GZIP(1) General Commands Manual GZIP(1)
gzip, gunzip, gzcat - compress and expand data (deflate mode)
gzip [-123456789cdfhLlNnOqrtVv] [-b bits] [-o filename] [-S suffix]
gunzip [-cfhLlNnqrtVv] [-o filename] [file ...]
gzcat [-fhqr] [file ...]
The gzip utility reduces the size of the named files using adaptive
Lempel-Ziv coding, in deflate mode. If invoked as gzip -O, the compress
mode of compression is chosen; see compress(1) for more information.
Each file is renamed to the same name plus the extension ".gz". As many
of the modification time, access time, file flags, file mode, user ID,
and group ID as allowed by permissions are retained in the new file. If
compression would not reduce the size of a file, the file is ignored
(unless -f is used).
The gunzip utility restores compressed files to their original form,
renaming the files by removing the extension (or by using the stored name
if the -N flag is specified). It has the ability to restore files
compressed by both gzip and compress(1), recognising the following
extensions: ".Z", "-Z", "_Z", ".gz", "-gz", "_gz", ".tgz", "-tgz",
"_tgz", ".taz", "-taz", and "_taz". Extensions ending in "tgz" and "taz"
are not removed when decompressing, instead they are converted to "tar".
The gzcat command is equivalent in functionality to gunzip -c.
If renaming the files would cause files to be overwritten and the
standard input device is a terminal, the user is prompted (on the
standard error output) for confirmation. If prompting is not possible or
confirmation is not received, the files are not overwritten.
If no files are specified, the standard input is compressed or
uncompressed to the standard output. If either the input or output files
are not regular files, the checks for reduction in size and file
overwriting are not performed, the input file is not removed, and the
attributes of the input file are not retained.
By default, when compressing, the original file name and time stamp are
stored in the compressed file. When uncompressing, this information is
not used. Instead, the uncompressed file inherits the time stamp of the
compressed version and the uncompressed file name is generated from the
name of the compressed file as described above. These defaults may be
overridden by the -N and -n flags, described below.
The options are as follows:
-1...9 Use the deflate scheme, with compression factor of -1 to -9.
Compression factor -1 is the fastest, but provides a poorer level
of compression. Compression factor -9 provides the best level of
compression, but is relatively slow. The default is -6.
Specify the bits code limit (see below).
-c Compressed or uncompressed output is written to the standard
output. No files are modified (force gzcat mode).
-d Decompress the source files instead of compressing them (force
-f Force compression of file, even if it is not actually reduced in
size. Additionally, files are overwritten without prompting for
confirmation. If the input data is not in a format recognized by
gzip and if the option -c is also given, copy the input data
without change to the standard output: let gzcat behave as
-h Print a short help message.
-L A no-op which exists for compatibility only. On GNU gzip, it
displays the program's license.
-l List information for the specified compressed files. The
following information is listed:
compressed size Size of the compressed file.
uncompressed size Size of the file when uncompressed.
compression ratio Ratio of the difference between the compressed
and uncompressed sizes to the uncompressed
uncompressed name Name the file will be saved as when
If the -v option is specified, the following additional
information is printed:
compression method Name of the method used to compress the file.
crc 32-bit CRC (cyclic redundancy code) of the
time stamp Date and time corresponding to the last data
modification time (mtime) of the compressed
file (if the -n option is specified, the time
stamp stored in the compressed file is
-N When uncompressing or listing, use the time stamp and file name
stored in the compressed file, if any, for the uncompressed
-n When compressing, do not store the original file name and time
stamp in the gzip header.
-O Use old compression method (force compress(1) mode).
Set the output file name.
-q Be quiet: suppress all messages.
-r Recursive mode: gzip will descend into specified directories.
Set the suffix for compressed files.
-t Test the integrity of each file leaving any files intact.
-V A no-op which exists for compatibility only. On GNU gzip, it
displays version information.
-v Print the percentage reduction of each file and other
gzip uses a modified Lempel-Ziv algorithm (LZW). Common substrings are
replaced by pointers to previous strings, and are found using a hash
table. Unique substrings are emitted as a string of literal bytes, and
compressed as Huffman trees. When code 512 is reached, the algorithm
switches to 10-bit codes and continues to use more bits until the limit
specified by the -b flag is reached. bits must be between 9 and 16 (the
default is 16).
After the bits limit is reached, gzip periodically checks the compression
ratio. If it is increasing, gzip continues to use the existing code
dictionary. However, if the compression ratio decreases, gzip discards
the table of substrings and rebuilds it from scratch. This allows the
algorithm to adapt to the next "block" of the file.
The -b flag is omitted for gunzip since the bits parameter specified
during compression is encoded within the output, along with a magic
number to ensure that neither decompression of random data nor
recompression of compressed data is attempted.
The amount of compression obtained depends on the size of the input, the
number of bits per code, and the distribution of common substrings.
Typically, text such as source code or English is reduced by 60 - 70%
using gzip. Compression is generally much better than that achieved by
Huffman coding (as used in the historical command pack), or adaptive
Huffman coding (as used in the historical command compact), and takes
less time to compute.
GZIP Options which are passed to gzip, gunzip, and gzcat
The gzip utility exits with one of the following values:
1 An error occurred.
2 At least one of the specified files was not compressed
since -f was not specified and compression would have
resulted in a size increase.
>2 An error occurred.
The gunzip and gzcat utilities exit 0 on success, and >0 if an error
compress(1), gzexe(1), zdiff(1), zforce(1), zmore(1), znew(1),
P. Deutsch and J-L. Gailly, ZLIB Compressed Data Format Specification
version 3.3, RFC 1950, May 1996.
P. Deutsch, DEFLATE Compressed Data Format Specification version 1.3, RFC
1951, May 1996.
P. Deutsch, GZIP file format specification version 4.3, RFC 1952, May
gzip compatibility was added to compress(1) in OpenBSD 3.4. The `g' in
this version of gzip stands for "gratis".
OpenBSD 6.2 October 7, 2014 OpenBSD 6.2
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