One good way to save disk space is to compress large files that are used only now and then or that are maintained on your servers solely as archives. These files can use up a LOT of free space and there's little, if any, advantage to keeping them uncompressed. In fact, the only advantage that I can think of is that you'll never have to worry if you have enough space to uncompress them when the time comes -- not much of a justification for anything but the most critical files.

There are quite a few different commands for compressing files. Some are included in the basic install of your operating system. Others have to be downloaded and installed as add-ons. Almost any compression utility, however, is going to give you better than a 50% compression savings on most files and this can make a BIG difference if there are a lot of them on your system and your disks are at risk of filling up. The standard compression utilities include compress, pack, gzip, zip and bzip2. In this and next week's columns, we're going to contrast these various tools and try to answer the question "Which should you use?".

To help answer this question, we're going to consider a number of factors which might influence you to favor one compression utility over another. These include:

1) whether or not it's already on your system (the status quo factor)

2) whether or not you already know how to use it (the momentum factor)

3) the compression ratio -- how much space you can expect to save (the effectiveness factor)

4) the speed with which the compression will take place (the efficiency factor)

5) whether or not the utility is cross-platform, in particular, whether it exists or can be built for both Unix and non-Unix systems (the portability factor) and

6) whether or not the algorithm is patented (let's call this the freedom factor)

The compress Command

In older (non-current) versions of Unix, you were likely to find that the only compression commands at your disposal were the old compress command and the even older pack command.

The compress command, which creates files with the .Z file extension is one that has been around a long time and is still used fairly extensively. Many archives sites, for example, will offer downloads in both the .Z and .gz (GNU zip) format. Some are offering .bz2 (bzip2) files as well.

Every compression command has a partner -- a command used to undo the compression that the compress command invoked. The undo command for compress is uncompress.

Once a compress command has finished compressing, the original file will have been replaced by its compressed (.Z extension) counterpart. Similarly, once an uncompress command has finished uncompressing, the .Z file will have been replaced by the original file.

During a compress and uncompress operation, you will see that both the .Z and the original version of the file exist simultaneously.

The compress and uncompress commands use adaptive Lempel-Ziv (sometimes referred to as

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