gunzip

GZIP(1) General Commands Manual GZIP(1)

NAME
gzip, gunzip, zcat - compress or expand files

SYNOPSIS
gzip [ -acdfhklLnNrtvV19 ] [--rsyncable] [-S suffix] [ name ... ]
gunzip [ -acfhklLnNrtvV ] [-S suffix] [ name ... ]
zcat [ -fhLV ] [ name ... ]

DESCRIPTION
Gzip reduces the size of the named files using Lempel-Ziv coding (LZ77). Whenever possi‐
ble, 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, Windows NT 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 com‐
press regular files. In particular, it will ignore symbolic links.

If the compressed 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 trun‐
cated. For example, if file names are limited to 14 characters, gzip.msdos.exe is com‐
pressed to gzi.msd.exe.gz. Names are not truncated on systems which do not have a limit
on file name length.

By default, gzip keeps the original file name and timestamp in the compressed file. These
are used when decompressing the file with the -N option. This is useful when the com‐
pressed file name was truncated or when the time stamp was not preserved after a file
transfer.

Compressed files can be restored to their original form using gzip -d or gunzip 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, -gz, .z, -z, or _z (ignoring case) and which begins with the correct magic num‐
ber 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 gzip, zip, compress, compress -H or pack.
The detection of the input format is automatic. When using the first two formats, gunzip
checks a 32 bit CRC. For pack and gunzip checks the uncompressed length. The standard com‐
press 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 com‐
plain. 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 com‐
pressed 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.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:

gzip -cd 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:

gzip -cd 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 or zip. GNU tar supports the
-z option to invoke gzip transparently. gzip is designed as a complement to tar, not as a
replacement.

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="-8v --name"; export GZIP
for csh: setenv GZIP "-8v --name"
for MSDOS: set GZIP=-8v --name

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), zip(1), unzip(1), compress(1)

The gzip file format is specified in P. Deutsch, GZIP file format specification version
4.3, , Internet RFC 1952 (May 1996). The zip defla‐
tion format is specified in P. Deutsch, DEFLATE Compressed Data Format Specification ver‐
sion 1.3, , Internet RFC 1951 (May 1996).

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

Usage: gzip [-cdfhklLnNrtvV19] [-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 com‐
presses 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 and -l.)

-- 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 gun‐
zip for decompression, gunzip detects that there is extra trailing garbage after the com‐
pressed data and emits a warning by default. You have to use the --quiet option to sup‐
press the warning. This option can be set in the GZIP environment variable as in:
for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0

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. (This example assumes you are using the GNU version of tar.)

BUGS
The gzip format represents the input size modulo 2^32, so the --list option reports incor‐
rect uncompressed sizes and compression ratios for uncompressed files 4 GB and larger. To
work around this problem, you can use the following command to discover a large uncom‐
pressed file's true size:

zcat file.gz | wc -c

The --list option reports sizes as -1 and crc as ffffffff if the compressed file is on a
non seekable media.

In some rare cases, the --best option gives worse compression than the default compression
level (-6). On some highly redundant files, compress compresses better than gzip.

Permission is granted to make and distribute verbatim copies of this manual provided the
copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the
conditions for verbatim copying, provided that the entire resulting derived work is dis‐
tributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another lan‐
guage, under the above conditions for modified versions, except that this permission
notice may be stated in a translation approved by the Foundation.

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