gzip

The data compression program

Edition 1.0.3, for Gzip Version 1.0.3

February 1993

Copyright © 1992-1993 Jean-loup Gailly

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation.

1 Overview

Gzip reduces the size of the named files using Lempel-Ziv coding (LZ77). Whenever possible, each file is replaced by one with the extension ".z", while keeping the same ownership modes, access and modification times. If no files are specified, the standard input is compressed to the standard output. If the new file name is too long, gzip truncates it and keeps the original file name in the compressed file. gzip will only attempt to compress regular files. In particular, it will ignore symbolic links.

Compressed files can be restored to their original form using "gzip -d" or gunzip or zcat.

gunzip takes a list of files on its command line and replaces each file whose name ends with ".z" or ".Z" and which begins with the correct magic number with an uncompressed file without the original extension.

gunzip can currently decompress files created by gzip, zip, compress or pack. The detection of the input format is automatic. When using the first two formats, gunzip checks a 32 bit CRC (cyclic redundancy check). For pack, gunzip checks the uncompressed length. The compress format was not designed to allow consistency checks. However gunzip is sometimes able to detect a bad .Z file. If you get an error when uncompressing a .Z file, do not assume that the .Z file is correct simply because the standard uncompress does not complain. This generally means that the standard uncompress does not check its input, and happily generates garbage output.

Files created by zip can be uncompressed by gzip only if they have a single member compressed with the ’deflation’ method. This feature is only intended to help conversion of tar.zip files to the tar.z format. To extract zip files with several members, use unzip instead of gunzip.

zcat is identical to "gunzip -c". zcat uncompresses either a list of files on the command line or its standard input and writes the uncompressed data on standard output. zcat will uncompress files that have the correct magic number whether they have a ".z" suffix or not.

gzip uses the Lempel-Ziv algorithm used in zip and PKZIP. The amount of compression obtained depends on the size of the input and the distribution of common substrings. Typically, text such as source code or English is reduced by 60-70%. Compression is generally much better than that achieved by LZW (as used in compress), Huffman coding (as used in pack), or adaptive Huffman coding (compact).

Compression is always performed, even if the compressed file is slightly larger than the original. The worst case expansion is a few bytes for the gzip file header, plus 5 bytes every 32K block, or an expansion ratio of 0.015% for large files. gzip preserves the mode, ownership and timestamps of files when compressing or decompressing.

2 Sample Output

Here are some realistic examples of running gzip.

This is the output of the command ‘gzip’:

usage: gzip [-cdfhLrv19] [file ...]
For more help, type: gzip -h

This is the output of the command ‘gzip -h’:

gzip 1.0.3 (11 Feb 93)
usage: gzip [-cdfhLrtvV19] [file ...]
 -c --stdout      write on standard output, keep original files unchanged
 -d --decompress  decompress
 -f --force       force overwrite of output file and compress links
 -h --help        give this help
 -L --license     display software license
 -r --recurse     recurse through directories
 -t --test        test compressed file integrity (implies -d)
 -v --verbose     verbose mode
 -V --version     display version number
 -1 --fast        compress faster
 -9 --best        compress better
 file...          files to (de)compress. If none given, use standard input

This is the output of the command ‘gzip -v gzip.c’:

gzip.c:                 69.8% -- replaced with gzip.c.z

3 Invoking gzip

The format for running the gzip program is:

gzip option …

gzip supports the following options:

‘--help’

‘-h’

Print an informative help message describing the options.

‘--stdout’

‘-c’

Write output on standard output; keep original files unchanged. If there are several input files, the output consists of a sequence of independently compressed members. To obtain better compression, concatenate all input files before compressing them.

‘--decompress’

‘-d’

Decompress.

‘--force’

‘-f’

Force compression even if the file has multiple links or the corresponding .z file already exists. If -f is not given, and when not running in the background, gzip prompts to verify whether an existing .z file should be overwritten.

‘--help’

‘-h’

Display a help screen.

‘--license’

‘-L’

Display the gzip license.

‘--recurse’

‘-r’

Travel the directory structure recursively. If any of the file names specified on the command line are directories, gzip will descend into the directory and compress all the files it finds there (or decompress them in the case of gunzip).

‘--test’

‘-t’

Test. Check the compressed file integrity.

‘--verbose’

‘-v’

Verbose. Display the name and percentage reduction for each file compressed.

‘--version’

‘-V’

Version. Display the version number and compilation options.

‘--fast’

‘--best’

‘-#’

Regulate the speed of compression using the specified digit #, where -1 or –fast indicates the fastest compression method (less compression) and –best or -9 indicates the slowest compression method (optimal compression). The default compression level is -5.

4 Advanced usage

Multiple compressed files can be concatenated. In this case, gunzip will extract all members at once. If one member is damaged, other members might still be recovered after removal of the damaged member. Better compression can be usually obtained if all members are decompressed then recompressed in a single step.

This is an example of concatenating gzip files:

gzip -c file1  > foo.z
gzip -c file2 >> foo.z

Then

gunzip -c foo

is equivalent to

cat file1 file2

In case of damage to one member of a .z file, other members can still be recovered (if the damaged member is removed). However, you can get better compression by compressing all members at once:

cat file1 file2 | gzip > foo.z

compresses better than

gzip -c file1 file2 > foo.z

If you want to recompress concatenated files to get better compression, do:

zcat old.z | gzip > new.z

5 Reporting Bugs

If you find a bug in gzip, please send electronic mail to ‘jloup@chorus.fr’ or, if this fails, to ‘bug-gnu-utils@prep.ai.mit.edu’. Include the version number, which you can find by running ‘gzip -V’. Also include in your message the hardware and operating system, the compiler used to compile, a description of the bug behavior, and the input to gzip that triggered the bug.

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