compress and expand data (deflate
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
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
compress(1), and zip(1), recognising the following extensions: “.Z”,
“-Z”, “_Z”, “.gz”,
“-gz”, “_gz”, “.tgz”,
“-tgz”, “_tgz”, “.taz”,
“-taz”, and “_taz”. The
-S option can be used to support other file
extensions. Extensions ending in “tgz” and “taz”
are not removed when decompressing, instead they are converted to
“tar”. Files in zip format are only supported if they contain
a single member either compressed with the deflate scheme or stored
gzcat command is equivalent in
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 flags, described below.
The options are as follows:
- Use the deflate scheme, with compression factor of
-9. Compression factor
-1is the fastest, but provides a poorer level of compression. Compression factor
-9provides the best level of compression, but is relatively slow. The default is
- Specify the bits code limit (see below).
- Compressed or uncompressed output is written to the standard output. No
files are modified (force
- Decompress the source files instead of compressing them (force
- 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
gzipand if the option
-cis also given, copy the input data without change to the standard output: let
gzcatbehave as cat(1).
- Print a short help message.
- Keep input files after compression or decompression.
- A no-op which exists for compatibility only. On GNU gzip, it displays the program's license.
- 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 size.
- uncompressed name
- Name the file will be saved as when uncompressing.
-voption is specified, the following additional information is printed:
- compression method
- Name of the method used to compress the file.
- 32-bit CRC (cyclic redundancy code) of the uncompressed file.
- time stamp
- Date and time corresponding to the last data modification time (mtime)
of the compressed file (if the
-noption is specified, the time stamp stored in the compressed file is printed instead).
- When uncompressing or listing, use the time stamp and file name stored in the compressed file, if any, for the uncompressed version.
- When compressing, do not store the original file name and time stamp in
- Use old compression method (force compress(1) mode).
- Set the output file name.
- Be quiet: suppress all messages.
- Recursive mode:
gzipwill descend into specified directories.
- When compressing, use the specified suffix as the extension when creating output files. When uncompressing, recognize file names with the specified suffix as compressed files.
- Test the integrity of each file leaving any files intact.
- A no-op which exists for compatibility only. On GNU gzip, it displays version information.
- Print the percentage reduction of each file and other information.
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.
-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
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.
- Options which are passed to
gzip utility exits with one of the
- An error occurred.
- At least one of the specified files was not compressed since
-fwas not specified and compression would have resulted in a size increase.
- An error occurred.
gzcat utilities exit 0 on success, and >0 if an
compress(1), gzexe(1), zdiff(1), zforce(1), zmore(1), znew(1), compress(3)
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 1996.
gzip compatibility was added to
compress(1) in OpenBSD 3.4. The
‘g’ in this version of
gzip stands for