Gzip (manpage)

NAME
,, - compress or expand files

DESCRIPTION
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, 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 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 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 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 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, gunzip checks the uncompressed length. The standard 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. 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 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 --to-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 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
 * }
 * 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:


 * file.Z | wc -c
 * In combination  with  the  --verbose option, the following 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.
 * 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  are those stored within  the compress file if present.
 * 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 transferred to other systems.  A null suffix forces gunzip to  try decompression on  all  given files regardless of suffix, as in:
 * -S "" *      (*.* for MSDOS)
 * Previous versions  of gzip used the .z suffix. This was changed to avoid a conflict with (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 (best compression).  The default compression level is -6 (that is, biased towards high compression at expense  of speed).
 * Previous versions  of gzip used the .z suffix. This was changed to avoid a conflict with (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 (best compression).  The default compression level is -6 (that is, biased towards high compression at expense  of speed).
 * -# --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 (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, 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

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:

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: 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.

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 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 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: 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 compression than the default compression level (-6). On some highly redundant files, compress compresses better than gzip.