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 will be 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 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 file name has more than 14 - 2 bytes
• the file has links
• the file is a directory
• the file cannot be opened
• the file is empty
• no disk storage blocks will be 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 contain no more than 14 - 2 bytes 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 may 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 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;
• the unpacked file cannot be created.
• the filename (excluding the .z extension) has more than 14 bytes.

To view a packed file named file.z use:

example% pcat file.z

or just:

example% pcat file

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