UNIX/LINUX Command – gzip


gzip, gunzip, zcatgzip, gunzip, zcat—Compress or expand files


gzip [ –acdfhlLnNrtvV19 ][–Ssuffix] [ name … ]
gunzip [ –acfhlLnNrtvV ][–Ssuffix] [ name … ]
zcat [ –fhLV ][name … ]


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 MS-DOS, OS/2 FAT, Windows NT FAT and Atari.) If no files are specified, or if a filename 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 filename is too long for its filesystem, gzip truncates it. gzip attempts to truncate only the parts of the filename 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 filenames 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 filename length.

By default, gzip keeps the original filename and timestamp in the compressed file. These are used when decompressing the file with the –N option. This is useful when the compressed filename 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 filesystem, 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, z, or .Z 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 tar.zip files to the tar.gz format. 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 to 70 percent. 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 32KB block, or an expansion ratio of 0.015 percent 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.


Options Description
–a –ascii ASCII text mode: convert end-of-lines using local conventions. This option is supported only on some non-UNIX systems. For MS-DOS, 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 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 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 (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 compressed 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 filename and timestamp by default. (The original name is always saved if the name had to be truncated.) When decompressing, do not restore the original filename if present (remove only the gzip suffix from the compressed filename) 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 filename and timestamp; this is the default. When decompressing, restore the original filename and timestamp if present. This option is useful on systems that have a limit on filename length or when the timestamp has been lost after a file transfer.
–q –quiet Suppress all warnings.
–r –recursive Travel the directory structure recursively. If any of the filenames 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 Use suffix .suf instead of .gz. Any suffix can be given, but suffixes other than .z and .gz should be avoided to avoid confusion when files are transferred to other systems. A null suffix forces gunzip to try decompression on all given files regardless of suffix, as in the following:
gunzip -S “” * (*.* for MS-DOS)
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 expense of speed).


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, use

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 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 MS-DOS: 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.

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