lester@lester-ThinkPad-T410:~$ man zcat
GZIP(1) GZIP(1)

NAME
gzip, gunzip, zcat - compress or expand【展开】 files

SYNOPSIS
       gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
       gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ]
       zcat [ -fhLV ] [ name ... ]

DESCRIPTION
       Gzip reduces the size of the named files using Lempel-Ziv coding (LZ77). Whenever possible[只要可能], each file is
       replaced by one with the extension .gz, while keeping the same ownership[所有者，所有权] 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[尤其，特别], it will ignore symbolic links.

If the compressed file name is too long for its file system, gzip truncates[缩短，截断] it. Gzip attempts to truncate
       only the parts of the file name longer than 3 characters. (A part is delimited by dots.[]一部分被圆点隔开) If the name con‐
       sists of[包含，由...组成] 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[保存] after a file transfer[传递，传输].

Compressed files can be restored to their original form[形式，格式] 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[构造，构建] 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, or _z (ignoring case) and which begins with the correct[正确的，正确] magic number with an uncompressed file without
       the original extension.   gunzip also recognizes the special extensions .tgz and .taz as shorthands[简写] for
       .tar.gz and .tar.Z respectively[分别的，各自的，独立的]. 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[探测，检测] of
       the input format is automatic. When using the first two formats, gunzip checks a 32 bit CRC. For pack, gun‐
       zip checks the uncompressed length. The standard compress format was not designed to allow consistency[一致性，相容性]
       checks. However gunzip is sometimes able to detect[察觉，发现，探测] 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 con‐
       sistency 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 <foo.zip or gunzip -S .zip foo.zip. To
       extract[提取，取出] zip files with several members, use unzip instead of gunzip.

zcat is identical[同等的] to gunzip -c. (On some systems, zcat may be installed as gzcat to preserve[保存] 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 num‐
       ber 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 pre‐
       serves the mode, ownership and timestamps of files when compressing or decompressing.

The gzip file format is specified in P. Deutsch, GZIP file format   specification   version   4.3,
       <http://www.ietf.org/rfc/rfc1952.txt>, Internet RFC 1952 (May 1996). The zip deflation format is specified
       in     P.     Deutsch,     DEFLATE     Compressed     Data     Format     Specification     version      1.3,
       <http://www.ietf.org/rfc/rfc1951.txt>, Internet RFC 1951 (May 1996).

OPTIONS
       -a --ascii
              Ascii text mode: convert end-of-lines using local conventions[惯例，会议，约定]. 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, con‐
              catenate[连接的] 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 output: let zcat behave as cat. If -f is not given, and when not run‐
              ning 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 for files not in gzip format, such as compressed .Z files. To get
the uncompressed size for such a file, you can use:

zcat file.Z | wc -c

In combination[结合，组合] 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[降低，减少，缩小], 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 decom‐
              pressing, 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[递归的]. 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 decom‐
              press them in the case of gunzip ).

-S .suf --suffix .suf
              When compressing, use suffix .suf instead of .gz. Any non-empty suffix can be given, but suffixes
              other than .z and .gz should be avoided to avoid confusion when files are transferred to other sys‐
              tems.

When decompressing, add .suf to the beginning of the list of suffixes to try, when deriving an output
file name from an input file name.

-t --test
Test. Check the compressed file integrity[完整，正直].

-v --verbose
Verbose[冗长的]. 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 compression). The default compression level is -6 (that is, biased towards high compres‐
              sion at expense of speed).

ADVANCED USAGE
Multiple compressed files can be concatenated[连在一起，连接]. 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[故障，损坏] 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 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="-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 GZIP_OPT, to avoid a conflict with the symbol set for
invocation of the program.

SEE ALSO
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(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 [-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

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 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 directory, (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
       boundary. 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 warning 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 --block-compress /dev/rst0
         for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0

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. (This example assumes you
are using the GNU version of tar.)

BUGS
       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 following command to discover a large uncompressed file's true size:

zcat file.gz | wc -c

The --list option reports sizes as -1 and crc as ffffffff if the compressed 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, compress compresses better than gzip.

COPYRIGHT NOTICE
Copyright © 1998, 1999, 2001, 2002 Free Software Foundation, Inc.
Copyright © 1992, 1993 Jean-loup Gailly

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and
this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verba‐
tim copying, provided that the entire resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the
above conditions for modified versions, except that this permission notice may be stated in a translation
approved by the Foundation.

lester@lester-ThinkPad-T410:~/shell/zdiff$ man zcmp
local GZIP(1)

NAME
zcmp, zdiff - compare compressed files

SYNOPSIS
zcmp [ cmp_options ] file1 [ file2 ]
zdiff [ diff_options ] file1 [ file2 ]

DESCRIPTION
       Zcmp and zdiff are used to invoke the cmp or the diff program on files compressed via gzip. All options
       specified are passed directly to cmp or diff. If only one file is specified, then it is compared to a file
       with the same name without the .gz extension. If two files are specified, then they are uncompressed if nec‐
       essary and fed to cmp or diff. The exit status from cmp or diff is preserved.

When both files must be uncompressed before comparison, the second is uncompressed to /tmp. In all other
cases, zdiff and zcmp use only a pipe.

SEE ALSO
cmp(1), diff(1), zmore(1), zgrep(1), znew(1), zforce(1), gzip(1), gzexe(1)

BUGS
Messages from the cmp or diff programs refer to temporary filenames instead of those specified.

ZDIFF(1)

以下内容转载自：http://www.thegeekstuff.com/2009/05/zcat-zless-zgrep-zdiff-zcmp-zmore-gzip-file-operations-on-the-compressed-files/#more-463

The Power of Z Commands – Zcat, Zless, Zgrep, Zdiff Examples

by SathiyaMoorthy onMay 14, 2009

In this article let us review[回顾] how to perform[执行] normal file operation on a compressed files using the powerful Linux Z commands.

Some of these z commands uncompresses[解压缩] the file temporarily[暂时，临时的，临时] in the /tmp directory to perform the specified operation. Some of the z commands uncompresses it on the fly to perfom the specified operation. But, under any case, z commands gives the peace of mind, as you don’t want to worry about the overhead【开销】 of uncompressing the compressed file to perform an operation.

You can do the following normal file operations on the compressed file

Viewing the compressed file with zcat.
Paging the compressed file with zless / zmore.
Searching inside the compressed file with zgrep / zegrep.
Comparison[比对，比较] of file using zdiff / zcmp

Example 1: View Compressed File and Uncompress with zcat

Compressing a file using gzip creates a compressed file with *.gz extension[扩展名]. You can view a compressed file with zcat with the following way. Which would be as same as the uncompressed file operation ‘cat filename’. zcat uncompresses the file and shows it in the stdout[标准输出].

$ zcat filename.gz | more

$ ls -l big-file.*
-rw-r--r-- 1 ramesh ramesh 24853275 May  9 15:14 big-file.txt$ gzip big-file.txt 
[Note: Compress the file]$ ls -l big-file.*
-rw-r--r-- 1 ramesh ramesh 9275204 May  9 15:14 big-file.txt.gz$ zcat big-file.txt.gz 
[Note: View the file without uncompressing it]zcat big-file.txt.gz > big-file.txt
[Note: Uncompress the file]

Example 2: View a gzipped file which don’t have the gz suffix.

You can uncompress[解压缩] a gzipped file which don’t have the gz suffix[]. If you try to uncompress a gzipped file which don’t have the gz suffix with “gunzip” or “gzip -d” command you will face the following error.

gunzip: auth.log: unknown suffix -- ignored

But this zcat will uncompress the file and shows the content as shown below.

$ cat > test-file.txt
This is a test file used for gunzip and zcat testingzcat is awesome command.  $ gzip test-file.txt$ mv test-file.txt.gz test-file-no-ext$ gzip -d test-file-no-ext
gzip: test-file-no-ext: unknown suffix -- ignored$ zcat test-file-no-ext
This is a test file used for gunzip and zcat testingzcat is awesome command.

Example 3: Display the file content without worrying about whether it is compressed or not

When you are not sure whether a file is compressed or not, you can still view the file without worrying about it’s compression status as shown below.

In this example, If the input-file is compressed zcat will display the content by uncompressing it. If the input-file is not compressed zcat will display the content as it is.

$ zcat -f input-file

Example 4: Paging the compressed file with zless / zmore.

You can paginate a compressed file with zless command or zmore command as shown below.

$ zcat filename.gz | more
$ zcat filename.gz | less(or)$ zless filename.gz
$ zmore filename.gz

Note: To open any kind of file type, refer to our previous articleOpen & View 10 Different File Types with Linux Less Command – The Ultimate Power of Less.

Example 5: Searching inside the compressed file with zgrep / zegrep.

You can search inside a compressed file with zgrep / zegrep as shown below. This would be as same as the uncompressed file operation ‘grep -i filename’. All the options to the zgrep command will be passed to grep, and the file will be fed to grep command. It may uncompress and feed the file to grep command if needed.

$ cat > test-file.txt
gzip, gunzip, zcat - compress or expand files
zless - file perusal filter for crt viewing of compressed text
zcmp, zdiff - compare compressed files$ grep -i less test-file.txt
zless - file perusal filter for crt viewing of compressed text$ gzip test-file.txt$ zgrep -i less test-file.txt.gz
zless - file perusal filter for crt viewing of compressed text

Note: Become familiar with the grep command by reading our earlier articleGet a Grip on the Grep! – 15 Practical Grep Command Examples.

Example 6: Comparison of file using zdiff / zcmp

You can compare two compressed files with zdiff / zcmp as shown below. This would be same as the uncompressed file operation ‘diff file1 file2′.

$ cat > file1.txt
This is line one
This is line two$ cat > file2.txt
This is line 1
This is line two$ diff file1.txt file2.txt
1c1
< This is line one
---
> This is line 1$ gzip file1.txt file2.txt $ zdiff file1.txt.gz file2.txt.gz
1c1
< This is line one
---
> This is line 1

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