Linux Commands Examples

A great documentation place for Linux commands


, gunzip, zcat compress or expand files

see also : gunzip - zcat - znew - zcmp - zmore - zforce - gzexe - zip - unzip


gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
[ -acfhlLnNrtvV ] [-S suffix] [ name ... ]
[ -fhLV ] [ name ... ]

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How to specify level of compression when using tar -zcvf?

Instead of using the gzip flag for tar, gzip the files manually after the tar process, then you can specify the compression level for the gzip program:

tar -cvf files.tar /path/to/file0 /path/to/file1 ; gzip -9 files.tar

Or you could use:

tar cvf - /path/to/file0 /path/to/file1 | gzip -9 - > files.tar.gz

The -9 in the gzip command line tells gzip to use the maximum possible compression level (default is -6).

Edit: Fixed pipe command line based on @depesz comment.


Why doesn't Gzip compression eliminate duplicate chunks of data?

gzip with no command line switches uses the lowest possible algorithm for compression.

Try using:

gzip -9 test.tar

You should get better results


How to gzip multiple files into one gz file?

You want to tar your files together and gzip the resulting tar file.

tar cfvz cvd.tar.gz cvd*.txt

To untar the gzip'd tar file you would do:

tar xfvz cvd.tar.gz -C /path/to/parent/dir

This would extract your files under the /path/to/parent/dir directory


Is there away to mount a file.tar.bz2 without extracting onto your fs?

The term mount in this context is ill-defined. I'm guessing you want to look inside the tarball without extracting it. One handy utility for this is Midnight Commander. See also Wikipedia Midnight Commander page.

This creates a sort of virtual filesystem for tarballs, rpms, deb and all sorts of other archives. Just fire it up, and navigate to your tarball, and hit Enter. To read a file use F3, F5 to copy a file, F10 quit. On Linux at least there is a convenient command help at the bottom of the screen.

So, to summarize you can read the files inside your tarball and copy them from to your regular filesystem. Hopefully that will do you.

I did a little more checking, and it looks like MC is basically only supported on Unix-like systems like Linux, though there is a Windows port here of some sort. However, you don't state what your OS is. I suggest you do so.


Rsync friendly gzip

I know that Ubuntu Linux applies a patch (gzip file) to gzip sources to allow for a --rsyncable flag. You can download that patch and use it yourself, or see if your distribution includes the patch.


Which archiving method is better for compressing text files on Linux?

Normally, bz2 has a better compression ratio, combined with better recoverability features.

OTOH, gz is faster.

xz is said to be even better than bz2, but I don't know the timing behaviour.


Files with same content but with different md5sums when gzip'd?

Just use gzip with '-n' flag:

tiagocruz@stark:~$ gzip -n Yippie-Ki-Yay.mp3 bla/Yippie-Ki-Yay.mp3 

tiagocruz@stark:~$ sha1sum Yippie-Ki-Yay.mp3.gz bla/Yippie-Ki-Yay.mp3.gz 
b44b21c5f414935f1ced1187bfafd989704474a5  Yippie-Ki-Yay.mp3.gz
b44b21c5f414935f1ced1187bfafd989704474a5  bla/Yippie-Ki-Yay.mp3.gz



Fastest GZIP utility

If you don't mind stepping away from DEFLATE, lzop is an implementation of LZO that favors speed over compression ratio.


How do I uncompress vmlinuz to vmlinux?

Maybe you misunderstood what the author of that post meant.

  1. The vmlinuz file contains other things besides the gzipped content, so you need to find out where the gzipped content starts. To do that, use:

    od -A d -t x1 vmlinuz | grep '1f 8b 08 00'

    What this does is to show you where in that file you can find the gzip header. The output looks like:

    0024576 24 26 27 00 ae 21 16 00 1f 8b 08 00 7f 2f 6b 45

    This means that at 0024576 (at least for the author of the post, yours might be somewhere completely different) in the vmlinuz file, you will find the binary values "24 26 27 00 ae 21 16 00 1f 8b 08 00 7f 2f 6b 45". You're looking for 1f 8b 08 00, which can be found from character 9 onwards, or, at 0024576 + 8 (start counting from 0) = 24584.

  2. Now that you know where the gzipped content starts (at position 24584) you can use dd to extract that gzipped content and ungzip it. To do that, use:

    dd if=vmlinuz bs=1 skip=24584 | zcat > vmlinux

    The first command will seek to that position and copy everything to stdout. zcat then will uncompress everything it gets from stdin and will output the uncompressed string to stdout. Then the > will redirect zcat's output to a new file named vmlinux.


gzip all files without deleting them

find . -type f | \
while read -r x
  gzip -c "$x" > "$x.gz"

The -c pushes the result to stdout and keeps the original alone. The disadvantage is, that you need to find the files yourself. For more sophisticated traversing, you can use find(1), however, like above: . searches starting from the current directory, and -type f returns the name of every regular file.


Why are there binary differences among compressed files generated exactly the same way from the exact same starting file?

Two possible causes:

  • different compression algorithm used by the same compression program, or
  • different compression programs

Unexpected end of file. Gzip compressed file

Did you by any chance transfer the file from Win* to Unix via ftp in ascii mode? That may explain it. Is the file the same size on Win* and Unix?


Does Linux GZip Zip the File in Place or create a new file

Try something like:

gzip --stdout textfile > /path/to/spacious/filesystem/textfile.gz

Critical gzip mistake (HELP!) - how to undo 'gzip -r ./'

To undo this, use the opposite command:

gunzip -r ./

Note that the original gzip command will skip over files that already have a .gz suffix, because there's no point in compressing them twice. However, the above gunzip command will decompress such files, because it doesn't know that gzip skipped them.


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 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, it will ignore symbolic links.

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 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 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, or _z (ignoring case) 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 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 files to the tar.gz format. To extract a zip file with a single member, use a command like gunzip < or gunzip -S .zip 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. 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 specification version 4.3, <>, Internet 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).


-a --ascii

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 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, concatenate all input files before compressing them.

-d --decompress --uncompress


-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 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 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 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 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 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.

-q --quiet

Suppress all warnings.

-r --recursive

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 ).

-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 systems.

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.

-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 compression). The default compression level is -6 (that is, biased towards high compression at expense of speed).

advanced usage

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


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 transparently. gzip is designed as a complement to tar, not as a replacement.


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.)

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 verbatim 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.


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.


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 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.

see also

znew , zcmp , zmore , zforce , gzexe , zip , unzip , compress

The gzip file format is specified in P. Deutsch, GZIP file format specification version 4.3, <>, Internet 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).

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