PACK(C) UNIX System V
Name
pack, pcat, unpack - compresses and expands files
Syntax
pack [ - ] name ...
pcat name ...
unpack name ...
Description
pack attempts to store the specified files in a compressed
form. Wherever possible, each input file name is replaced
by a packed file name.z with the same access modes, access
and modified dates, and the owner of name. If pack is
successful, name will be removed. Packed files can be
restored to their original form using unpack or pcat.
pack uses Huffman (minimum redundancy) codes on a byte-by-
byte basis. If the - argument is used, an internal flag is
set that causes pack to display information about the file
compression. Additional occurrences of - in place of name
will cause the internal flag to be set and reset.
The amount of compression obtained depends on the size of
the input file and the character frequency distribution.
Because a decoding tree forms the first part of each .z
file, it is usually not worthwhile to pack files smaller
than three blocks, unless the character frequency
distribution is very scattered, which may occur with printer
plots or pictures.
Typically, text files are reduced to 60-75% of their
original size. Load modules, which use a larger character
set and have a more uniform distribution of characters, show
little compression, the packed versions being about 90% of
the original size.
pack returns a value that is the number of files that it
failed to compress.
No packing will occur if:
- The file appears to be already packed
- The filename has more than 12 characters
- The file has links
- The file is a directory
- The file cannot be opened
- No disk storage blocks will be saved by packing
- A file called name.z already exists
- The .z file cannot be created
- An I/O error occurred during processing
The last segment of the filename must contain no more than
12 characters to allow space for the appended .z extension.
Directories cannot be compressed.
Pcat does for packed files what cat(C) does for ordinary
files. The specified files are unpacked and written to the
standard output. Thus to view a packed file named name.z
use:
pcat name.z
or just:
pcat name
To make an unpacked copy, say nnn, of a packed file named
name.z without destroying name.z, enter the command:
pcat name >nnn
Pcat returns the number of files it was unable to unpack.
Failure may occur if:
-
The filename (exclusive of the .z) has more than 12
characters
-
The file cannot be opened
-
The file does not appear to be the output of pack
unpack expands files created by pack. For each file name
specified in the command, a search is made for a file called
name.z (or just name, if name ends in .z). If this file
appears to be a packed file, it is replaced by its expanded
version. The new file has the .z suffix stripped from its
name, and has the same access modes, access and modification
dates, and owner as those of the packed file.
unpack returns a value that is the number of files it was
unable to unpack. Failure may occur for the same reasons
that it may in pcat, as well as in a file where the
``unpacked'' name already exists, or if the unpacked file
cannot be created.
Standards Conformance
pack, pcat and unpack are conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 8/28/89) PACK(C)