MAN page from RedHat Other gzip-1.2.4-11.i386.rpm

GZIP

Section: User Commands (1)
Updated: local
Index

NAME

gzip, gunzip, zcat - compress or expand files

SYNOPSIS

gzip[ -acdfhlLnNrtvV19 ][-S suffix][name ...]
gunzip[ -acfhlLnNrtvV ][-S suffix][name ...]
zcat[ -fhLV ][name ...]

DESCRIPTION

Gzipreduces 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-gzfor VMS,zfor MSDOS, OS/2 FAT, Windows NT FAT and Atari.)If no files are specified, or if a file name is "-", the standard input iscompressed to the standard output.Gzipwill 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,gziptruncates it.Gzipattempts 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 limitedto 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 namelength.

By default,gzipkeeps the original file name and timestamp in the compressed file. Theseare used when decompressing the file with the-Noption. This is useful when the compressed file name was truncated orwhen the time stamp was not preserved after a file transfer.

Compressed files can be restored to their original form usinggzip -dorgunziporzcat.If the original name saved in the compressed file is not suitable for itsfile system, a new name is constructed from the original one to make itlegal.

gunziptakes a list of files on its command line and replaces eachfile whose name ends with .gz, -gz, .z, -z, _z or .Zand which begins with the correct magic number with an uncompressedfile without the original extension.gunzipalso recognizes the special extensions.tgzand.tazas shorthands for.tar.gzand.tar.Zrespectively.When compressing,gzipuses the.tgzextension if necessary instead of truncating a file with a.tarextension.

gunzipcan currently decompress files created bygzip, zip, compress, compress -Horpack.The detection of the input format is automatic. When usingthe first two formats,gunzipchecks a 32 bit CRC. Forpack, gunzipchecks the uncompressed length. The standardcompressformat was not designed to allow consistency checks. Howevergunzipis sometimes able to detect a bad .Z file. If you get an errorwhen uncompressing a .Z file, do not assume that the .Z file iscorrect simply because the standarduncompressdoes not complain. This generally means that the standarduncompressdoes not check its input, and happily generates garbage output.The SCO compress -H format (lzh compression method) does not include a CRCbut also allows some consistency checks.

Files created byzipcan be uncompressed by gzip only if they have a single member compressedwith the 'deflation' method. This feature is only intended to helpconversion of tar.zip files to the tar.gz format. To extract zip fileswith several members, useunzipinstead ofgunzip.

zcatis identical togunzip-c.(On some systems,zcatmay be installed asgzcatto preserve the original link tocompress.)zcatuncompresses either a list of files on the command line or itsstandard input and writes the uncompressed data on standard output.zcatwill uncompress files that have the correct magic number whetherthey have a.gzsuffix or not.

Gzipuses the Lempel-Ziv algorithm used inzipand PKZIP.The amount of compression obtained depends on the size of theinput and the distribution of common substrings.Typically, text such as source code or Englishis reduced by 60-70%.Compression is generally much better than that achieved byLZW (as used in compress),Huffman coding (as used inpack),or adaptive Huffman coding(compact).

Compression is always performed, even if the compressed file isslightly larger than the original. The worst case expansion isa 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 actualnumber of used disk blocks almost never increases.gzippreserves the mode, ownership and timestamps of files when compressingor decompressing.

OPTIONS

-a --ascii

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

-c --stdout --to-stdout

Write output on standard output; keep original files unchanged.If there are several input files, the output consists of a sequence ofindependently 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 linksor the corresponding file already exists, or if the compressed datais read from or written to a terminal. If the input data is not ina format recognized bygzip,and if the option --stdout is also given, copy the input data without changeto the standard ouput: letzcatbehave ascat.If-fis not given,and when not running in the background,gzipprompts to verify whether an existing file should be overwritten.

-h --help

Display a help screen and quit.

-l --list

For each compressed file, list the following fields:

    compressed size: size of the compressed file
    uncompressed size: size of the uncompressed file
    ratio: compression ratio (0.0% if unknown)
    uncompressed_name: name of the uncompressed file

The uncompressed size is given as -1 for files not in gzip format,such as compressed .Z files. To get the uncompressed size for such a file,you can use:

zcat file.Z | wc -c

In combination with the --verbose option, the following fields are alsodisplayed:

    method: compression method
    crc: the 32-bit CRC of the uncompressed data
    date & time: time stamp for the uncompressed file

The compression methods currently supported are deflate, compress, lzh(SCO compress -H) and pack. The crc is given as ffffffff for a filenot in gzip format.

With --name, the uncompressed name, date and time arethose stored within the compress file if present.

With --verbose, the size totals and compression ratio for all filesis also displayed, unless some sizes are unknown. With --quiet,the title and totals lines are not displayed.

-L --license

Display thegziplicense and quit.

-n --no-name

When compressing, do not save the original file name and time stamp bydefault. (The original name is always saved if the name had to betruncated.) When decompressing, do not restore the original file nameif present (remove only thegzipsuffix from the compressed file name) and do not restore the originaltime stamp if present (copy it from the compressed file). This optionis the default when decompressing.

-N --name

When compressing, always save the original file name and time stamp; thisis the default. When decompressing, restore the original file name andtime stamp if present. This option is useful on systems which havea limit on file name length or when the time stamp has been lost aftera file transfer.

-q --quiet

Suppress all warnings.

-r --recursive

Travel the directory structure recursively. If any of the file namesspecified on the command line are directories, gzipwill descend into the directory and compress all the files it finds there(or decompress them in the case ofgunzip).

-S .suf --suffix .suf

Use suffix .suf instead of .gz. Any suffix can be given, but suffixesother than .z and .gz should be avoided to avoid confusion when filesare transferred to other systems. A null suffix forces gunzip to trydecompression on all given files regardless of suffix, as in:

gunzip -S "" * (*.* for MSDOS)

Previous versions of gzip usedthe .z suffix. This was changed to avoid a conflict withpack(1).

-t --test

Test. Check the compressed file integrity.

-v --verbose

Verbose. Display the name and percentage reduction for each file compressedor 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-1or--fastindicates the fastest compression method (less compression)and-9or--bestindicates the slowest compression method (best compression).The default compression level is-6(that is, biased towards high compression at expense of speed).

ADVANCED USAGE

Multiple compressed files can be concatenated. In this case,gunzipwill 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 canstill 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 uncompressedsize and CRC reported by the --list option applies to the last memberonly. 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 sothat members can later be extracted independently, use an archiversuch as tar or zip. GNU tar supports the -z option to invoke gziptransparently. gzip is designed as a complement to tar, not as areplacement.

ENVIRONMENT

The environment variableGZIPcan hold a set of default options forgzip.These options are interpreted first and can be overwritten byexplicit 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, toavoid a conflict with the symbol set for invocation of the program.

DIAGNOSTICS

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

Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]Invalid options were specified on the command line.file:not in gzip formatThe file specified togunziphas 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 failurecan be recovered usingzcat file > recoverfile:compressed with xxbits, can only handle yybitsFilewas compressed (using LZW) by a program that could deal withmore bitsthan the decompress code on this machine.Recompress the file with gzip, which compresses better and usesless memory.file:already has .gz suffix -- no changeThe file is assumed to be already compressed.Rename the file and try again.filealready 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 inputA SIGSEGV violation was detected which usually means that the input file hasbeen corrupted.xx.x%Percentage of the input saved by compression.(Relevant only for-vand-l.)-- not a regular file or directory: ignoredWhen the input file is not a regular file or directory,(e.g. a symbolic link, socket, FIFO, device file), it isleft unaltered.-- has xx other links: unchangedThe input file has links; it is left unchanged. Seeln(1)for more information. Use the-fflag to force compression of multiply-linked files.

CAVEATS

When writing compressed data to a tape, it is generally necessary topad the output with zeroes up to a block boundary. When the data isread and the whole block is passed togunzipfor decompression,gunzipdetects that there is extra trailing garbage after the compressed dataand emits a warning by default. You have to use the --quiet option tosuppress the warning. This option can be set in theGZIPenvironment 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 ofGNU tar. Make sure that the same block size (-b option of tar) is usedfor reading and writing compressed data on tapes. (This exampleassumes you are using the GNU version of tar.)

BUGS

The --list option reports incorrect sizes if they exceed 2 gigabytes.The --list option reports sizes as -1 and crc as ffffffff if thecompressed file is on a non seekable media.

In some rare cases, the --best option gives worse compression thanthe default compression level (-6). On some highly redundant files,compresscompresses better thangzip.

This document was created byman2html,using the manual pages.