Manual: zcat

GZIP(1) General Commands Manual GZIP(1)

NAME
gzip, gunzip, zcat - compress or expand files

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

DESCRIPTION
The gzip command 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, ac‐
cess and modification times. (The default extension is 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. The gzip command will only attempt to compress regular files. In particular, it
will ignore symbolic links.

If the compressed file name is too long for its file system, gzip truncates it. The gzip command at‐
tempts 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 truncated. For example, if
file names are limited to 14 characters, gzip.msdos.exe is compressed 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 compressed file name was
truncated or when the timestamp 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 con‐
structed from the original one to make it valid.

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 number with an uncom‐
pressed 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 exten‐
sion 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 de‐
tection 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 compress format was not de‐
signed 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 be‐
cause the standard uncompress does not complain. This generally means that the standard uncompress
does not check its input, and happily generates garbage output. The SCO compress -H format (lzh com‐
pression 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 compressed 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 ex‐
tracted 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 inter‐
preted first and can be overridden by explicit command line parameters. As this can cause problems
when using scripts, this feature is supported only for --rsyncable, --synchronous, and options like
-9 that set the compression level; any other options or operands in GZIP are silently ignored.

You can use an alias or script instead. For example, instead of setting GZIP="-9" in the environ‐
ment, if gzip is in the directory /usr/bin you can prepend $HOME/bin to your PATH and create an exe‐
cutable script $HOME/bin/gzip containing the following:

#! /bin/sh
export PATH=/usr/bin
exec gzip -9 "$@"

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 deflation format is
specified in P. Deutsch, DEFLATE Compressed Data Format Specification version 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 decom‐
press code on this machine. Recompress the file with gzip, which compresses better and uses
less memory.

file: already has .gz suffix -- unchanged
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 gunzip for decompression,
gunzip detects that there is extra trailing garbage after the compressed data and emits a warning by
default. You can use the --quiet option to suppress the warning.

BUGS
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.

REPORTING BUGS
Report bugs to: bug-gzip@gnu.org
GNU gzip home page:
General help using GNU software:

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 distributed under the terms
of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, 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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Manual: zcat

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