GNU Gzip: General file (de)compression
Table of Contents
This manual is for GNU Gzip (version 1.10, 7 August 2018), and documents commands for compressing and decompressing data.
Copyright © 1998-1999, 2001-2002, 2006-2007, 2009-2018 Free Software Foundation, Inc.
Copyright © 1992, 1993 Jean-loup Gailly
gzip reduces the size of the named files using Lempel–Ziv coding (LZ77). Whenever possible, each file is replaced by one with the extension ‘.gz’, while keeping the same ownership modes, access and modification times. (The default extension is ‘z’ for MSDOS, OS/2 FAT and Atari.) If no files are specified or if a file name is —, the standard input is compressed to the standard output.
gzip will only attempt to compress regular files. In particular, it will ignore symbolic links.
If the new file name is too long for its file system,
gzip truncates it.
gzip attempts to truncate only the parts of the file name longer than 3 characters. (A part is delimited by dots.) If the name consists of small parts only, the longest parts are truncated. For example, if file names are limited to 14 characters, gzip.msdos.exe is compressed to gzi.msd.exe.gz. Names are not truncated on systems which do not have a limit on file name length.
gzip keeps the original file name in the compressed file. This can be useful when decompressing the file with -N if the compressed file name was truncated after a file transfer.
If the original is a regular file,
gzip by default keeps its timestamp in the compressed file. This can be useful when decompressing the file with -N if the timestamp was not preserved after a file transfer. However, due to limitations in the current
gzip file format, fractional seconds are discarded. Also, timestamps must fall within the range 1970-01-01 00:00:01 through 2106-02-07 06:28:15 UTC, and hosts whose operating systems use 32-bit timestamps are further restricted to timestamps no later than 2038-01-19 03:14:07 UTC. The upper bounds assume the typical case where leap seconds are ignored.
Compressed files can be restored to their original form using ‘gzip -d’ or
zcat. If the original name saved in the compressed file is not suitable for its file system, a new name is constructed from the original one to make it legal.
gunzip takes a list of files on its command line and replaces each file whose name ends with ‘.gz’, ‘.z’ ‘-gz’, ‘-z’, or ‘_z’ (ignoring case) and which begins with the correct magic number with an uncompressed file without the original extension.
gunzip also recognizes the special extensions ‘.tgz’ and ‘.taz’ as shorthands for ‘.tar.gz’ and ‘.tar.Z’ respectively. When compressing,
gzip uses the ‘.tgz’ extension if necessary instead of truncating a file with a ‘.tar’ extension.
gunzip can currently decompress files created by
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
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. The SCO ‘compress -H’ format (LZH compression method) does not include a CRC but also allows some consistency checks.
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.gz format. To extract a
zip file with a single member, use a command like ‘gunzip <foo.zip’ or ‘gunzip -S .zip foo.zip’. To extract
zip files with several members, use
unzip instead of
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 ‘.gz’ 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 (
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. Note that the actual number of used disk blocks almost never increases.
gzip normally preserves the mode, ownership and timestamps of files when compressing or decompressing.
gzip file format is specified in P. Deutsch, GZIP file format specification version 4.3, Internet RFC 1952 (May 1996). The
zip deflation format is specified in P. Deutsch, DEFLATE Compressed Data Format Specification version 1.3, Internet RFC 1951 (May 1996).
2 Sample output
Here are some realistic examples of running
This is the output of the command ‘gzip -h’:
Usage: gzip [OPTION]... [FILE]... Compress or uncompress FILEs (by default, compress FILES in-place). Mandatory arguments to long options are mandatory for short options too. -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 -k, --keep keep (don't delete) input files -l, --list list compressed file contents -L, --license display software license -n, --no-name do not save or restore the original name and timestamp -N, --name save or restore the original name and timestamp -q, --quiet suppress all warnings -r, --recursive operate recursively on directories --rsyncable make rsync-friendly archive -S, --suffix=SUF use suffix SUF on compressed files --synchronous synchronous output (safer if system crashes, but slower) -t, --test test compressed file integrity -v, --verbose verbose mode -V, --version display version number -1, --fast compress faster -9, --best compress better With no FILE, or when FILE is -, read standard input. Report bugs to <email@example.com>.
This is the output of the command ‘gzip -v texinfo.tex’:
texinfo.tex: 69.3% -- replaced with texinfo.tex.gz
The following command will find all regular ‘.gz’ files in the current directory and subdirectories (skipping file names that contain newlines), and extract them in place without destroying the original, stopping on the first failure:
find . -name '* *' -prune -o -name '*.gz' -type f -print | sed " s/'/'\\\\''/g s/^\\(.*\\)\\.gz$/gunzip <'\\1.gz' >'\\1'/ " | sh -e
The format for running the
gzip program is:
gzip option …
gzip supports the following options:
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.
Force compression or decompression even if the file has multiple links or the corresponding file already exists, or if the compressed data is read from or written to a terminal. If the input data is not in a format recognized by
gzip, and if the option —stdout is also given, copy the input data without change to the standard output: let
cat. If -f is not given, and when not running in the background,
gzipprompts to verify whether an existing file should be overwritten.
Print an informative help message describing the options then quit.
Keep (don’t delete) input files during compression or decompression.
For each compressed file, list the following fields:
compressed size: size of the compressed file uncompressed size: size of the uncompressed file ratio: compression ratio (0.0% if unknown) uncompressed_name: name of the uncompressed file
The uncompressed size is given as -1 for files not in
gzipformat, such as compressed ‘.Z’ files. To get the uncompressed size for such a file, you can use:
zcat file.Z | wc -c
In combination with the —verbose option, the following fields are also displayed:
method: compression method (deflate,compress,lzh,pack) crc: the 32-bit CRC of the uncompressed data date & time: timestamp for the uncompressed file
The CRC is given as ffffffff for a file not in gzip format.
With —verbose, the size totals and compression ratio for all files is also displayed, unless some sizes are unknown. With —quiet, the title and totals lines are not displayed.
gzipformat represents the input size modulo 2^32, so the uncompressed size and compression ratio are listed incorrectly for uncompressed files 4 GiB and larger. To work around this problem, you can use the following command to discover a large uncompressed file’s true size:
zcat file.gz | wc -c
gziplicense then quit.
When compressing, do not save the original file name and timestamp by default. (The original name is always saved if the name had to be truncated.) When decompressing, do not restore the original file name if present (remove only the
gzipsuffix from the compressed file name) and do not restore the original timestamp if present (copy it from the compressed file). This option is the default when decompressing.
When compressing, always save the original file name, and save the original timestamp if the original is a regular file; this is the default. When decompressing, restore the original file name and timestamp if present. This option is useful on systems which have a limit on file name length or when the timestamp has been lost after a file transfer.
Suppress all warning messages.
Travel the directory structure recursively. If any of the file names specified on the command line are directories,
gzipwill descend into the directory and compress all the files it finds there (or decompress them in the case of
Cater better to the
rsyncprogram by periodically resetting the internal structure of the compressed data stream. This lets the
rsyncprogram take advantage of similarities in the uncompressed input when synchronizing two files compressed with this flag. The cost: the compressed output is usually about one percent larger.
- —suffix suf
- -S suf
Use suffix suf instead of ‘.gz’. Any suffix can be given, but suffixes other than ‘.z’ and ‘.gz’ should be avoided to avoid confusion when files are transferred to other systems. A null suffix forces gunzip to try decompression on all given files regardless of suffix, as in:
gunzip -S "" * (*.* for MSDOS)
Previous versions of gzip used the ‘.z’ suffix. This was changed to avoid a conflict with
Use synchronous output, by transferring output data to the output file’s storage device when the file system supports this. Because file system data can be cached, without this option if the system crashes around the time a command like ‘gzip FOO’ is run the user might lose both FOO and FOO.gz; this is the default with
gzip, just as it is the default with most applications that move data. When this option is used,
gzipis safer but can be considerably slower.
Test. Check the compressed file integrity.
Verbose. Display the name and percentage reduction for each file compressed.
Version. Display the version number and compilation options, then quit.
Regulate the speed of compression using the specified digit n, 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 -6 (that is, biased towards high compression at expense of speed).
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 and then recompressed in a single step.
This is an example of concatenating
gzip -c file1 > foo.gz gzip -c file2 >> foo.gz
gunzip -c foo
is equivalent to
cat file1 file2
In case of damage to one member of a ‘.gz’ 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.gz
compresses better than
gzip -c file1 file2 > foo.gz
If you want to recompress concatenated files to get better compression, do:
zcat old.gz | gzip > new.gz
If a compressed file consists of several members, the uncompressed size and CRC reported by the —list option applies to the last member only. If you need the uncompressed size for all members, you can use:
zcat file.gz | wc -c
If you wish to create a single archive file with multiple members so that members can later be extracted independently, use an archiver such as
tar supports the -z option to invoke
gzip is designed as a complement to
tar, not as a replacement.
The obsolescent environment variable
GZIP can hold a set of default options for
gzip. These options are interpreted first and can be overwritten by explicit command line parameters. As this can cause problems when using scripts, this feature is supported only for options that are reasonably likely to not cause too much harm, and
gzip warns if it is used. This feature will be removed in a future release of
You can use an alias or script instead. For example, if
gzip is in the directory ‘/usr/bin’ you can prepend $HOME/bin to your
PATH and create an executable script $HOME/bin/gzip containing the following:
#! /bin/sh export PATH=/usr/bin exec gzip -9 "$@"
gzip on tapes
When writing compressed data to a tape, it is generally necessary to pad the output with zeroes up to a block boundary. When the data is read and the whole block is passed to
gunzip for decompression,
gunzip detects that there is extra trailing garbage after the compressed data and emits a warning by default if the garbage contains nonzero bytes. You can use the —quiet option to suppress the warning.
7 Reporting Bugs
If you find a bug in
gzip, please send electronic mail to firstname.lastname@example.org. 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
gzip, a description of the bug behavior, and the input to
gzip that triggered the bug.