
GZIP(1)                  USER COMMANDS                    GZIP(1)

NAME
     gzip, gunzip, zcat - compress or expand files

SYNOPSIS
     gzip [ -acdfhLrtvV19 ] [-S suffix] [ name ... ]
     gunzip [ -acfhLrtvV ] [-S suffix] [ name ... ]
     zcat [ -hLV ] [ 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 exten-
     sion is -gz for VMS, z for MSDOS, OS/2  and  Atari.)  If  no
     files are specified, the standard input is compressed to the
     standard output. If the new file  name  is  too  long,  gzip
     truncates  it  and  keeps  the  original  file  name  in the
     compressed file.  Gzip will only attempt to compress regular
     files.  In particular, it will ignore symbolic links.

     Compressed files can be  restored  to  their  original  form
     using gzip -d or gunzip or zcat.

     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.

     gunzip can currently decompress files created by gzip,  zip,
     compress  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 compress format was not designed to  allow  con-
     sistency  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 gen-
     erally means that the standard uncompress does not check its
     input, and happily generates garbage output.

     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.   Typi-
     cally,  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 expan-
     sion 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. Notng'dr .cesctual  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  con-
          ventions.  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
          Write output on standard output;  keep  original  files
          unchanged.   If there are several input files, the out-
          put 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 writ-
          ten  to  a  terminal.  If -f is not given, and when not
          running in  r .cbackground,  gzip  prompts  to  verify
          whether an existing file should be overwritten.

     -h --help
          Display a help screen and quit.
       
     -l --list        list .gz file contents

     -L --license
          Display the gzip license.

     -q --quiet
          Suppress all warnings.

     -r --recurse
          Travel the directory structure recursively. If  any  of
          the file names specified on the command line are direc-
          tories,  gzip  will  descend  into  the  directory  and
          compress  all  the  files it finds there (or decompress
          them in the case of gunzip ).

     -S .z --suffix .z
          Use suffix .z 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.  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.

     -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  (optimal
          compression).  The default compression level is -5.

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:

           zcat old.gz | gzip > new.gz

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="-8 -v"; export GZIP
           for csh:   setenv GZIP "-8 -v"
           for MSDOS: set GZIP=-8 -v

     On  Vax/VMS,  the  name  of  r .cenvironment  variable   is
     GZIP_OPT,  to avoid a conflict with the symbol set for invo-
     cation of the program.

The options system variable is <Gzip$Otions> ,referred as GZIP or GZIP_OPT
under DOS/OS-2 and in the docs.

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 [-cdfhLrtvV19] [-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  r .coutput  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  r .cinput  saved  by   compression.
             (Relevant only for -v.)
     -- not a regular file or directory: ignored
             When the input file is not a regular file or  direc-
             tory,  (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 boun-
     dary. 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 warn-
     ing  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 /dev/rmt/datn
         for csh:   (setenv GZIP "-q"; tar xfz /dev/rmt/datn)

     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.
*gunzip -c ram:$.readme
This is the file README for the gzip distribution, version 1.1.1.

gzip (GNU zip) is a compression utility designed to be a replacement
for 'compress'. Its main advantages over compress are much better
compression and freedom from patented algorithms.  The GNU Project
uses it as the standard compression program for its system.

gzip currently uses by default the LZ77 algorithm used in zip 1.9 (the
portable pkzip compatible archiver). The gzip format was however
designed to accommodate several compression algorithms. See below
for a comparison of zip and gzip.

gunzip can currently decompress files created by gzip, compress or
pack. The detection of the input format is automatic.  For the
gzip format, gunzip checks a 32 bit CRC. For pack, gunzip checks the
uncompressed length.  The 'compress' format was not designed to allow
consistency checks. However gunzip is sometimes able to detect a bad
.Z file because there is some redundancy in the .Z compression format.
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.

gzip produces files with a .gz extension. Previous versions of gzip
used the .z extension, which was already used by the 'pack'
Huffman encoder. gunzip is able to decompress .z files (packed
or gzip'ed).

Several planned features are not yet supported (see the file TODO).
See the file NEWS for a summary of changes since 0.5.  See the file
INSTALL for installation instructions. Some answers to frequently
asked questions are given in the file INSTALL, please read it. (In
particular, please don't ask me once more for an /etc/magic entry.)

WARNINGS about broken optimizers:

- on the NeXT, "cc -finline-functions" is broken.  gzip produces
  valid .z files but they are much too large because the string
  matching code misses most matches. Use "cc -O" instead.

- on the Mips R4000, gcc -O (version 2.3.1) generates bad code, use cc
  or just gcc -g instead.

- gcc 2.3.3 on the SGI Indigo IRIX 4.0.5 also produces bad code. Use
  instead: make CC='cc -O2' or gcc without -O.

- on Sparc with SunOS 4.1.1 and the SC1.0 compiler, the optimizer
  works up to -O3 but -O4 does not work.

- MSC 5.1 with -Ox and -DDYN_ALLOC generates bad code in inflate.c.
  The default is static allocation (no DYN_ALLOC) and -Ox works on inflate.c.
  But -Ox does not work on util.c, so you must use -Oait -Gs.

- On dnix 5.3 2.2 cc version 2.37c is buggy. Version 2.38d works.

For all machines, Use "make check" to check that gzip was compiled correctly.

Please send all comments and bug reports by electronic mail to:
   Jean-loup Gailly <jloup@chorus.fr>

or, if this fails, to bug-gnu-utils@prep.ai.mit.edu.
Bug reports should ideally include:

    * The complete output of "gzip -V" (or the contents of revision.h
      if you can't get gzip to compile)
    * The hardware and operating system (try "uname -a")
    * The compiler used to compile (if it is gcc, use "gcc -v")
    * A description of the bug behavior
    * The input to gzip, that triggered the bug

The package crypt++.el is highly recommended to manipulate gzip'ed
file from emacs. It recognizes automatically encrypted and compressed
files when they are first visited or written. It is available via
anonymous ftp to roebling.poly.edu [128.238.5.31] in /pub/crypt++.el.
The same directory contains also patches to dired, ange-ftp and info.
GNU tar 1.11.2 has a -z option to invoke directly gzip, so you don't have
to patch it.

The znew and gzexe shell scripts provided with gzip benefit from
(but do not require) the cpmod utility to transfer file attributes.
It is available by anonymous ftp on gatekeeper.dec.com in
/.0/usenet/comp.sources.unix/volume11/cpmod.Z.

The sample programs zread.c, sub.c and add.c are provided as examples
of useful complements to gzip. Read the comments inside each source file.
The perl script ztouch is also provided as example (not installed
by default since it relies on perl).


gzip is free software, you can redistribute it and/or modify it under
the terms of the GNU General Public License, a copy of which is
provided under the name COPYING. The latest version of gzip are always
available by ftp in prep.ai.mit.edu:/pub/gnu, or in any of the prep
mirror sites:

- sources in gzip-*.tar (or .shar or .tar.z)
- Solaris 2 executables in sparc-sun-solaris2/gzip-binaries-*.tar
- MSDOS lha self-extracting exe in gzip-msdos-*.exe. Once extracted,
  copy gzip.exe to gunzip.exe and zcat.exe, or use "gzip -d" to decompress.

A VMS executable is available in ftp.spc.edu:[.macro32.savesets]gzip-1-*.zip
(use [.macro32]unzip.exe to extract).

Many thanks to those who provided me with bug reports and feedback.
See the files THANKS and ChangeLog for more details.


                Note about zip vs. gzip:

The name 'gzip' was a very unfortunate choice, because zip and gzip
are two really different programs, although the actual compression and
decompression sources were written by the same persons. A different
name should have been used for gzip, but it is too late to change now.

zip is an archiver: it compresses several files into a single archive
file. gzip is a simple compressor: each file is compressed separately.
Both share the same compression and decompression code for the
'deflate' method.  unzip can also decompress old zip archives
(implode, shrink and reduce methods). gunzip can also decompress files
created by compress and pack. zip 1.9 and gzip do not support
compression methods other than deflation. (zip 1.0 supports shrink and
implode). Better compression methods may be added in future versions
of gzip. zip will always stick to absolute compatibility with pkzip,
it is thus constrained by PKWare, which is a commercial company.  The
gzip header format is deliberately different from that of pkzip to
avoid such a constraint.

On Unix, gzip is mostly useful in combination with tar. GNU tar
1.11.2 has a -z option to invoke gzip automatically.  "tar -z"
compresses better than zip, since gzip can then take advantage of
redundancy between distinct files. The drawback is that you must
scan the whole tar.z file in order to extract a single file near
the end; unzip can directly seek to the end of the zip file. There
is no overhead when you extract the whole archive anyway.
If a member of a .zip archive is damaged, other files can still
be recovered. If a .tar.z file is damaged, files beyond the failure
point cannot be recovered. (Future versions of gzip will have
error recovery features.)

gzip and gunzip are distributed as a single program. zip and unzip
are, for historical reasons, two separate programs, although the
authors of these two programs work closely together in the info-zip
team. zip and unzip are not associated with the GNU project.
The sources are available by ftp in

         oak.oakland.edu:/pub/misc/unix/zip19p1.zip
         oak.oakland.edu:/pub/misc/unix/unz50p1.tar-z
*the stt