3.1 Lesson 1

How much disk space is saved by compressing files depends on a few factors. The nature of the data you are compressing, the algorithm used to compress the data, and the compression level. Not all algorithms support different compression levels.

Let’s start with setting up some test files to compress:

Now we create three copies of this file:

Now we are going to compress the files with each aforementioned compression tool:

Compare the sizes of the compressed files to the uncompressed file named bigfile. Also notice how the compression tools added extensions to the file names and removed the uncompressed files.

Use bunzip2, gunzip, or unxz to decompress the files:

Notice again that now the compressed file is deleted once it is decompressed.

Some compression tools support different compression levels. A higher compression level usually requires more memory and CPU cycles, but results in a smaller compressed file. The opposite is true for a lower level. Below is a demonstration with xz and gzip:

It is not necessary to decompress a file every time you use it. Compression tools typically come with special versions of common tools used to read text files. For example, gzip has a version of cat, grep, diff, less, more, and a few others. For gzip, the tools are prefixed with a z, while the prefix bz exists for bzip2 and xz exists for xz. Below is an example of using zcat to read display a file compressed with gzip:

The tar program is probably the most widely used archiving tool on Linux systems. In case you are wondering why it is named how it is, it as an abbreviation for “tape archive”. Files created with tar are often called tar balls. It is very common for applications distributed as source code to be in tar balls.

The GNU version of tar that Linux distributions ship with has a lot of options. This lesson is going to cover the most commonly used subset.

Let’s start off by creating an archive of the files used for compression:

The c option instructs tar to create a new archive file and the f option is the name of the file to create. The argument immediately following the options is always going to be the name of the file to work on. The rest of the arguments are the paths to any files or directories you wish to add to, list, or extract from the file. In the example, we are adding the directory compression and all of its contents to the archive.

To view the contents of a tar ball, use the t option of tar:

Notice how the options are preceded with -. Unlike most programs, with tar, the - isn’t required when specifying options, although it doesn’t cause any harm if it is used.

Now let’s extract the file:

Suppose you only need one file out of the archive. If this is the case, you can specify it after the archive’s file name. You can specify multiple files if necessary:

With the exception of absolute paths (paths beginning with /), tar files preserve the entire path to files when they are created. Since the file 3.1.tar was created with a single directory, that directory will be created relative to your current working directory when extracted. Another example should clarify this:

The tar program can also manage compression and decompression of archives on the fly. tar does so by calling one of the compression tools discussed earlier in this section. It is as simple as adding the option appropriate to the compression algorithm. The most commonly used ones are j, J, and z for bzip2, xz, and gzip, respectively. Below are examples using the aforementioned algorithms:

Notice how in the example the .tar files have different sizes. This shows that they were successfully compressed. If you create compressed .tar archives, you should always add a second file extension denoting the algorithm you used. They are .xz, .bz, and .gz for xz, bzip2, and gzip, respectively. Sometimes shortened extensions such as .tgz are used.

It is possible to add files to already existing uncompressed tar archives. Use the u option to do this. If you attempt to add to a compressed archive, you will get an error.

Windows machines often don’t have applications to handle tar balls or many of the compression tools commonly found on Linux systems. If you need to interact with Windows systems, you can use ZIP files. A ZIP file is an archive file similar to a compressed tar file.

The zip and unzip programs can be used to work with ZIP files on Linux systems. The example below should be all you need to get started using them. First we create a set of files:

Now we use zip to pack these files into a ZIP file:

Finally, we unpack the ZIP file again:

When adding directories to ZIP files, the -r option causes zip to include a directory’s contents. Without it, you would have an empty directory in the ZIP file.