compression" -- Technical Information" "

The amount of compression will vary, depending upon the type of file being compressed, the compression algorithm used, and the level of compression requested. In general, files that show a great deal of repetition internally will compress more thoroughly than those that are largely random; thus, in general a text file will compress more thoroughly than will a digitized sound sample or image (although there are exceptions). The higher the level of compression you request, the more thoroughly the file will be compressed, but the longer the machine will have to work to achieve it. In most instances, raising the level of compression very high will save only a few bytes at a great cost in computer time.

You should note, too, that although compression algorithms try very hard not to lose information, it is possible that compressing some very complex files may result in a loss of information: that is, if you compress a file and de-compress it, the de-compressed file may be exactly the same as it was before you first compressed it. These programs will not affect most everyday varieties of data; but you should be aware of this fact.

COHERENT comes with the following tools for compressing and uncompressing files:

ccoommpprreessss

This program compresses files uses the Lempel-Ziv algorithm. By default, it creates a file with the suffix ..ZZ. It replaces the uncompressed original with its compressed analogue.

ggttaarr

This program creates tape archives. Its options --zz and --ZZ invoke, respectively, the programs gzip and compress to compress the archive as it is being built, thus permitting you to build a compressed archive automatically.

gguunnzziipp

This de-compresses files that had been compressed by the program gzip. By default, it works only with files that have the suffices ..zz or ..ggzz. It replaces the compressed file with its uncompressed analogue.

ggzziipp

This program compresses files into the zziipp format. In general, it is faster and more thorough than compress, although it may not work as well on some files. It replaces the uncompressed original file with its compressed analogue.

uunnccoommpprreessss

This uncompresses files that had been compressed by compress. It works with files that have the suffix ..ZZ. It replaces the compressed file with its uncompressed analogue.

zzccaatt

This program de-compresses ``on the fly'' programs that had been compressed by compress, and writes the decompressed form to the standard output device. This is useful if you want to look at the contents of a compressed file but do not want to bother with de- compressing all of it.

Default Suffixes

If you have received a compressed file from a third-party source, you may have no idea what tool was used to compress the file; fortunately, however, most compression tools use standard suffices to ``stamp'' the files they compress. The following table gives commonly used suffices, plus examples of how to uncompress files that bear them:

          _C_o_m_p_r_e_s_s_i_o_n      _D_e_c_o_m_p_r_e_s_s_i_o_n     _E_x_a_m_p_l_e
_S_u_f_f_i_x    _P_r_o_g_r_a_m          _P_r_o_g_r_a_m
..ZZ        ccoommpprreessss         uunnccoommpprreessss        uunnccoommpprreessss ffoooo..ZZ
..ttaarr..ZZ    ttaarr||ccoommpprreessss     uunnccoommpprreessss||ttaarr    zzccaatt ffoooo..ttaarr..ZZ || ttaarr xxvvff --
..zz        ggzziipp             gguunnzziipp            gguunnzziipp ffoooo..zz
..ttaarr..zz    ttaarr||ggzziipp         gguunnzziipp||ttaarr        gguunnzziipp ffoooo..ttaarr..zz ;; ttaarr xxvvff ffoooo..ttaarr
..ttggzz      ggttaarr --cczz         ggttaarr --xxzz          ggttaarr --xxvvzzff ffoooo..zz
..ggttzz      Same as ..ttggzz
..ttaazz      Same as ..ttggzz

See Also

• compress,
• gtar,
• gunzip,
• gzip,
• uncompress,
• Using COHERENT,
• zcat