gzip, gunzip, uncompress, zcat - compress or expand files


gzip [ -cdfhlLnNqrtvV19 ] [-S suffix ] [name ...]

gunzip [ -cdfhlLnNqrtvV19] [ -S suffix ] [ name ... ]
uncompress [-cCdf?hkKvV] [-b maxbits] [-Iinpath] [-Ooutpath] [filenames...]
zcat [-cCdf?hkKvV][-b maxbits][-Iinpath][-Ooutpath][filenames...]


The gzip(1) utility 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 Windows file allocation table (FAT). If no files are specified, or if a file name is "-", the standard input is compressed to the standard output. The gzip(1) utility will attempt to compress regular files only. In particular, it will ignore symbolic links.

If the compressed file name is too long for its file system, gzip truncates it. The gzip(1) utility attempts to truncate only the parts of the file name longer than three 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 that do not have a limit on file-name length.

By default, gzip keeps the original file name and time stamp 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 gunzip(1), uncompress(1), or zcat(1). 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.

The gunzip(1) utility takes a list of files on its command line and replaces with an uncompressed file without the original extension each file whose name ends with .gz or .Z and begins with the correct magic number. The gunzip(1) utility also recognizes the special extensions .tgz and .taz as abbreviations for .tar.gz and .tar.Z, respectively. When compressing, gzip(1) uses the .tgz extension if necessary instead of truncating a file with a .tar extension.

The gunzip(1) utility can currently decompress files created by gzip(1), zip(1), compress(1), compress -H(1) or pack(1). The detection of the input format is automatic. When using the first two formats, gunzip(1) performs a 32-bit cyclical redundancy check (CRC). For pack(1), gunzip(1) checks the uncompressed length. The standard compress(1) format was not designed to allow consistency checks. However, gunzip(1) 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(1) does not indicate a problem. This generally means that the standard uncompress(1) does not check its input and generates unreliable output. The Santa Cruz Operation (SCO) compress -H format (lzh compression method) does not include a CRC, but does allow some consistency checks.

Files created by zip(1) can be uncompressed by gzip(1) 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 zip files with several members, use unzip(1) instead of gunzip(1). The uncompress(1) utility is identical to gunzip(1) except that zcat(1) is identical to gunzip(1) -c. (On some systems, zcat(1) can be installed as gzcat(1) to preserve the original link to compress)(1). The zcat(1) utility uncompresses either a list of files on the command line or its standard input and writes the uncompressed data on standard output. The zcat(1) utility uncompresses files that have the correct magic number, whether they have a .gz suffix or not.

The gzip(1) utility uses the Lempel-Ziv algorithm used in zip(1) 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(1)), Huffman coding (as used in pack(1)), or adaptive Huffman coding (compact(1)).

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(1) file header, plus five 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. The gzip(1) utility preserves the mode, ownership, and time stamps of files when compressing or decompressing.


The gzip(1) utility takes the following options. (The long form of the option letter is listed in parentheses; the long forms are preceded by two dashes (--).)

-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(1), and if the option --stdout is also given, copy the input data without change to the standard output: let zcat(1) behave as cat(1). If -f is not given, and when not running in the background, gzip(1) 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:
Size of the compressed file
Size of the uncompressed file
Compression ratio (0.0% if unknown)
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:
Compression method
The 32-bit CRC of the uncompressed data
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 are also displayed unless some sizes are unknown. With --quiet, the title and totals lines are not displayed.

-L (--license)
Display the gzip(1) 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(1) 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 that 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(1) will descend into the directory and compress all the files it finds there (or decompress them in the case of gunzip(1)).
-S .suf (--suffix.suf)
Use suffix .suf instead of .gz. Any suffix can be given, but avoid using suffixes other than .z and .gz to prevent confusion when files are transferred to other systems. A null suffix forces gunzip(1) to try decompression on all given files regardless of suffix, as in:
gunzip -S "" *
Previous versions of gzip used the .z suffix. This was changed to avoid a conflict with pack(1).
-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 the expense of speed).

The uncompress(1) and zcat(1) commands use the following options:

print Version
do_decomp default = on
force overwrite of output file default = off
no header: useful to uncompress old files
cat all output to stdout default = off
generate output compatible with compress 2.0.
keep input file, default == kill (erase)
keep output file on error, default == kill (erase)
-b maxbits
default == 16 bits, max == 16 bits
-I pathname
infile path ==
-O pathname
outfile path ==
-? | -h
help usage.


Multiple compressed files can be concatenated. In this case, gunzip(1) 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). You can get better compression by compressing all members at once, however. For example:

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, use the following:

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

To create a single archive file with multiple members so that members can later be extracted independently, use an archiver, such as tar(1) or zip(1). GNU tar(1) supports the -z option to invoke gzip(1) transparently. The gzip(1) utility is designed as a complement to tar(1), not as a replacement.


The environment variable GZIP can hold a set of default options for gzip(1). These options are interpreted first, and can be overwritten by explicit command-line parameters, as shown in the following example:

for sh: GZIP="-8v --name"; export GZIP
for csh: setenv GZIP "-8v --name"


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(1) 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 the following:
zcat file > recover
file: compressed with xx bits, can only handle yy bits
The file was compressed (using LZW) by a program that could handle more bits than the decompress code on this computer. 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 want 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.
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, (for example, a symbolic link, socket, first-in/first-out [FIFO], or 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.


When writing compressed data to a tape, it is usually 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(1) for decompression, gunzip(1) 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(1) is invoked implicitly by the -z option of GNU tar(1). Ensure that the same block size (the -b option of tar(1)) is used for reading and writing compressed data on tapes. (This example assumes you are using the GNU version of tar(1).)


The --list option reports incorrect sizes if they exceed two gigabytes. The --list option reports sizes as -1 and CRC as ffffffff if the compressed file is on a non-seekable medium.

In rare cases, the --best option gives worse compression than the default compression level(-6). On some highly redundant files, traditional compress(1) compresses better than gzip(1); this is not true, hhowever on Interix, where compress(1) is a link to gzip(1).