PACK(1) INTERACTIVE UNIX System PACK(1)
NAME
pack, pcat, unpack - compress and expand files
SYNOPSIS
pack [ - ] [ -f ] name ...
pcat name ...
unpack name ...
DESCRIPTION
The pack command attempts to store the specified files in a
compressed form. Wherever possible (and useful), each input
file name is replaced by a packed file name.z with the same
access modes, access and modified dates, and owner as those
of name. The -f option will force packing of name. This is
useful for causing an entire directory to be packed even if
some of the files will not benefit. If pack is successful,
name will be removed. Packed files can be restored to their
original form using unpack or pcat.
The pack command uses Huffman (minimum redundancy) codes on
a byte-by-byte basis. If the - argument is used, an inter-
nal flag is set that causes the number of times each byte is
used, its relative frequency, and the code for the byte to
be printed on the standard output. 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 distribu-
tion is very skewed, which may occur with printer plots or
pictures.
Typically, text files are reduced to 60-75% of their origi-
nal 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.
The pack command 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 file name has more than 12 characters;
the file has links;
the file is a directory;
the file cannot be opened;
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PACK(1) INTERACTIVE UNIX System PACK(1)
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 file name must contain no more than
12 characters to allow space for the appended .z extension.
Directories cannot be compressed.
The pcat command does for packed files what cat(1) does for
ordinary files, except that pcat cannot be used as a filter.
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), use the command:
pcat name >nnn
The pcat command returns the number of files it was unable
to unpack. Failure may occur if:
the file name (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 for the following:
a file with the ``unpacked'' name already exists;
if the unpacked file cannot be created.
SEE ALSO
cat(1).
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