use on my Mac and Linux machines.
ZIP

Zip is probably the most commonly used archiving format out there today. Its biggest advantage is the fact that it is available on all operating system platforms such as Linux, Windows, and Mac OS, and generally supported out of the box. The downside of the zip format is that it does not offer the best level of compression. Tar.gz and tar.bz2 are far superior in that respect. Let’s move on to usage now.

To compress a directory with zip do the following:

# zip -r archive_name.zip directory_to_compress

Here’s how you extract a zip archive:

# unzip archive_name.zip
TAR

Tar is a very commonly used archiving format on Linux systems. The advantage with tar is that it consumes very little time and CPU to compress files, but the compression isn’t very much either. Tar is probably the Linux/UNIX version of zip – quick and dirty. Here’s how you compress a directory:

# tar -cvf archive_name.tar directory_to_compress

And to extract the archive:

# tar -xvf archive_name.tar.gz

This will extract the files in the archive_name.tar archive in the current directory. Like with the tar format you can optionally extract the files to a different directory:

# tar -xvf archive_name.tar -C /tmp/extract_here/
TAR.GZ

This format is my weapon of choice for most compression. It gives very good compression while not utilizing too much of the CPU while it is compressing the data. To compress a directory use the following syntax:

# tar -zcvf archive_name.tar.gz directory_to_compress

To decompress an archive use the following syntax:

# tar -zxvf archive_name.tar.gz

This will extract the files in the archive_name.tar.gz archive in the current directory. Like with the tar format you can optionally extract the files to a different directory:

# tar -zxvf archive_name.tar.gz -C /tmp/extract_here/
TAR.BZ2

This format has the best level of compression among all of the formats I’ve mentioned here. But this comes at a cost – in time and in CPU. Here’s how you compress a directory using tar.bz2:

# tar -jcvf archive_name.tar.bz2 directory_to_compress

This will extract the files in the archive_name.tar.bz2 archive in the current directory. To extract the files to a different directory use:

# tar -jxvf archive_name.tar.bz2 -C /tmp/extract_here/

Data compression is very handy particularly for backups. So if you have a shell script that takes a backup of your files on a regular basis you should think about using one of the compression formats you learned about here to shrink your backup size.

Over time you will realize that there is a trade-off between the level of compression and the the time and CPU taken to compress. You will learn to judge where you need a quick but less effective compression, and when you need the compression to be of a high level and you can afford to wait a little while longer.

 Posted by at 11:11 pm

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