home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Skunkware 98
/
Skunkware 98.iso
/
src
/
shellutil
/
gzip-1.2.4.tar
/
gzip-1.2.4
/
vms
/
gzip.hlp
next >
Wrap
Text File
|
1993-08-18
|
14KB
|
298 lines
1 GZIP
NAME
gzip, gunzip, zcat - compress or expand files
SYNOPSIS
gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ]
zcat [ -fhLV ] [ name ... ]
2 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 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 out-
put. Gzip will only attempt to compress regular files.
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 del-
imited 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 out-
put. 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. 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. 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.
2 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 --to-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 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 stan-
dard 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 on VMS because it
it is not possible to seek reliably to the end of the
compressed file, where this size is stored.
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 on VMS for the reason given
above about the uncompressed size.
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 decompress-
ing, 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 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 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. A null suffix forces gunzip to try
decompression on all given files regardless of suffix,
as in:
gunzip --suffix "" *.*
Previous versions of gzip used the -z suffix. This was
changed to avoid a conflict with pack on Unix.
-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 compres-
sion). The default compression level is -6 (that is,
biased towards high compression at expense of speed).
2 ENVIRONMENT
The environment variable GZIP_OPT 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:
define GZIP_OPT "-8 -v"
2 SEE ALSO
compress, zip, unzip
2 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
define /user sys$output file.recover
zcat file
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 direc-
tory, it is left unaltered.
2 CAVEATS
On VMS:
- upper case options need quotes: gzip "-V".
- restoration of timestamps and version numbers is not supported
- If a compressed file already exists, gzip -f overwrites it, it
does not create a new version.
- multi-part gzip files are not supported.
- gunzip does not preserve the input file format. You can use a
separate utility to restore the original format.
- gunzip and zcat can be used only if you have created the
links to gzip as documented in makegzip.com. Otherwise
you must use explicit parameters ("gzip -c" or "gzip -dc").
- gzip --list cannot give the uncompressed size and crc.
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_OPT
environment variable as in:
define GZIP_OPT "-q"
2 BUGS
On VMS, files in VFC record format are not correctly handled by
the C runtime library (the linefeed character is suppressed).
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.