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 (--).)
In combination with the --verbose option, the following fields are also displayed:
$ zcat file.Z | wc -c
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.
Previous versions of gzip used the .z suffix. This was changed to avoid a conflict with pack(1).
gunzip -S "" *
The uncompress(1) and zcat(1) commands use the following options:
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:
Exit status is normally 0; if an error occurs, exit status is 1. If a warning occurs, exit status is 2.
zcat file > recover
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).