[apple/file_cmds.git] / compress / compress.1

.\"	$NetBSD: compress.1,v 1.6 1997/09/15 10:58:37 lukem Exp $
.\"
.\" Copyright (c) 1986, 1990, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" James A. Woods, derived from original work by Spencer Thomas
.\" and Joseph Orost.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)compress.1	8.2 (Berkeley) 4/18/94
.\"
.Dd April 18, 1994
.Dt COMPRESS 1
.Os BSD 4.3
.Sh NAME
.Nm compress ,
.\".Nm uncompress ,
.Nm uncompress
.\".Nm zcat
.Nd compress and expand data
.Sh SYNOPSIS
.Nm
.Op Fl cfv
.Op Fl b Ar bits
.Op Ar
.Nm uncompress
.Op Fl cfv
.Op Ar 
.\".Nm zcat
.\".Op Ar
.Sh DESCRIPTION
.Nm
reduces the size of the named files using adaptive Lempel-Ziv coding.
Each
.Ar file
is renamed to the same name plus the extension
.Dq .Z .
As many of the modification time, access time, file flags, file mode,
user ID, and group ID as allowed by permissions are retained in the
new file.
If compression would not reduce the size of a
.Ar file ,
the file is ignored.
.Pp
.Nm uncompress
restores the compressed files to their original form, renaming the
files by deleting the
.Dq .Z
extension.
.\".Pp
.\".Nm Zcat
.\"is an alias for
.\".Dq "uncompress -c" .
.Pp
If renaming the files would cause files to be overwritten and the standard
input device is a terminal, the user is prompted (on the standard error
output) for confirmation.
If prompting is not possible or confirmation is not received, the files
are not overwritten.
.Pp
If no files are specified, the standard input is compressed or uncompressed
to the standard output.
If either the input and output files are not regular files, the checks for
reduction in size and file overwriting are not performed, the input file is
not removed, and the attributes of the input file are not retained.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl b
Specify the
.Ar bits
code limit (see below).
.It Fl c
Compressed or uncompressed output is written to the standard output.
No files are modified.
.It Fl f
Force compression of
.Ar file ,
even if it is not actually reduced in size.
Additionally, files are overwritten without prompting for confirmation.
.It Fl v
Print the percentage reduction of each file.
.El
.Pp
.Nm
uses a modified Lempel-Ziv algorithm.
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
.Fl b
flag is reached (the default is 16).
.Ar Bits
must be between 9 and 16.
.Pp
After the
.Ar bits
limit is reached,
.Nm
periodically checks the compression ratio.
If it is increasing,
.Nm
continues to use the existing code dictionary.
However, if the compression ratio decreases,
.Nm
discards the table of substrings and rebuilds it from scratch.  This allows
the algorithm to adapt to the next "block" of the file.
.Pp
The
.Fl b
flag is omitted for
.Ar uncompress
since the
.Ar bits
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.
.Pp
.ne 8
The amount of compression obtained depends on the size of the
input, the number of
.Ar bits
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 the historical command pack), or adaptive Huffman
coding (as used in the historical command compact), and takes less
time to compute.
.Pp
The
.Nm
utility exits 0 on success, and >0 if an error occurs.
.Sh SEE ALSO
.Xr zcat 1
.Rs
.%A Welch, Terry A.
.%D June, 1984
.%T "A Technique for High Performance Data Compression"
.%J "IEEE Computer"
.%V 17:6
.%P pp. 8-19
.Re
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 .
Commit	Line	Data
44a7a5ab A	1	.\" $NetBSD: compress.1,v 1.6 1997/09/15 10:58:37 lukem Exp $
	2	.\"
	3	.\" Copyright (c) 1986, 1990, 1993
	4	.\" The Regents of the University of California. All rights reserved.
	5	.\"
	6	.\" This code is derived from software contributed to Berkeley by
	7	.\" James A. Woods, derived from original work by Spencer Thomas
	8	.\" and Joseph Orost.
	9	.\"
	10	.\" Redistribution and use in source and binary forms, with or without
	11	.\" modification, are permitted provided that the following conditions
	12	.\" are met:
	13	.\" 1. Redistributions of source code must retain the above copyright
	14	.\" notice, this list of conditions and the following disclaimer.
	15	.\" 2. Redistributions in binary form must reproduce the above copyright
	16	.\" notice, this list of conditions and the following disclaimer in the
	17	.\" documentation and/or other materials provided with the distribution.
	18	.\" 3. All advertising materials mentioning features or use of this software
	19	.\" must display the following acknowledgement:
	20	.\" This product includes software developed by the University of
	21	.\" California, Berkeley and its contributors.
	22	.\" 4. Neither the name of the University nor the names of its contributors
	23	.\" may be used to endorse or promote products derived from this software
	24	.\" without specific prior written permission.
	25	.\"
	26	.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	27	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	28	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	29	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	30	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	31	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	32	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	33	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	34	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	35	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	36	.\" SUCH DAMAGE.
	37	.\"
	38	.\" @(#)compress.1 8.2 (Berkeley) 4/18/94
	39	.\"
	40	.Dd April 18, 1994
	41	.Dt COMPRESS 1
	42	.Os BSD 4.3
	43	.Sh NAME
	44	.Nm compress ,
	45	.\".Nm uncompress ,
	46	.Nm uncompress
	47	.\".Nm zcat
	48	.Nd compress and expand data
	49	.Sh SYNOPSIS
	50	.Nm
	51	.Op Fl cfv
	52	.Op Fl b Ar bits
	53	.Op Ar
	54	.Nm uncompress
	55	.Op Fl cfv
	56	.Op Ar
	57	.\".Nm zcat
	58	.\".Op Ar
	59	.Sh DESCRIPTION
	60	.Nm
	61	reduces the size of the named files using adaptive Lempel-Ziv coding.
	62	Each
	63	.Ar file
	64	is renamed to the same name plus the extension
65	.Dq .Z .
66	As many of the modification time, access time, file flags, file mode,
67	user ID, and group ID as allowed by permissions are retained in the
68	new file.
69	If compression would not reduce the size of a
70	.Ar file ,
71	the file is ignored.
72	.Pp
73	.Nm uncompress
74	restores the compressed files to their original form, renaming the
75	files by deleting the
76	.Dq .Z
77	extension.
78	.\".Pp
79	.\".Nm Zcat
80	.\"is an alias for
81	.\".Dq "uncompress -c" .
82	.Pp
83	If renaming the files would cause files to be overwritten and the standard
84	input device is a terminal, the user is prompted (on the standard error
85	output) for confirmation.
86	If prompting is not possible or confirmation is not received, the files
87	are not overwritten.
88	.Pp
89	If no files are specified, the standard input is compressed or uncompressed
90	to the standard output.
91	If either the input and output files are not regular files, the checks for
92	reduction in size and file overwriting are not performed, the input file is
93	not removed, and the attributes of the input file are not retained.
94	.Pp
95	The options are as follows:
96	.Bl -tag -width Ds
97	.It Fl b
98	Specify the
99	.Ar bits
100	code limit (see below).
101	.It Fl c
102	Compressed or uncompressed output is written to the standard output.
103	No files are modified.
104	.It Fl f
105	Force compression of
106	.Ar file ,
107	even if it is not actually reduced in size.
108	Additionally, files are overwritten without prompting for confirmation.
109	.It Fl v
110	Print the percentage reduction of each file.
111	.El
112	.Pp
113	.Nm
114	uses a modified Lempel-Ziv algorithm.
115	Common substrings in the file are first replaced by 9-bit codes 257 and up.
116	When code 512 is reached, the algorithm switches to 10-bit codes and
117	continues to use more bits until the
118	limit specified by the
119	.Fl b
120	flag is reached (the default is 16).
121	.Ar Bits
122	must be between 9 and 16.
123	.Pp
124	After the
125	.Ar bits
126	limit is reached,
127	.Nm
128	periodically checks the compression ratio.
129	If it is increasing,
130	.Nm
131	continues to use the existing code dictionary.
132	However, if the compression ratio decreases,
133	.Nm
134	discards the table of substrings and rebuilds it from scratch. This allows
135	the algorithm to adapt to the next "block" of the file.
136	.Pp
137	The
138	.Fl b
139	flag is omitted for
140	.Ar uncompress
141	since the
142	.Ar bits
143	parameter specified during compression
144	is encoded within the output, along with
145	a magic number to ensure that neither decompression of random data nor
146	recompression of compressed data is attempted.
147	.Pp
148	.ne 8
149	The amount of compression obtained depends on the size of the
150	input, the number of
151	.Ar bits
152	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
155	coding (as used in the historical command pack), or adaptive Huffman
156	coding (as used in the historical command compact), and takes less
157	time to compute.
158	.Pp
159	The
160	.Nm
161	utility exits 0 on success, and >0 if an error occurs.
162	.Sh SEE ALSO
163	.Xr zcat 1
164	.Rs
165	.%A Welch, Terry A.
166	.%D June, 1984
167	.%T "A Technique for High Performance Data Compression"
168	.%J "IEEE Computer"
169	.%V 17:6
170	.%P pp. 8-19
171	.Re
172	.Sh HISTORY
173	The
174	.Nm
175	command appeared in
176	.Bx 4.3 .