PACK(1) User Commands PACK(1)


pack, pcat, unpack - compress and expand files


pack [-f/] [-] file...

pcat file...

unpack [-/] file...


The pack command attempts to store the specified files in a compressed
form. Wherever possible (and useful), each input file file is replaced by
a packed file file.z with the same access modes, access and modified
dates, and owner as those of file. If pack is successful, file is

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 skewed, which can occur with printer plots or

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.

The pack utility returns a value that is the number of files that it
failed to compress. If that number exceeds 255, 255 is returned.

No packing occurs if:

o the file appears to be already packed

o the file name is too long to add the .z suffix

o the file has links

o the file is a directory

o the file cannot be opened

o the file is empty

o no disk storage blocks are saved by packing

o a file called file.z already exists

o the .z file cannot be created

o an I/O error occurred during processing.

The last segment of the file name must be short enough 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.

pcat returns the number of files it was unable to unpack. Failure can
occur if:

o the file cannot be opened;

o the file does not appear to be the output of pack.

The unpack command expands files created by pack. For each file specified
in the command, a search is made for a file called file.z (or just file,
if file 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 can occur for the same reasons that it can in pcat, as
well as for the following:

o a file with the unpacked name already exists;

o the unpacked file cannot be created.


The following options are supported by pack:

Forces packing of file. This is useful for causing an entire
directory to be packed even if some of the files do not benefit.
Packed files can be restored to their original form using unpack or

The following options are supported by pack and unpack:

When packing or unpacking, copies any ACL and extended system
attributes associated with the source file to the target file. If
an ACL or extended system attributes cannot be copied, the original
file is retained, a diagnostic message is written to stderr, and
the final exit status is non-zero.


The following operands are supported:

A path name of a file to be packed, unpacked, or pcated; file can
include or omit the .z suffix.

pack uses Huffman (minimum redundancy) codes on a byte-by-byte
basis. If the - argument is used, an internal 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 file
causes the internal flag to be set and reset.


See largefile(7) for the description of the behavior of pack, pcat, and
unpack when encountering files greater than or equal to 2 Gbyte (2^31


Example 1: Viewing a Packed File

To view a packed file named file.z use:

example% pcat file.z

or just:

example% pcat file

Example 2: Making an Unpacked Copy:

To make an unpacked copy, say nnn, of a packed file named file.z (without
destroying file.z) use the command:

example% pcat file >nnn


See environ(7) for descriptions of the following environment variables
that affect the execution of pack, pcat, and unpack: LC_CTYPE,


The following exit values are returned:

Successful completion.

An error occurred. The number of files the command failed to
pack/unpack is returned. If the number of failures exceeds 255,
then 255 is returned.


See attributes(7) for descriptions of the following attributes:

|CSI | Enabled |


cat(1), compress(1), zcat(1), fgetattr(3C), fsetattr(3C), attributes(7),
environ(7), largefile(7)

February 5, 2020 PACK(1)