gzip

Compress or decompress files

Syntax
       gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ...  ]

       gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ...  ]

       zcat [ -fhLV ] [ name ...  ]

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

The gzip format represents the input size modulo 2^32, so the --list option reports incorrect uncompressed sizes and compression ratios for uncompressed files 4 GB and larger. To work around this problem, you can use the wc command to discover a large uncompressed file's true size:

                  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 trans-
              ferred to other systems.  A null suffix forces gun-
              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).

       -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 speci-
              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-
              sion at expense of speed).

       In some rare cases, the --best option gives worse compres-
       sion  than  the  default  compression  level (-6). On some
       highly redundant files, compress  compresses  better  than
       gzip.

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 time stamp 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.

To gzip an application bundle put it in a tarball first, otherwise you'll get the error: "foo.app/ is a directory; ignored"

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.

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 indepen-
dently, 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/bash: GZIP="-8v --name"; export GZIP
             for csh:     setenv GZIP "-8v --name"

"Small is beautiful" - Schumacher's Dictum

Related:


znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1), pack(1), compact(1)


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