4.4BSD/usr/src/contrib/gzip-1.0.7/vms/gzip.hlp
1 GZIP
NAME
gzip, gunzip, zcat - compress or expand files
SYNOPSIS
gzip [ -cdfhLrtvV19 ] [ name ... ]
gunzip [ -cfhLrtvV ] [ name ... ]
zcat [ -hLV ] [ 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 an extension *-z, while keeping the same ownership
modes, access and modification times. 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 par-
ticular, it will ignore devices.
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 -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 shorthand for .tar-z.
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.z format. To extract zip files with
several members, use unzip instead of gunzip.
zcat is identical to gunzip -c. 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 -z 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. gzip preserves the mode, ownership and timestamps of
files when compressing or decompressing.
2 OPTIONS
-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
Decompress.
-f --force
Force compression even if the corresponding file already
exists. 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.
-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 ).
-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.
-# --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.
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:
GZIP_OPT == "-8 -v"
2 SEE ALSO
compress, zip, unzip
2 DIAGNOSTICS
Usage: gzip [-cdfhLrtvV19] [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 z suffix -- no change
The file is assumed to be already compressed.
Rename the file and try again or use zcat.
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.)
-- 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.
- wildcards in file names are not supported.
- multi-part gzip files are not supported.
- gunzip does not preserver 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").
2 BUGS
On VMS, files in VFC record format are not correctly handled by
the C runtime library (the linefeed character is suppressed).