1COMPRESS(1) General Commands Manual COMPRESS(1)
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6 compress, uncompress, zcat - compress and expand data
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9 compress [ -f ] [ -v ] [ -c ] [ -b bits ] [ name ... ]
10 uncompress [ -f ] [ -v ] [ -c ] [ name ... ]
11 zcat [ name ... ]
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14 Compress reduces the size of the named files using adaptive Lempel-Ziv
15 coding. Whenever possible, each file is replaced by one with the
16 extension .Z, while keeping the same ownership modes, access and modi‐
17 fication times. If no files are specified, the standard input is com‐
18 pressed to the standard output. Compressed files can be restored to
19 their original form using uncompress or zcat.
20 The -f option will force compression of name, even if it does not actu‐
22 ally shrink or the corresponding name.Z file already exists. Except
23 when run in the background under /bin/sh, if -f is not given the user
24 is prompted as to whether an existing name.Z file should be overwrit‐
25 ten.
26 The -c (``cat'') option makes compress/uncompress write to the standard
28 output; no files are changed. The nondestructive behavior of zcat is
29 identical to that of uncompress -c.
30 Compress uses the modified Lempel-Ziv algorithm popularized in "A Tech‐
32 nique for High Performance Data Compression", Terry A. Welch, IEEE Com‐
33 puter, vol. 17, no. 6 (June 1984), pp. 8-19. Common substrings in the
34 file are first replaced by 9-bit codes 257 and up. When code 512 is
35 reached, the algorithm switches to 10-bit codes and continues to use
36 more bits until the limit specified by the -b flag is reached (default
37 16). Bits must be between 9 and 16. The default can be changed in the
38 source to allow compress to be run on a smaller machine.
39 After the bits limit is attained, compress periodically checks the com‐
41 pression ratio. If it is increasing, compress continues to use the
42 existing code dictionary. However, if the compression ratio decreases,
43 compress discards the table of substrings and rebuilds it from scratch.
44 This allows the algorithm to adapt to the next "block" of the file.
45 Note that the -b flag is omitted for uncompress, since the bits parame‐
47 ter specified during compression is encoded within the output, along
48 with a magic number to ensure that neither decompression of random data
49 nor recompression of compressed data is attempted.
50 The amount of compression obtained depends on the size of the input,
52 the number of bits per code, and the distribution of common substrings.
53 Typically, text such as source code or English is reduced by 50-60%.
54 Compression is generally much better than that achieved by Huffman cod‐
55 ing (as used in pack), or adaptive Huffman coding (compact), and takes
56 less time to compute.
57 The -v option causes the printing of the percentage reduction of each
59 file.
60 If an error occurs, exit status is 1, else if the last file was not
62 compressed because it became larger, the status is 2; else the status
63 is 0.
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66 Usage: compress [-fvc] [-b maxbits] [file ...]
67 Invalid options were specified on the command line.
68 Missing maxbits
69 Maxbits must follow -b.
70 file: not in compressed format
71 The file specified to uncompress has not been compressed.
72 file: compressed with xx bits, can only handle yy bits
73 File was compressed by a program that could deal with more bits
74 than the compress code on this machine. Recompress the file
75 with smaller bits.
76 file: already has .Z suffix -- no change
77 The file is assumed to be already compressed. Rename the file
78 and try again.
79 file: filename too long to tack on .Z
80 The file cannot be compressed because its name is longer than
81 12 characters. Rename and try again. This message does not
82 occur on BSD systems.
83 file already exists; do you wish to overwrite (y or n)?
84 Respond "y" if you want the output file to be replaced; "n" if
85 not.
86 uncompress: corrupt input
87 A SIGSEGV violation was detected which usually means that the
88 input file is corrupted.
89 Compression: xx.xx%
90 Percentage of the input saved by compression. (Relevant only
91 for -v.)
92 -- not a regular file: unchanged
93 When the input file is not a regular file, (e.g. a directory),
94 it is left unaltered.
95 -- has xx other links: unchanged
96 The input file has links; it is left unchanged. See ln(1) for
97 more information.
98 -- file unchanged
99 No savings is achieved by compression. The input remains vir‐
100 gin.
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103 Although compressed files are compatible between machines with large
104 memory, -b12 should be used for file transfer to architectures with a
105 small process data space (64KB or less, as exhibited by the DEC PDP
106 series, the Intel 80286, etc.)
107 compress should be more flexible about the existence of the `.Z' suf‐
109 fix.
1104.3 Berkeley Distribution May 11, 1986 COMPRESS(1)