Linux gzip, gunzip, and zcat commands

On Unix-like operating systems, the gzip, gunzip, and zcat commands are used to compress or expand files in the GNU GZIP format.

Description

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 file name is "-", the standard input is compressed to the standard output. gzip only attempts to compress regular files. In particular, it ignores 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 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 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 setting 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, 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 because the standard uncompress does not complain. This generally indicates 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 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 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 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 32 K 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.

Syntax

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

gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ...  ]

zcat [ -fhLV ] [ name ...  ]

Options

zcat file.Z | wc -c

Advanced usage

Multiple compressed files can be concatenated. In this example, gunzip will extract all members at once. For example:

gzip -c file1  > foo.gz

gzip -c file2 >> foo.gz

Then:

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

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

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.

Environment

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.

Examples

zcat myfiles.tar.gz | more

Prints the contents of all files in the gzipped tar archive myfiles.tar.gz, piping the output to the more command, which pages the output (pauses at the end of each page).

gzip backup.tar

Compresses the tar archive backup.tar, and renames it backup.tar.gz.

gunzip backup.tar.gz

Uncompresses the gzipped file backup.tar.gz, renaming it backup.tar.

gzip -r backupfolder

Recursively compresses all files in the folder backupfolder, and all files in any subdirectories of that folder, and gives them the extension .gz.

gunzip -r backupfolder

Recursively uncompresses all gzipped files in the folder backupfolder, and removes the extension .gz from the file names.

gzip -c myfile.txt > myfile.txt.gz

Compresses the file myfile.txt, writing the compressed file data to standard output, which is redirected to a file, myfile.txt.gz. The result is that the original file and the gzipped file will both exist after the command is run.

zcat myfile.txt.gz

Prints the uncompressed contents of the compressed file myfile.txt.gz. The output is identical to printing the contents of the uncompressed file with the command cat myfile.txt.

gunzip -c myfile.txt.gz > myfile.txt

Uncompresses the gzipped file myfile.txt.gz to standard output, which is redirected to the file myfile.txt. If myfile.txt already exists, the shell prompt you to overwrite it (or not).

Related commands

cat — Output the contents of a file.
compress — Compress a file or files.
tar — Create, modify, list the contents of, and extract files from tar archives.
uncompress — Extract files from compressed archives.
unzip — List, test and extract compressed files in a zip archive.
zip — A compression and archiving utility.