CCCCOOOOMMMMPPPPRRRREEEESSSSSSSS((((1111)))) UUUUNNNNIIIIXXXX 5555....0000 ((((llllooooccccaaaallll)))) CCCCOOOOMMMMPPPPRRRREEEESSSSSSSS((((1111)))) NNNNAAAAMMMMEEEE compress, uncompress, zcat - compress and expand data SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS ccccoooommmmpppprrrreeeessssssss [ ----ffff ] [ ----vvvv ] [ ----cccc ] [ ----VVVV ] [ ----bbbb _b_i_t_s ] [ _n_a_m_e ... ] uuuunnnnccccoooommmmpppprrrreeeessssssss [ ----ffff ] [ ----vvvv ] [ ----cccc ] [ ----VVVV ] [ _n_a_m_e ... ] zzzzccccaaaatttt [ ----VVVV ] [ _n_a_m_e ... ] DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN _C_o_m_p_r_e_s_s reduces the size of the named files using adaptive Lempel-Ziv coding. Whenever possible, each file is replaced by one with the extension ....ZZZZ,,,, while keeping the same ownership modes, access and modification times. If no files are specified, the standard input is compressed to the standard output. Compressed files can be restored to their original form using _u_n_c_o_m_p_r_e_s_s or _z_c_a_t. The ----ffff option will force compression of _n_a_m_e. This is useful for compressing an entire directory, even if some of the files do not actually shrink. If ----ffff is not given and _c_o_m_p_r_e_s_s is run in the foreground, the user is prompted as to whether an existing file should be overwritten. The ----cccc option makes _c_o_m_p_r_e_s_s/_u_n_c_o_m_p_r_e_s_s write to the standard output; no files are changed. The nondestructive behavior of _z_c_a_t is identical to that of _u_n_c_o_m_p_r_e_s_s ----cccc.... _C_o_m_p_r_e_s_s uses the modified Lempel-Ziv algorithm popularized in "A Technique for High Performance Data Compression", Terry A. Welch, _I_E_E_E _C_o_m_p_u_t_e_r, vol. 17, no. 6 (June 1984), pp. 8-19. Common substrings in the file are first replaced by 9-bit codes 257 and up. When code 512 is reached, the algorithm switches to 10-bit codes and continues to use more bits until the limit specified by the ----bbbb flag is reached (default 16). _B_i_t_s must be between 9 and 16. The default can be changed in the source to allow _c_o_m_p_r_e_s_s to be run on a smaller machine. After the _b_i_t_s limit is attained, _c_o_m_p_r_e_s_s periodically checks the compression ratio. If it is increasing, _c_o_m_p_r_e_s_s continues to use the existing code dictionary. However, if the compression ratio decreases, _c_o_m_p_r_e_s_s discards the table of substrings and rebuilds it from scratch. This allows the algorithm to adapt to the next "block" of the file. Note that the ----bbbb flag is omitted for _u_n_c_o_m_p_r_e_s_s, since the _b_i_t_s parameter specified during compression is encoded within the output, along with a magic number to ensure that neither decompression of random data nor recompression of compressed data is attempted. Page 1 (printed 1/1/86) CCCCOOOOMMMMPPPPRRRREEEESSSSSSSS((((1111)))) UUUUNNNNIIIIXXXX 5555....0000 ((((llllooooccccaaaallll)))) CCCCOOOOMMMMPPPPRRRREEEESSSSSSSS((((1111)))) The amount of compression obtained depends on the size of the input, the number of _b_i_t_s per code, and the distribution of common substrings. Typically, text such as source code or English is reduced by 50-60%. Compression is generally much better than that achieved by Huffman coding (as used in _p_a_c_k), or adaptive Huffman coding (_c_o_m_p_a_c_t), and takes less time to compute. Under the ----vvvv option, a message is printed yielding the percentage of reduction for each file compressed. If the ----VVVV option is specified, the current version and compile options are printed on stderr. Exit status is normally 0; if the last file is larger after (attempted) compression, the status is 2; if an error occurs, exit status is 1. SSSSEEEEEEEE AAAALLLLSSSSOOOO pack(1), compact(1) DDDDIIIIAAAAGGGGNNNNOOOOSSSSTTTTIIIICCCCSSSS Usage: compress [-dfvcV] [-b maxbits] [file ...] Invalid options were specified on the command line. Missing maxbits Maxbits must follow ----bbbb. _f_i_l_e: not in compressed format The file specified to _u_n_c_o_m_p_r_e_s_s has not been compressed. _f_i_l_e: compressed with _x_x bits, can only handle _y_y bits _F_i_l_e was compressed by a program that could deal with more _b_i_t_s than the compress code on this machine. Recompress the file with smaller _b_i_t_s. _f_i_l_e: already has .Z suffix -- no change The file is assumed to be already compressed. Rename the file and try again. _f_i_l_e: filename too long to tack on .Z The file cannot be compressed because its name is longer than 12 characters. Rename and try again. This message does not occur on BSD systems. _f_i_l_e already exists; do you wish to overwrite (y or n)? Respond "y" if you want the output file to be replaced; "n" if not. uncompress: corrupt input A SIGSEGV violation was detected which usually means that the input file has been corrupted. Compression: _x_x._x_x% Percentage of the input saved by compression. (Relevant only for ----vvvv.) -- not a regular file: unchanged When the input file is not a regular file, (e.g. a directory), it is left unaltered. Page 2 (printed 1/1/86) CCCCOOOOMMMMPPPPRRRREEEESSSSSSSS((((1111)))) UUUUNNNNIIIIXXXX 5555....0000 ((((llllooooccccaaaallll)))) CCCCOOOOMMMMPPPPRRRREEEESSSSSSSS((((1111)))) -- has _x_x other links: unchanged The input file has links; it is left unchanged. See _l_n(1) for more information. -- file unchanged No savings is achieved by compression. The input remains virgin. BBBBUUUUGGGGSSSS Although compressed files are compatible between machines with large memory, ----bbbb12 should be used for file transfer to architectures with a small process data space (64KB or less, as exhibited by the DEC PDP series, the Intel 80286, etc.) Page 3 (printed 1/1/86)