gzip command help

 gzip, gunzip, zcat - compress or expand files
 

SYNOPSIS

 gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
 gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ]
 zcat [ -fhLV ] [ 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 ownュ
 ership 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 compress reguュ
 lar files. In particular, it will ignore symbolic links.
 
 If the compressed file name is too long for its file sysュ
 tem, 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 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 timesュ
 tamp in the compressed file. These are used when decomュ
 pressing the file with the -N option. This is useful when
 the compressed 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,
 _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.
 When compressing, gzip uses the .tgz extension if necesュ
 sary 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 forュ
 mats, gunzip checks a 32 bit CRC. For pack, gunzip checks
 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. The SCO compress -H format (lzh compresュ
 sion 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 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 uncomュ
 press 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. Typュ
 ically, 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 --to-stdout
 Write output on standard output; keep original
 files unchanged. If there are several input files,
 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 the input data is
 not in a format recognized by gzip, and if the
 option --stdout is also given, copy the input data
 without change to the standard ouput: let zcat
 behave as cat. 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 --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 folュ
 lowing fields are also displayed:
 
 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 file not in gzip
 format.
 
 With --name, the uncompressed name, date and time
 
 With --verbose, the size totals and compression
 ratio for all files is also displayed, unless some
 sizes are unknown. With --quiet, the title and
 totals lines are not displayed.
 
 -L --license
 Display the gzip license and quit.
 
 -n --no-name
 When compressing, do not save the original file
 name and time stamp by default. (The original name
 is always saved if the name had to be truncated.)
 When decompressing, do not restore the original
 file name if present (remove only the gzip suffix
 from the compressed file name) and do not restore
 the original time stamp if present (copy it from
 the compressed file). This option is the default
 when decompressing.
 
 -N --name
 When compressing, always save the original file
 name and time stamp; this is the default. When
 decompressing, restore the original file name and
 time stamp if present. This option is useful on
 systems which have a limit on file name length or
 when the time stamp has been lost after a file
 transfer.
 
 -q --quiet
 Suppress all warnings.
 
 -r --recursive
 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 transュ
 ferred to other systems. A null suffix forces gunュ
 zip to try decompression on all given files
 regardless of suffix, as in:
 
 gunzip -S "" * (*.* for MSDOS)
 
 Previous versions of gzip used the .z suffix. This
 was changed to avoid a conflict with pack(1).
 
 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 speciュ
 fied digit #, where -1 or --fast indicates the
 fastest compression method (less compression) and
 -9 or --best indicates the slowest compression
 method (best compression). The default compression
 level is -6 (that is, biased towards high compresュ
 sion at expense of speed).
 

ADVANCED USAGE

 Multiple compressed files can be concatenated. In this
 case, gunzip will extract all members at once. For examュ
 ple:
 
 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 memュ
 bers can still be recovered (if the damaged member is
 removed). However, you can get better compression by comュ
 pressing 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
 
 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 indepenュ
 dently, use an archiver such as tar or zip. GNU tar supュ
 ports 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) , pack(1) , compact(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 [-cdfhlLnNrtvV19] [-S suffix] [file ...]
 Invalid options were specified on the command
 line.
 file: not in gzip format
 The file specified to gunzip has not been comュ
 pressed.
 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
 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 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 --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 --list option reports incorrect sizes if they exceed 2
 gigabytes. 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 compresュ
 sion than the default compression level (-6). On some
 highly redundant files, compress compresses better than
 gzip.