1COMPRESS(1) General Commands Manual COMPRESS(1)
2
6 compress, uncompress, zcat - compress and expand data (version 4.1)
7
9 compress [ -f ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ name ... ]
10 uncompress [ -f ] [ -v ] [ -c ] [ -V ] [ name ... ]
11 zcat [ -V ] [ name ... ]
12
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. Compress will only attempt to compress
19 regular files. In particular, it will ignore symbolic links. If a file
20 has multiple hard links, compress will refuse to compress it unless the
21 -f flag is given.
22 If -f is not given and compress is run in the foreground, the user is
24 prompted as to whether an existing file should be overwritten.
25 Compressed files can be restored to their original form using uncom‐
27 press or zcat.
28 uncompress takes a list of files on its command line and replaces each
30 file whose name ends with .Z and which begins with the correct magic
31 number with an uncompressed file without the .Z. The uncompressed file
32 will have the mode, ownership and timestamps of the compressed file.
33 The -c option makes compress/uncompress write to the standard output;
35 no files are changed.
36 zcat is identical to uncompress -c. zcat uncompresses either a list of
38 files on the command line or its standard input and writes the uncom‐
39 pressed data on standard output. zcat will uncompress files that have
40 the correct magic number whether they have a .Z suffix or not.
41 If the -r flag is specified, compress will operate recursively. If any
43 of the file names specified on the command line are directories, com‐
44 press will descend into the directory and compress all the files it
45 finds there.
46 The -V flag tells each of these programs to print its version and
48 patchlevel, along with any preprocessor flags specified during compila‐
49 tion, on stderr before doing any compression or uncompression.
50 Compress uses the modified Lempel-Ziv algorithm popularized in "A Tech‐
52 nique for High Performance Data Compression", Terry A. Welch, IEEE Com‐
53 puter, vol. 17, no. 6 (June 1984), pp. 8-19. Common substrings in the
54 file are first replaced by 9-bit codes 257 and up. When code 512 is
55 reached, the algorithm switches to 10-bit codes and continues to use
56 more bits until the limit specified by the -b flag is reached (default
57 16). Bits must be between 9 and 16. The default can be changed in the
58 source to allow compress to be run on a smaller machine.
59 After the bits limit is attained, compress periodically checks the com‐
61 pression ratio. If it is increasing, compress continues to use the
62 existing code dictionary. However, if the compression ratio decreases,
63 compress discards the table of substrings and rebuilds it from scratch.
64 This allows the algorithm to adapt to the next "block" of the file.
65 Note that the -b flag is omitted for uncompress, since the bits parame‐
67 ter specified during compression is encoded within the output, along
68 with a magic number to ensure that neither decompression of random data
69 nor recompression of compressed data is attempted.
70 The amount of compression obtained depends on the size of the input,
72 the number of bits per code, and the distribution of common substrings.
73 Typically, text such as source code or English is reduced by 50-60%.
74 Compression is generally much better than that achieved by Huffman cod‐
75 ing (as used in pack), or adaptive Huffman coding (compact), and takes
76 less time to compute.
77 Under the -v option, a message is printed yielding the percentage of
79 reduction for each file compressed.
80 Exit status is normally 0; if the last file is larger after (attempted)
82 compression, the status is 2; if an error occurs, exit status is 1.
83
85 pack(1), compact(1)
86
88 Usage: compress [-dfvcVr] [-b maxbits] [file ...]
89 Invalid options were specified on the command line.
90 Missing maxbits
91 Maxbits must follow -b.
92 file: not in compressed format
93 The file specified to uncompress has not been compressed.
94 file: compressed with xx bits, can only handle yy bits
95 File was compressed by a program that could deal with more bits
96 than the compress code on this machine. Recompress the file
97 with smaller bits.
98 file: already has .Z suffix -- no change
99 The file is assumed to be already compressed. Rename the file
100 and try again.
101 file: filename too long to tack on .Z
102 The file cannot be compressed because its name is longer than
103 12 characters. Rename and try again. This message does not
104 occur on BSD systems.
105 file already exists; do you wish to overwrite (y or n)?
106 Respond "y" if you want the output file to be replaced; "n" if
107 not.
108 uncompress: corrupt input
109 A SIGSEGV violation was detected which usually means that the
110 input file has been corrupted.
111 Compression: xx.xx%
112 Percentage of the input saved by compression. (Relevant only
113 for -v.)
114 -- not a regular file or directory: ignored
115 When the input file is not a regular file or directory, (e.g. a
116 symbolic link, socket, FIFO, device file), it is left unal‐
117 tered.
118 -- has xx other links: unchanged
119 The input file has links; it is left unchanged. See ln(1) for
120 more information. Use the -f flag to force compression of mul‐
121 tiply-linked files.
122 -- file unchanged
123 No savings is achieved by compression. The input remains vir‐
124 gin.
125
127 Although compressed files are compatible between machines with large
128 memory, -b12 should be used for file transfer to architectures with a
129 small process data space (64KB or less, as exhibited by the DEC PDP
130 series, the Intel 80286, etc.)
131 Invoking compress with a -r flag will occasionally cause it to produce
133 spurious error warnings of the form
134 "<filename>.Z already has .Z suffix - ignored"
136 These warnings can be ignored. See the comments in compress.c:compdir()
138 for an explanation.
139 local COMPRESS(1)