Manual Pages for UNIX Darwin command on man gzcat

GZIP(1) GZIP(1)

NAME

gzip, gunzip, zcat - compress or expand files

SYNOPSIS

ggzziipp [ -aaccddffhhllLLnnNNrrttvvVV1199 ] [-SS ssuuffffiixx] [ name ... ]

gguunnzziipp [ -aaccffhhllLLnnNNrrttvvVV ] [-SS ssuuffffiixx] [ name ... ]

zzccaatt [ -ffhhLLVV ] [ 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 ..ggzz,, while keeping the same ownership modes, access and modi-

fication times. (The default extension is -ggzz for VMS, zz 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 particu-

lar, it will ignore symbolic links.

If the compressed file name is too long for its file system, gzip trun-

cates 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 com-

pressed file. These are used when decompressing the file with the -NN

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 ..ttggzz and ..ttaazz as shorthands for ..ttaarr..ggzz and ..ttaarr..ZZ respectively. When compressing, gzip uses the ..ttggzz extension if necessary instead of truncating a file with a ..ttaarr 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 a zip file with a single member, use a command like gunzip

members, use unzip instead of gunzip.

zcat is identical to gunzip -cc.. (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 ..ggzz suffix or not. In POSIX mode zcat will append ..ZZ to all filenames that do not have that suffix.

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 dis-

tribution 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.

The gzip file format is specified in P. Deutsch, GZIP file format spec-

ification version 4.3, , Inter-

net RFC 1952 (May 1996). The zip deflation format is specified in P. Deutsch, DEFLATE Compressed Data Format Specification version 1.3,

, Internet RFC 1951 (May 1996).

OOPPTTIIOONNSS

-aa --aasscciiii

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 con-

verted to CR LF when decompressing.

-cc --ssttddoouutt --ttoo-ssttddoouutt

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.

-dd --ddeeccoommpprreessss --uunnccoommpprreessss

Decompress.

-ff --ffoorrccee

Force compression or decompression even if the file has multiple

links or the corresponding file already exists, or if the com-

pressed 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 -ff is not given,

and when not running in the background, gzip prompts to verify whether an existing file should be overwritten.

-hh --hheellpp

Display a help screen and quit.

-ll --lliisstt

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)

uncompressedname: name of the uncompressed file

The uncompressed size is given as -1 for files not in gzip for-

mat, 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, com-

press, 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.

-LL --lliicceennssee

Display the gzip license and quit.

-nn --nnoo-nnaammee

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.

-NN --nnaammee

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.

-qq --qquuiieett

Suppress all warnings.

-rr --rreeccuurrssiivvee

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 ).

-SS ..ssuuff --ssuuffffiixx ..ssuuff

Use suffix .suf instead of .gz. Any suffix can be given, but

suffixes other than .z and .gz should be avoided to avoid confu-

sion when files are transferred to other systems. A null suffix

forces gunzip to try decompression on all given files regard-

less 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).

-tt --tteesstt

Test. Check the compressed file integrity.

-vv --vveerrbboossee

Verbose. Display the name and percentage reduction for each file compressed or decompressed.

-VV --vveerrssiioonn

Version. Display the version number and compilation options then quit.

-## --ffaasstt --bbeesstt

Regulate the speed of compression using the specified digit #,

where -11 or --ffaasstt indicates the fastest compression method

(less compression) and -99 or --bbeesstt indicates the slowest com-

pression method (best compression). The default compression

level is -66 (that is, biased towards high compression at expense

of speed). AADDVVAANNCCEEDD UUSSAAGGEE 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 transpar-

ently. gzip is designed as a complement to tar, not as a replacement. ENVIRONMENT The environment variable GGZZIIPP 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 GZIPOPT, to avoid a conflict with the symbol set for invocation of the program.

BUGS

The gzip format represents the 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 following command to discover a large uncom-

pressed file's true size:

zcat file.gz | wc -c

The -list option reports sizes as -1 and crc as ffffffff if the com-

pressed 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, com-

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entire resulting derived work is distributed under the terms of a per-

mission notice identical to this one.

Permission is granted to copy and distribute translations of this man-

ual into another language, under the above conditions for modified ver-

sions, except that this permission notice may be stated in a transla-

tion approved by the Foundation. local GZIP(1)