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tar and GZip Reference

Working on UNIX systems

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).

Listing Contents of an Archive

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

Quick Reference

Zipping & UnZipping
     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 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 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

     -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

     -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).

Advanced Usage

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, 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
     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.
             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.

Caveats & Bugs

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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