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 removed.

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 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.

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:

the file appears to be already packed
the file name is too long to add the .z suffix
the file has links
the file is a directory
the file cannot be opened
the file is empty
no disk storage blocks are saved by packing
a file called file.z already exists
the .z file cannot be created
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:

the file cannot be opened;
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:

a file with the unpacked name already exists;
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 pcat.

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

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(5) for descriptions of the following environment variables that affect the execution of pack, pcat, and unpack: LC_CTYPE, LC_MESSAGES, and NLSPATH.

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(5) for descriptions of the following attributes:

CSI Enabled

cat(1), compress(1), zcat(1), fgetattr(3C), fsetattr(3C), attributes(5), environ(5), largefile(5)
February 5, 2020