bunzip2 Linux Command: How to Use It

Open bz2 files on Linux

What to Know

  • Use the command option -c --stdout to compress/decompress to standard output and -d --decompress to force decompression.
  • Use -t --test to check the integrity of specified files. Use -f --force to force overwrite of output files. Use -k --keep to retain input files.
  • Use -s --small to lower memory usage and -q --quiet to suppress warning messages. Use -v --verbose to show file compression ratios.

This article explains how to use the Linux bunzip2 command, which is a block-sorting file compressor that can be used to both compress and decompress data. When you execute the bzip2 command without any options, the system compresses the data by default. bunzip2 decompresses by default, and bzcat decompresses to stdout.

Bunzip2 Linux Command Options

List of bunzip2 Command Options
Option Explanation
-c --stdout Compress or decompress to standard output.
-d --decompress Force decompression. bzip2, bunzip2 and bzcat are really the same program, and the decision about what actions to take is done on the basis of which name is used. This flag overrides that mechanism, and forces bzip2 to decompress.
-z --compress The complement to -d: forces compression, regardless of the invocation name.
-t --test Check integrity of the specified file(s), but don't decompress them. This really performs a trial decompression and throws away the result.
-f --force

Force overwrite of output files. Normally, bzip2 will not overwrite existing output files. Also forces bzip2 to break hard links to files, which it otherwise wouldn't do.

bzip2 normally declines to decompress files which don't have the correct magic header bytes. If forced (-f), however, it will pass such files through unmodified. This is how GNU gzip behaves.

-k --keep Keep (don't delete) input files during compression or decompression.
-s --small

Reduce memory usage, for compression, decompression and testing. Files are decompressed and tested using a modified algorithm which only requires 2.5 bytes per block byte. This means any file can be decompressed in 2300k of memory, albeit at about half the normal speed.

During compression, -s selects a block size of 200k, which limits memory use to around the same figure, at the expense of your compression ratio. In short, if your machine is low on memory (8 megabytes or less), use -s for everything. See MEMORY MANAGEMENT below.

-q --quiet Suppress non-essential warning messages. Messages pertaining to I/O errors and other critical events will not be suppressed.
-v --verbose Verbose mode -- show the compression ratio for each file processed. Further -v's increase the verbosity level, spewing out lots of information which is primarily of interest for diagnostic purposes.
-L --license Display the software version, license terms and conditions.
-V --version
Display the software version, license terms and conditions.
-1 or -9

Set the block size to 100 k, 200 k .. 900 k when compressing. Has no effect when decompressing.

--fast (alias for -1) and --best (alias for -9) are primarily for GNU gzip compatibility. In particular, --fast doesn't make things significantly faster. And --best merely selects the default behavior.

Treats all subsequent arguments as file names, even if they start with a dash. This is so you can handle files with names beginning with a dash, for example: bzip2 -- -myfilename.

--repetitive-fast and --repetitive-best are redundant in versions 0.9.5 and above. They provided some coarse control over the behavior of the sorting algorithm in earlier versions, which was sometimes useful. 0.9.5 and above have an improved algorithm which renders these flags irrelevant.

Use the man command (% man) to see how a command is used on your particular computer.

Exploring the Compression of bunzip2

bzip2 compresses files using the Burrows-Wheeler block sorting text compression algorithm, and Huffman coding. Compression is generally considerably better than that achieved by more conventional LZ77/LZ78-based compressors and approaches the performance of the PPM family of statistical compressors.

The command line options are deliberately very similar to those of GNU gzip, but they are not identical.

bzip2 expects a list of file names to accompany the command line flags. Each file is replaced by a compressed version of itself, with the name "original_name.bz2". Each compressed file has the same modification date, permissions, and, when possible, ownership as the corresponding original so that these properties can be correctly restored at decompression time.

Linux bunzip2

When to Use the -f Flag

File name handling is naive in the sense that there is no mechanism for preserving original file names, permissions, ownerships, or dates in filesystems which lack these concepts or have serious file name length restrictions, such as MS-DOS.

bzip2 and bunzip2 will by default not overwrite existing files. If you want this to happen, specify the -f flag.

If no file names are specified, bzip2 compresses from standard input to standard output. In this case, bzip2 will decline to write compressed output to a terminal since this would be entirely incomprehensible and therefore pointless.

Linux bunzip2 multiple files

bunzip2 (or bzip2 -d) decompresses all specified files. Files which were not created by bzip2 will be detected and ignored, and a warning issued. bzip2 attempts to guess the filename for the decompressed file from that of the compressed file as follows:

  • filename.bz2 becomes filename
  • filename.bz becomes filename
  • filename.tbz2 becomes filename.tar
  • filename.tbz becomes filename.tar
  • anyothername becomes anyothername.out

If the file does not end in one of the recognized file extensionsBZ2, BZ, TBZ2, or TBZbzip2 complains that it cannot guess the name of the original file, and uses the original name with .out appended.

Linux bunzip2 decompress tarball

Like with compression, not supplying a filename causes decompression from standard input to standard output.

bunzip2 will correctly decompress a file which is the concatenation of two or more compressed files. The result is the concatenation of the corresponding uncompressed files. Integrity testing (-t) of concatenated compressed files is also supported.

Using the -c Flag for Standard Output

You can also compress or decompress files to the standard output by giving the -c flag. Multiple files may be compressed and decompressed like this. The resulting outputs are fed sequentially to stdout. Compression of multiple files in this manner generates a stream containing multiple compressed file representations.

Such a stream can be decompressed correctly only by bzip2version 0.9.0 or later. Earlier versions of bzip2 will stop after decompressing the first file in the stream.

bzip2 Decompression

bzcat (or bzip2 -dc) decompresses all specified files to the standard output.

bzip2 will read arguments from the environment variables BZIP2 and BZIP, in that order, and will process them before any arguments read from the command line. This gives a convenient way to supply default arguments.

Compression is always performed, even if the compressed file is slightly larger than the original. Files of less than about 100 bytes tend to get larger because the compression mechanism has a constant overhead in the region of 50 bytes. Random data (including the output of most file compressors) is coded at about 8.05 bits per byte, giving an expansion of around 0.5 percent.

bzip2 and CRCs

As a self-check for your protection, bzip2 uses 32-bit CRCs to make sure that the decompressed version of a file is identical to the original. This guards against corruption of the compressed data and against undetected bugs in bzip2 (hopefully very unlikely).

The chances of data corruption going undetected is microscopic, about one chance in four billion for each file processed. Be aware, however, that the check occurs upon decompression, so it can only tell you that something is wrong. It can't help you recover the original uncompressed data.

You can use bzip2recover to try to recover data from damaged files.

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