bzip2

bzip2, by Julian Seward, compresses with the Burrows–Wheeler transform in independent blocks of 100–900 kB. It beats gzip on ratio, particularly for text, but is slow in both directions, single-threaded, and is outperformed on speed, and usually on ratio, by xz and zstd. Today it is mainly something you decompress: old source tarballs (.tar.bz2) and some datasets. For new files, use zstd or xz.

Common commands

bzip2 -k file              # file.bz2, 900k blocks, keep the original
bzip2 -1 file              # 100k blocks: faster, less memory, worse ratio
bzip2 -d file.bz2          # decompress (or bunzip2); bzcat to stdout
bzip2 -t file.bz2          # test integrity

bzip2 deletes the input unless you pass -k. The -1 … -9 options set the block size to 100 kB … 900 kB; the default is -9, and smaller blocks only help when memory is limited. Compression at -9 needs about 7.6 MB of memory and decompression about 3.7 MB (-s reduces it to at most 2.5 MB, at some cost in speed).

Archives and parallelism

tar -cjf backup.tar.bz2 dir/
tar -xf backup.tar.bz2

The reference bzip2 uses a single thread. Because blocks are independent, parallel implementations such as pbzip2 and lbzip2 can speed it up and also decompress .bz2 files in parallel. 7-Zip can also create bzip2 with multiple threads (7z a -tbzip2).

Pitfalls

• Block independence helps recovery. Damage to one block loses only that block, and bzip2recover can extract the intact ones. gzip has no equivalent.
• Poor on binary or already-compressed data and slower than modern alternatives; the gain over gzip rarely justifies the time.
• .bz2 is a single stream, not an archive. Combine it with tar for directories.