Bzip2

Bzip2 is a high-quality, patent free file compressor using the Burrows-Wheeler algorithm. Files are typically compressed to anywhere between 10% - 15% of their original size^[1] while maintaining a speed of roughly twice the speed of compression and up to six times the speed of decompression of the PPM compression family. Any machine with an ANSI C compiler should be able to support bzip2, making it a great choice for those who run systems with portable demands.

In 2019, an experimental Rust rewrite of bzip2 started ^[2] with the blessing of the bzip2 maintainer, but progress was slow. A new maintainer took over the nascent Rust rewrite repository in 2021 ^[3] but there does not appear to be movement towards a release.

The original bzip2 is still maintained on Sourceware despite the initial plans otherwise.

Installation

USE flags

Emerge

After adjusting USE flags:

Usage

Invocation

Displays the default bzip2 arguments summary. The command line options are similar to gzip, however they aren't identical and so a little revision on bzip2's arguments go a long way. For more information, see the bzip2 man page.

A short summary of usage flags can be found below:

bzip2, a block-sorting file compressor. Version 1.0.8, 13-Jul-2019.
 usage: bzip2 [flags and input files in any order]
 -h --help print this message
 -d --decompress force decompression
 -z --compress force compression
 -k --keep keep (don't delete) input files
 -f --force overwrite existing output files
 -t --test test compressed file integrity
 -c --stdout output to standard out
 -q --quiet suppress noncritical error messages
 -v --verbose be verbose (a 2nd -v gives more)
 -L --license display software version & license
 -V --version display software version & license
 -s --small use less memory (at most 2500k)
 -1 .. -9 set block size to 100k .. 900k
 --fast alias for -1
 --best alias for -9
 If invoked as `bzip2', default action is to compress.
 as `bunzip2', default action is to decompress.
 as `bzcat', default action is to decompress to stdout.
 If no file names are given, bzip2 compresses or decompresses
 from standard input to standard output. You can combine
 short flags, so `-v -4' means the same as -v4 or -4v, &c.

Compression

The default behavior of bzip2 is to compress a file when given as an argument.

If forcing a compression operation is desired, bzip2 can be invoked with the -z option:

Decompression

If invoked as bunzip2, the default action is to decompress a given file:

A file can also be decompressed to stdout:

If no arguments are given, bzip2 will compress/decompress from standard input to standard output.

Memory management

bzip2 compresses large files in blocks. The block size is proportionate to the compression ratio achieved and the total amount of memory needed for compression and decompression. Block size is read from the header of the file and can be specified manually when compressing.

When decompressing bzip2 will read this and allocate just enough space to decompress the file. Because during decompression, the block size is read from the header, the -1 to -9 flags are ignored.^[4] For more information, visit the bzip2 man page.

Parallelization

bzip2 itself does not support parallel compression or decompression. app-arch/lbzip2 however can be used for parallel compression by some tools, either opportunistically (e.g. Dracut), or if configured (like tar). Another option is app-arch/pbzip2 .

As a result of some tools using lbzip2 opportunistically, simply installing lbzip2 can be useful even without configuration. To maximize the usage of lbzip2 (or pbzip2), enable one of the lbzip2 or pbzip2 USE flags on app-alternatives/bzip2 which will create a symlink at bzip2 to lbzip2 (or pbzip2, respectively).

Configuration

Environment variables

Removal

Unmerge

See also

• Gzip — a data compression utility maintained by the GNU project that uses Lempel-Ziv (LZ77) encoding
• Zstd — a fast, lossless compression algorithm and tool suite
• Xz-utils — LZMA2-based data compression utility.

References

• Command line tools
• Archive utilities