1COMPRESS(1P) POSIX Programmer's Manual COMPRESS(1P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 compress - compress data
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15 compress [-fv][-b bits][file ...]
16 compress [-cfv][-b bits][file]
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21 The compress utility shall attempt to reduce the size of the named
22 files by using adaptive Lempel-Ziv coding algorithm.
23 Note: Lempel-Ziv is US Patent 4464650, issued to William Eastman,
25 Abraham Lempel, Jacob Ziv, Martin Cohn on August 7th, 1984, and
26 assigned to Sperry Corporation.
27 Lempel-Ziv-Welch compression is covered by US Patent 4558302, issued to
29 Terry A. Welch on December 10th, 1985, and assigned to Sperry Corpora‐
30 tion.
31 On systems not supporting adaptive Lempel-Ziv coding algorithm, the
33 input files shall not be changed and an error value greater than two
34 shall be returned. Except when the output is to the standard output,
35 each file shall be replaced by one with the extension .Z. If the invok‐
36 ing process has appropriate privileges, the ownership, modes, access
37 time, and modification time of the original file are preserved. If
38 appending the .Z to the filename would make the name exceed {NAME_MAX}
39 bytes, the command shall fail. If no files are specified, the standard
40 input shall be compressed to the standard output.
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43 The compress utility shall conform to the Base Definitions volume of
44 IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
45 The following options shall be supported:
47 -b bits
49 Specify the maximum number of bits to use in a code. For a con‐
50 forming application, the bits argument shall be:
51 9 <= bits <= 14
54 The implementation may allow bits values of greater than 14. The
56 default is 14, 15, or 16.
57 -c Cause compress to write to the standard output; the input file
59 is not changed, and no .Z files are created.
60 -f Force compression of file, even if it does not actually reduce
62 the size of the file, or if the corresponding file .Z file
63 already exists. If the -f option is not given, and the process
64 is not running in the background, the user is prompted as to
65 whether an existing file .Z file should be overwritten.
66 -v Write the percentage reduction of each file to standard error.
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71 The following operand shall be supported:
72 file A pathname of a file to be compressed.
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77 The standard input shall be used only if no file operands are speci‐
78 fied, or if a file operand is '-' .
79
81 If file operands are specified, the input files contain the data to be
82 compressed.
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85 The following environment variables shall affect the execution of com‐
86 press:
87 LANG Provide a default value for the internationalization variables
89 that are unset or null. (See the Base Definitions volume of
90 IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
91 ables for the precedence of internationalization variables used
92 to determine the values of locale categories.)
93 LC_ALL If set to a non-empty string value, override the values of all
95 the other internationalization variables.
96 LC_CTYPE
98 Determine the locale for the interpretation of sequences of
99 bytes of text data as characters (for example, single-byte as
100 opposed to multi-byte characters in arguments).
101 LC_MESSAGES
103 Determine the locale that should be used to affect the format
104 and contents of diagnostic messages written to standard error.
105 NLSPATH
107 Determine the location of message catalogs for the processing of
108 LC_MESSAGES .
109
112 Default.
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115 If no file operands are specified, or if a file operand is '-', or if
116 the -c option is specified, the standard output contains the compressed
117 output.
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120 The standard error shall be used only for diagnostic and prompt mes‐
121 sages and the output from -v.
122
124 The output files shall contain the compressed output. The format of
125 compressed files is unspecified and interchange of such files between
126 implementations (including access via unspecified file sharing mecha‐
127 nisms) is not required by IEEE Std 1003.1-2001.
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130 None.
131
133 The following exit values shall be returned:
134 0 Successful completion.
136 1 An error occurred.
138 2 One or more files were not compressed because they would have
140 increased in size (and the -f option was not specified).
141 >2 An error occurred.
143
146 The input file shall remain unmodified.
147 The following sections are informative.
149
151 The amount of compression obtained depends on the size of the input,
152 the number of bits per code, and the distribution of common substrings.
153 Typically, text such as source code or English is reduced by 50-60%.
154 Compression is generally much better than that achieved by Huffman cod‐
155 ing or adaptive Huffman coding ( compact), and takes less time to com‐
156 pute.
157 Although compress strictly follows the default actions upon receipt of
159 a signal or when an error occurs, some unexpected results may occur. In
160 some implementations it is likely that a partially compressed file is
161 left in place, alongside its uncompressed input file. Since the general
162 operation of compress is to delete the uncompressed file only after the
163 .Z file has been successfully filled, an application should always
164 carefully check the exit status of compress before arbitrarily deleting
165 files that have like-named neighbors with .Z suffixes.
166 The limit of 14 on the bits option-argument is to achieve portability
168 to all systems (within the restrictions imposed by the lack of an
169 explicit published file format). Some implementations based on 16-bit
170 architectures cannot support 15 or 16-bit uncompression.
171
173 None.
174
176 None.
177
179 None.
180
182 uncompress, zcat
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185 Portions of this text are reprinted and reproduced in electronic form
186 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
187 -- Portable Operating System Interface (POSIX), The Open Group Base
188 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
189 Electrical and Electronics Engineers, Inc and The Open Group. In the
190 event of any discrepancy between this version and the original IEEE and
191 The Open Group Standard, the original IEEE and The Open Group Standard
192 is the referee document. The original Standard can be obtained online
193 at http://www.opengroup.org/unix/online.html .
194IEEE/The Open Group 2003 COMPRESS(1P)