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GZIP(1)                 FreeBSD General Commands Manual                GZIP(1)

NAME
       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 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 output: 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 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.

              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:

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

              Previous versions of gzip used the .z suffix. This was changed
              to avoid a conflict with pack(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).

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

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

                                                                       GZIP(1)

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ADVANCED USAGE | ENVIRONMENT | SEE ALSO | DIAGNOSTICS | CAVEATS | BUGS

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