||FreeBSD General Commands Manual
gzip, gunzip, zcat - compress or expand files
gzip [ -acdfhklLnNrtvV19 ] [-S suffix] [ name
gunzip [ -acfhklLnNrtvV ] [-S suffix] [ name
zcat [ -fhLV ] [ name ... ]
The gzip command 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 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. The
gzip command 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. The gzip command 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 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 timestamp 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 and
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
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 zip files with several members, use unzip instead of
The zcat command 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
The gzip command 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
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.
Multiple compressed files can be concatenated. In this case, gunzip will
extract all members at once. For example:
- -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
- -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.
- -k --keep
- Keep (don't delete) input files during compression or decompression.
- -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: timestamp 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 timestamp 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 timestamp 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 timestamp; this
is the default. When decompressing, restore the original file name and
timestamp if present. This option is useful on systems which have a limit
on file name length or when the timestamp has been lost after a file
- -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
- Use synchronous output. With this option, gzip is less likely to
lose data during a system crash, but it can be considerably slower.
- -t --test
- Test. Check the compressed file integrity then quit.
- -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
- -# --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
- When you synchronize a compressed file between two computers, this option
allows rsync to transfer only files that were changed in the archive
instead of the entire archive. Normally, after a change is made to any
file in the archive, the compression algorithm can generate a new version
of the archive that does not match the previous version of the archive. In
this case, rsync transfers the entire new version of the archive to the
remote computer. With this option, rsync can transfer only the changed
files as well as a small amount of metadata that is required to update the
archive structure in the area that was changed.
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
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.
The obsolescent 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. As this can cause problems
when using scripts, this feature is supported only for options that are
reasonably likely to not cause too much harm, and gzip warns if it is
used. This feature will be removed in a future release of gzip.
You can use an alias or script instead. For example, if
gzip is in the directory /usr/bin you can prepend
$HOME/bin to your PATH and create an executable script
$HOME/bin/gzip containing the following:
exec gzip -9 "$@"
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1)
The gzip file format is specified in P. Deutsch, GZIP file
format specification version 4.3,
<https://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,
<https://www.ietf.org/rfc/rfc1951.txt>, Internet RFC 1951 (May
Exit status is normally 0; if an error occurs, exit status is 1. If a warning
occurs, exit status is 2.
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 can use the --quiet option to suppress the warning.
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:
- Usage: gzip [-cdfhklLnNrtvV19] [-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
- 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 -- unchanged
- The file is assumed to be already compressed. Rename the file and try
- 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
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.
Report bugs to: email@example.com
GNU gzip home page: <https://www.gnu.org/software/gzip/>
General help using GNU software: <https://www.gnu.org/gethelp/>
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Copyright © 1992, 1993 Jean-loup Gailly
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