NAME
gzip, gunzip, zcat - compress or expand files

SYNOPSIS
gzip [ -acdfhLrtvV19 ] [-S suffix] [ name ... ]
gunzip [ -acfhLrtvV ] [-S suffix] [ name ... ]
zcat [ -hLV ] [ name ... ]

DESCRIPTION
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 -gz for VMS, z for
MSDOS, OS/2 FAT and Atari.) 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 .gz, -gz, .z, -z, _z or .Z 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.

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. 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.gz format. To extract zip files with several members, use unzip
instead of gunzip.

zcat is identical to gunzip -c. (On some systems, zcat may be
installed as gzcat to preserve the original link to compress.) 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 (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. Note that the
actual number of used disk blocks almost never increases. gzip
preserves the mode, ownership and timestamps of files when
compressing or decompressing.

OPTIONS
-a --ascii
Ascii text mode: convert end-of-lines using local conventions.
This option is supported only on some non-Unix systems. For
MSDOS, CR LF is converted to LF when compressing, and LF is
converted to CR LF when decompressing.

-c --stdout
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.

-d --decompress --uncompress
Decompress.

-f --force
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
-f is not given, and when not running in the background, gzip
prompts to verify whether an existing file should be
overwritten.

-h --help
Display a help screen and quit.

-L --license
Display the gzip license.

-q --quiet
Suppress all warnings.

-r --recurse
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 ).

-S .suf --suffix .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.
Previous versions of gzip used the .z suffix. This was changed
to avoid a conflict with pack(1).

-t --test
Test. Check the compressed file integrity.

-v --verbose
Verbose. Display the name and percentage reduction for each
file compressed or decompressed.

-V --version
Version. Display the version number and compilation options
then quit.

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

ADVANCED USAGE
Multiple compressed files can be concatenated. In this case, gunzip
will extract all members at once. For example:

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

Then
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

ENVIRONMENT
The 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. For example:
for sh: GZIP="-8 -v"; export GZIP
for csh: setenv GZIP "-8 -v"
for MSDOS: set GZIP=-8 -v

On Vax/VMS, the name of the environment variable is GZIP_OPT, to
avoid a conflict with the symbol set for invocation of the program.

SEE ALSO
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), compress(1)

DIAGNOSTICS
Exit status is normally 0; if an error occurs, exit status is 1. If
a warning occurs, exit status is 2.

Usage: gzip [-cdfhLrtvV19] [-S suffix] [file ...]
Invalid options were specified on the command line.
file: not in gzip format
The file specified to gunzip has not been compressed.
file: Corrupt input. Use zcat to recover some data.
The compressed file has been damaged. The data up to the
point of failure can be recovered using
zcat file > recover
file: compressed with xx bits, can only handle yy bits
File was compressed (using LZW) by a program that could
deal with more bits than the decompress code on this
machine. Recompress the file with gzip, which compresses
better and uses less memory.
file: already has .gz suffix -- no change
The file is assumed to be already compressed. Rename the
file and try again.
file already exists; do you wish to overwrite (y or n)?
Respond "y" if you want the output file to be replaced; "n"
if not.
gunzip: corrupt input
A SIGSEGV violation was detected which usually means that
the input file has been corrupted.
xx.x%
Percentage of the input saved by compression. (Relevant
only for -v.)
-- not a regular file or directory: ignored
When the input file is not a regular file or directory,
(e.g. a symbolic link, socket, FIFO, device file), it is
left unaltered.
-- has xx other links: unchanged
The input file has links; it is left unchanged. See ln(1)
for more information. Use the -f flag to force compression
of multiply-linked files.

CAVEATS
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. You have to use the
--quiet option to suppress the warning. This option can be set in
the GZIP environment variable as in:
for sh: GZIP="-q" tar xfz /dev/rmt/datn
for csh: (setenv GZIP "-q"; tar xfz /dev/rmt/datn)

In the above example, gzip is invoked implicitly by the -z option
of GNU tar. Make sure that the same block size (-b option of tar)
is used for reading and writing compressed data on tapes.