tar is a quirky archive utility available on all UNIX systems. It is based on the idea of making a "Tape ARchive" (thus the name), which is just streamed data (no compression). GZip is a UNIX-standard zip/unzip utility (via the Free Software Foundation). Both are available for Windows.
The general usage for the tar program is as follows:
C:\>tar -options filename ...
Where filename ... represents either the files that you want to de-archive or the files to be contained in the archive. Which of the two it is depends on the options specified.
To get a quick reminder of the format of the command you can just type tar without any arguments:
C:\> tar
tar: you must specify exactly one of the c, t, r, u or x options
Usage: tar -c[bfvw] device block filename..
tar -r[bvw] device block [filename...]
tar -t[vf] device
tar -u[bvw] device block [filename...]
tar -x[flmovw] device [filename...]
-c -- create
-r -- replace
-t -- table of contents
-u -- update
-x -- extract
b -- blocking factor
f -- file name
v -- verbose mode
w -- what (await user confirmation)
l -- link (output error message if links fail)
m -- modify
o -- ownership (owner is the user running tar)
-I -- include file
See also (on the unix system) man tar
To archive a number of different files we will use the c option and the f option as follows:
C:\> tar -cvf arc.tar c:\docs
This will archive all the files (and directories) under c:\docs and put them into one file called arc.tar. The meaning of each of the options cvf are:
c This states that we are going to compact many files into one file.
Note: There is in fact no file compression going on here!
v This specifies verbose operation. A message will be printed to the screen as each file is compacted into the archive.
f This specifies the output filename of the archive as the next argument, arc.tar in this case.
If the f option is left off then the contents of the archive will be written to the screen as they are joined into one (rather than being written to a file).
To list the files contained in the archive arc.tar you would use a command like the following:
tar -tvf arc.tar
[file listing removed]
This will print out to the screen a listing of the files contained in the archive arc.tar. The meaning of the options provided are as follows:
t Type out to the screen the contents of the archive being examined. v This specifies verbose operation, providing more screen information. f This specifies the name of the input file to be the next argument arc.tar in this case.
This listing will be contain dates, times, sizes and permissions associated with each file in the archive. If you are just interested in the files themselves then you would remove the v option.
C:\> tar -xvf arc.tar
This will un-archive all the files (and directories) stored in arc.tar. The files will be placed in the current directory, and any additional directories specified in the archive. The meaning of each of the options xvf are:
x This states that we are going to extract many files from one file.
Note: There is in fact no file de-compression going on here!
v This specifies verbose operation. A message will be printed to the screen as
each file is removed from the archive.
f This specifies the name of the input filename of the archive as the next
argument, arc.tar in this case.
gzip, gunzip, zcat - compress or expand files
gzip (e.g., gzip docsteve.tar)
gunzip (e.g., gunzip docsteve.tar.gz)
gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
gunzip [ -acfhlLnNrtvV ] [-S suffix] [ 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 exten- sion is -gz for VMS, 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 out- put. Gzip 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. Gzip 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 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 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, _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 out- put. 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. Typi- cally, 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 (compact).
Compression is always performed, even if the compressed file is slightly larger than the original. The worst case expan- sion 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.
-a --ascii
Ascii text mode: convert end-of-lines using local con-
ventions. 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
decompressing.
-c --stdout --to-stdout
Write output on standard output; keep original files
unchanged. If there are several input files, the out-
put 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 writ-
ten 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 stan-
dard ouput: 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: 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 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 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 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 decompress-
ing, restore the original file name and time stamp if
present. This option is useful on systems which 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 direc-
tories, 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:
gunzip -S "" * (*.* for MSDOS)
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 compres-
sion). 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
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
If you want 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
If you wish to create a single archive file with multiple members so that members can later be extracted indepen- dently, 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 MSDOS: set 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 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 bits
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 -- no change
The file is assumed to be already compressed.
Rename the file and try again.
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 direc-
tory, (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 multiply-linked files.
When writing compressed data to a tape, it is generally necessary to pad the output with zeroes up to a block boun- dary. 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 warn- ing 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 is invoked implicitly by the -z option of GNU tar. Make sure that the same block size (-b option of tar) is used for reading and writing compressed data on tapes. (This example assumes you are using the GNU version of tar.)
The --list option reports incorrect sizes if they exceed 2 gigabytes. 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 compres- sion than the default compression level (-6). On some highly redundant files, compress compresses better than gzip.
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