freeze (1) Linux Manual Page
freeze, unfreeze, melt, fcat – compress and uncompress files
Synopsis
freeze [ -cdfvVgx ] [ filename | type … ]unfreeze [ -cfvV ] [ filename … ]
melt [ -cfvV ] [ filename … ]
fcat [ filename … ]
Description
Compresses the specified files or standard input. Each file is replaced by a file with the extension .F, but only if the file got smaller. If no files are specified, the compression is applied to the standard input and is written to standard output regardless of the results. Compressed files can be restored to their original form by specifying the -d option, or by running melt or unfreeze (both linked to freeze), on the .F files or the standard input.If the output file exists, it will not be overwritten unless the -f flag is given. If -f is not specified and freeze is run in the foreground, the user is prompted as to whether the file should be overwritten.
If the -g flag is given, a slightly less powerful (compression rate is 1.5% less), but somewhat faster heuristic is used. This flag can be used more than once (this mode is quite useful when freezing bitmaps) for additional speedup.
If you want to improve compression rate at the cost of speed, use -x flag. It means "maximum compression" (the speed may degrade substantially when freezing bitmaps).
If the -f flag is given, all files specified are replaced with .F files – even if the file didn’t get smaller.
When file names are given, the ownership (if run by root), modes, accessed and modified times are maintained between the file and its .F version. In this respect, freeze can be used for archival purposes, yet can still be used with make(1) after melting.
The -c option causes the results of the freeze/melt operation to be written to stdout; no files are changed. The fcat program is the same as specifying -c to melt (all files are unpacked and written to stdout).
The -v (verbose) option causes the diagnostics (at the end of each file processing) to be printed to stderr, and the -vv option causes the progress indicator to be drawn to the same place.
Type is a token preceded by a ‘+’ or a ‘–‘, which defines the type of following files in the command string. An explicite definition of the file’s type can give up to 2% of additional compression. The list of types is stored in file /usr/lib/freeze.cnf. Types may be abbreviated while not ambigious. You can also determine values for the static Huffman table by using a list of 8 numbers separated by commas instead of type.
Freeze uses the Lempel-Ziv algorithm on the first pass and the dynamic Huffman algorithm on the second one. The size of sliding window is 8K, and the maximum length of matched string is 256. The positions on the window are coded using a static Huffman table.
A two byte magic number is prepended to the file to ensure that neither melting of random text nor refreezing of already frozen text are attempted. In addition, the characteristics of the static Huffman table being used during freeze is written to the file so that these characteristics may be adapted to concrete conditions.
The amount of compression obtained depends on the size of the input file and the distribution of character substrings and their probabilities. Typically, text files, such as C programs, are reduced by 60-75%, executable files are reduced by 50%. Compression is generally much better than that achieved by LZW coding (as used in compress), or Huffman coding (pack), though takes more time to compute.
If the -V (version) flag is given, the program’s version number and compilation options are printed.
The exit status is normally 0; if the last file gets bigger after freezing, the exit status is 2; if an error occurs, the exit status is 1.