compress, uncompress, zcat − compress and expand data
(version 4.1)

compress [ −f ] [ −v ] [ −c ] [ −V ] [ −r ] [ −b bits ] [
name ...  ]
uncompress [ −f ] [ −v ] [ −c ] [ −V ] [ name ...  ]
zcat [ −V ] [ name ...  ]

Compress reduces the size of the named files using adaptive
Lempel‐Ziv coding.  Whenever possible, each file is replaced
by one with the extension .Z, while keeping the same
ownership modes, access and modification times.  If no files
are specified, the standard input is compressed to the
standard output.  Compress will only attempt to compress
regular files.  In particular, it will ignore symbolic
links. If a file has multiple hard links, compress will
refuse to compress it unless the −f flag is given.

     If −f is not given and compress is run in the
foreground, the user is prompted as to whether an existing
file should be overwritten.

     Compressed files can be restored to their original form
using uncompress or zcat.

     uncompress takes a list of files on its command line
and replaces each file whose name ends with .Z and which
begins with the correct magic number with an uncompressed
file without the .Z.  The uncompressed file will have the
mode, ownership and timestamps of the compressed file.

     The −c option makes compress/uncompress write to the
standard output; no files are changed.

     zcat is identical to uncompress −c.  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 .Z suffix or not.

     If the −r flag is specified, compress will operate
recursively. If any of the file names specified on the
command line are directories, compress will descend into the
directory and compress all the files it finds there.

     The −V flag tells each of these programs to print its
version and patchlevel, along with any preprocessor flags
specified during compilation, on stderr before doing any
compression or uncompression.

     Compress uses the modified Lempel‐Ziv algorithm
popularized in "A Technique for High Performance Data
Compression", Terry A. Welch, IEEE Computer, vol. 17, no. 6
(June 1984), pp. 8‐19.  Common substrings in the file are


first replaced by 9‐bit codes 257 and up.  When code 512 is
reached, the algorithm switches to 10‐bit codes and
continues to use more bits until the limit specified by the
−b flag is reached (default 16).  Bits must be between 9 and
16.  The default can be changed in the source to allow
compress to be run on a smaller machine.

     After the bits limit is attained, compress periodically
checks the compression ratio.  If it is increasing, compress
continues to use the existing code dictionary.  However, if
the compression ratio decreases, compress discards the table
of substrings and rebuilds it from scratch.  This allows the
algorithm to adapt to the next "block" of the file.

     Note that the −b flag is omitted for uncompress, since
the bits parameter specified during compression is encoded
within the output, along with a magic number to ensure that
neither decompression of random data nor recompression of
compressed data is attempted.

     The amount of compression obtained depends on the size
of the input, the number of bits per code, and the
distribution of common substrings.  Typically, text such as
source code or English is reduced by 50−60%.  Compression is
generally much better than that achieved by Huffman coding
(as used in or adaptive Huffman coding and takes less time
to compute.

     Under the −v option, a message is printed yielding the
percentage of reduction for each file compressed.

     Exit status is normally 0; if the last file is larger
after (attempted) compression, the status is 2; if an error
occurs, exit status is 1.

pack(1), compact(1)

Usage: compress [−dfvcVr] [−b maxbits] [file ...]
        Invalid options were specified on the command line.
Missing maxbits
        Maxbits must follow
not in compressed format
        The file specified to uncompress has not been
compressed with xx bits, can only handle yy bits
        File was compressed by a program that could deal
        with more bits than the compress code on this
        machine.  Recompress the file with smaller
already has .Z suffix ‐‐ no change
        The file is assumed to be already compressed.
        Rename the file and try again.
filename too long to tack on .Z
        The file cannot be compressed because its name is
        longer than 12 characters.  Rename and try again.


        This message does not occur on BSD systems.
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.
uncompress: corrupt input
        A SIGSEGV violation was detected which usually means
        that the input file has been corrupted.
Compression: xx.xx%
        Percentage of the input saved by compression.
        (Relevant only for
‐‐ not a regular file or directory: ignored
        When the input file is not a regular file or
        directory, (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
        for more information. Use the −f flag to force
        compression of multiply‐linked files.
‐‐ file unchanged
        No savings is achieved by compression.  The input
        remains virgin.

Although compressed files are compatible between machines
with large memory, should be used for file transfer to
architectures with a small process data space (64KB or less,
as exhibited by the DEC PDP series, the Intel 80286, etc.)

     Invoking compress with a −r flag will occasionally
cause it to produce spurious error warnings of the form

        "<filename>.Z already has .Z suffix ‐ ignored"

     These warnings can be ignored. See the comments in
compress.c:compdir() for an explanation.