[email protected]:~$ man zcat

GZIP(1) GZIP(1)

NAME

gzip, gunzip, zcat - compress or expand【展開】 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[隻要可能], each file is

replaced by one with the extension .gz, while keeping the same ownership[所有者，所有權] 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[縮短，截斷] it. Gzip attempts to truncate

only the parts of the file name longer than 3 characters. (A part is delimited by dots.[]一部分被圓點隔開) If the name con‐

sists of[包含，由...組成] 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[儲存] after a file transfer[傳遞，傳輸].

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[構造，建構] 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[簡寫] for

.tar.gz and .tar.Z respectively[分别的，各自的，獨立的]. 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[探測，檢測] 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[一緻性，相容性]

checks. However gunzip is sometimes able to detect[察覺，發現，探測] 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[儲存] 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[慣例，會議，約定]. 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[連接配接的] 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[結合，組合] 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，減少，縮小], 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[遞歸的]. 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[冗長的]. 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[連在一起，連接配接]. 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，損壞] 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.

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

by SathiyaMoorthy onMay 14, 2009

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

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

Some of these z commands uncompresses[解壓縮] the file temporarily[暫時，臨時的，臨時] 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【開銷】 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[比對，比較] 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[擴充名]. 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[标準輸出].

$ 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[解壓縮] 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 testing

zcat 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 testing

zcat 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

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

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

Example 1: View Compressed File and Uncompress with zcat

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

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

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

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

Example 6: Comparison of file using zdiff / zcmp

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