path: root/usr.bin/compress/gzip.1

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.\" This code is derived from software contributed to Berkeley by
.\" James A. Woods, derived from original work by Spencer Thomas
.\" and Joseph Orost.
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.\"     @(#)compress.1	8.2 (Berkeley) 4/18/94
.\"
.Dd $Mdocdate: September 22 2011 $
.Dt GZIP 1
.Os
.Sh NAME
.Nm gzip ,
.Nm gunzip ,
.Nm gzcat
.Nd compress and expand data (deflate mode)
.Sh SYNOPSIS
.Nm gzip
.Op Fl 123456789cdfghLlNnOqrtVv
.Op Fl b Ar bits
.Op Fl o Ar filename
.Op Fl S Ar suffix
.Op Ar
.Nm gunzip
.Op Fl cfhlNnqrtVv
.Op Fl o Ar filename
.Op Ar
.Nm gzcat
.Op Fl fghqr
.Op Ar
.Sh DESCRIPTION
The
.Nm
utility
reduces the size of the named files using adaptive Lempel-Ziv coding,
in deflate mode.
If invoked as
.Nm gzip -O ,
the compress mode of compression is chosen;
see
.Xr compress 1
for more information.
Each file is renamed to the same name plus the extension
.Dq .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
.Fl f
is used).
.Pp
The
.Nm gunzip
utility restores compressed files to their original form, renaming the
files by removing the extension (or by using the stored name if the
.Fl N
flag is specified).
It has the ability to restore files compressed by both
.Nm
and
.Xr compress 1 ,
recognising the following extensions:
.Dq .Z ,
.Dq -Z ,
.Dq _Z ,
.Dq .gz ,
.Dq -gz ,
.Dq _gz ,
.Dq .tgz ,
.Dq -tgz ,
.Dq _tgz ,
.Dq .taz ,
.Dq -taz ,
and
.Dq _taz .
Extensions ending in
.Dq tgz
and
.Dq taz
are not removed when decompressing, instead they are converted to
.Dq tar .
.Pp
The
.Nm gzcat
command is equivalent in functionality to
.Nm gunzip
.Fl c .
.Pp
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.
.Pp
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.
.Pp
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
.Fl N
and
.Fl n
flags, described below.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl 1...9
Use the deflate scheme, with compression factor of
.Fl 1
to
.Fl 9 .
Compression factor
.Fl 1
is the fastest, but provides a poorer level of compression.
Compression factor
.Fl 9
provides the best level of compression, but is relatively slow.
The default is
.Fl 6 .
This option implies
.Fl g .
.It Fl b Ar bits
Specify the
.Ar bits
code limit
.Pq see below .
.It Fl c
Compressed or uncompressed output is written to the standard output.
No files are modified (force
.Nm gzcat
mode).
.It Fl d
Decompress the source files instead of compressing them (force
.Nm gunzip
mode).
.It Fl f
Force compression of
.Ar 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
.Nm
and if the option
.Fl c
is also given, copy the input data without change
to the standard output: let
.Nm gzcat
behave as
.Xr cat 1 .
.It Fl g
Use the deflate scheme, which reportedly provides better compression rates
(the default).
.It Fl h
Print a short help message.
.It Fl L
In other compatible programs, this displays program license.
.It Fl l
List information for the specified compressed files.
The following information is listed:
.Bl -tag -width "compression ratio"
.It compressed size
Size of the compressed file.
.It uncompressed size
Size of the file when uncompressed.
.It compression ratio
Ratio of the difference between the compressed and uncompressed
sizes to the uncompressed size.
.It uncompressed name
Name the file will be saved as when uncompressing.
.El
.Pp
If the
.Fl v
option is specified, the following additional information is printed:
.Bl -tag -width "compression method"
.It compression method
Name of the method used to compress the file.
.It crc
32-bit CRC
.Pq cyclic redundancy code
of the uncompressed file.
.It "time stamp"
Date and time corresponding to the last data modification time
(mtime) of the compressed file (if the
.Fl n
option is specified, the time stamp stored in the compressed file
is printed instead).
.El
.It Fl N
When uncompressing or listing, use the time stamp and file name stored
in the compressed file, if any, for the uncompressed version.
.It Fl n
When compressing, do not store the original file name and time stamp
in the
.Nm
header.
.It Fl O
Use old compression method
(force
.Xr compress 1
mode).
.It Fl o Ar filename
Set the output file name.
.It Fl q
Be quiet: suppress all messages.
.It Fl r
Recursive mode:
.Nm
will descend into specified directories.
.It Fl S Ar suffix
Set the suffix for compressed files.
.It Fl t
Test the integrity of each file leaving any files intact.
.It Fl V
In other compatible programs, this displays version information.
.It Fl v
Print the percentage reduction of each file and other information.
.El
.Pp
.Nm
uses a modified Lempel-Ziv algorithm
.Pq 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
.Fl b
flag is reached.
.Ar bits
must be between 9 and 16
.Pq the default is 16 .
.Pp
After the
.Ar bits
limit is reached,
.Nm
periodically checks the compression ratio.
If it is increasing,
.Nm
continues to use the existing code dictionary.
However, if the compression ratio decreases,
.Nm
discards the table of substrings and rebuilds it from scratch.
This allows the algorithm to adapt to the next
.Dq block
of the file.
.Pp
The
.Fl b
flag is omitted for
.Nm gunzip
since the
.Ar 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.
.Pp
The amount of compression obtained depends on the size of the
input, the number of
.Ar bits
per code, and the distribution of common substrings.
Typically, text such as source code or English is reduced by 60 \- 70% using
.Nm .
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.
.Pp
The
.Nm gzip ,
.Nm gunzip ,
and
.Nm gzcat
utilities exit with 0 on success; 1 if an error occurred;
or 2 if a warning occurred.
.Sh ENVIRONMENT
.Bl -tag -width Ds
.It Ev GZIP
Options which are passed to
.Nm ,
.Nm gunzip ,
and
.Nm gzcat
automatically.
.El
.Sh SEE ALSO
.Xr compress 1 ,
.Xr gzexe 1 ,
.Xr gzsig 1 ,
.Xr zdiff 1 ,
.Xr zforce 1 ,
.Xr zmore 1 ,
.Xr znew 1 ,
.Xr compress 3
.Pp
.Bl -tag -width 12n -compact
.It RFC 1950
ZLIB Compressed Data Format Specification.
.It RFC 1951
DEFLATE Compressed Data Format Specification.
.It RFC 1952
GZIP File Format Specification.
.El
.Sh HISTORY
.Nm gzip
compatibility was added to
.Xr compress 1
in
.Ox 3.4 .
The
.Sq g
in this version of
.Nm gzip
stands for
.Dq gratis .