-.\" $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.
.\"
.\" 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.
.\" SUCH DAMAGE.
.\"
.\" @(#)compress.1 8.2 (Berkeley) 4/18/94
+.\" $FreeBSD: src/usr.bin/compress/compress.1,v 1.23 2010/12/11 08:32:16 joel Exp $
.\"
-.Dd April 18, 1994
+.Dd May 17, 2002
.Dt COMPRESS 1
-.Os BSD 4.3
+.Os
.Sh NAME
.Nm compress ,
-.\".Nm uncompress ,
.Nm uncompress
-.\".Nm zcat
.Nd compress and expand data
.Sh SYNOPSIS
.Nm
-.Op Fl cfv
+.Op Fl fv
+.Op Fl b Ar bits
+.Op Ar
+.Nm
+.Fl c
.Op Fl b Ar bits
+.Op Ar file
+.Nm uncompress
+.Op Fl fv
.Op Ar
.Nm uncompress
-.Op Fl cfv
-.Op Ar
-.\".Nm zcat
-.\".Op Ar
+.Fl c
+.Op Ar file
.Sh DESCRIPTION
+The
.Nm
-reduces the size of the named files using adaptive Lempel-Ziv coding.
+utility reduces the size of 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.
+.Pa .Z .
+A
+.Ar file
+argument with a
+.Pa .Z
+extension will be ignored except it will cause an
+error exit after other arguments are processed.
If compression would not reduce the size of a
.Ar file ,
the file is ignored.
.Pp
+The
.Nm uncompress
-restores the compressed files to their original form, renaming the
+utility restores compressed files to their original form, renaming the
files by deleting the
-.Dq .Z
+.Pa .Z
+extensions.
+A file specification need not include the file's
+.Pa .Z
extension.
-.\".Pp
-.\".Nm Zcat
-.\"is an alias for
-.\".Dq "uncompress -c" .
+If a file's name in its file system does not have a
+.Pa .Z
+extension, it will not be uncompressed and it will cause
+an error exit after other arguments are processed.
.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
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.
+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.
+.Pp
+If no files are specified or a
+.Ar file
+argument is a single dash
+.Pq Sq Fl ,
+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.
+not removed, and the attributes of the input file are not retained
+in the output file.
.Pp
The options are as follows:
-.Bl -tag -width Ds
-.It Fl b
-Specify the
-.Ar bits
-code limit (see below).
+.Bl -tag -width ".Fl b Ar bits"
+.It Fl b Ar bits
+The code size (see below) is limited to
+.Ar bits ,
+which must be in the range 9..16.
+The default is 16.
.It Fl c
Compressed or uncompressed output is written to the standard output.
No files are modified.
+The
+.Fl v
+option is ignored.
+Compression is attempted even if the results will be larger than the
+original.
.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.
+Files are overwritten without prompting for confirmation.
+Also, for
+.Nm compress ,
+files are compressed even if they are not actually reduced in size.
.It Fl v
Print the percentage reduction of each file.
+Ignored by
+.Nm uncompress
+or if the
+.Fl c
+option is also used.
.El
.Pp
+The
.Nm
-uses a modified Lempel-Ziv algorithm.
+utility 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.
+option or its default is reached.
.Pp
-After the
-.Ar bits
-limit is reached,
+After the limit is reached,
.Nm
periodically checks the compression ratio.
If it is increasing,
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
+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
+option is unavailable for
+.Nm uncompress
since the
.Ar bits
parameter specified during compression
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
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.
+.Sh EXIT STATUS
+.Ex -std compress uncompress
.Pp
The
-.Nm
-utility exits 0 on success, and >0 if an error occurs.
+.Nm compress
+utility exits 2 if attempting to compress a file would not reduce its size
+and the
+.Fl f
+option was not specified and if no other error occurs.
.Sh SEE ALSO
-.Xr zcat 1
+.Xr gunzip 1 ,
+.Xr gzexe 1 ,
+.Xr gzip 1 ,
+.Xr zcat 1 ,
+.Xr zmore 1 ,
+.Xr znew 1
.Rs
.%A Welch, Terry A.
.%D June, 1984
.%V 17:6
.%P pp. 8-19
.Re
+.Sh STANDARDS
+The
+.Nm compress
+and
+.Nm uncompress
+utilities conform to
+.St -p1003.1-2001 .
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 .
+.Sh BUGS
+Some of these might be considered otherwise-undocumented features.
+.Pp
+.Nm compress :
+If the utility does not compress a file because doing so would not
+reduce its size, and a file of the same name except with an
+.Pa .Z
+extension exists, the named file is not really ignored as stated above;
+it causes a prompt to confirm the overwriting of the file with the extension.
+If the operation is confirmed, that file is deleted.
+.Pp
+.Nm uncompress :
+If an empty file is compressed (using
+.Fl f ) ,
+the resulting
+.Pa .Z
+file is also empty.
+That seems right, but if
+.Nm uncompress
+is then used on that file, an error will occur.
+.Pp
+Both utilities: If a
+.Sq Fl
+argument is used and the utility prompts the user, the standard input
+is taken as the user's reply to the prompt.
+.Pp
+Both utilities:
+If the specified file does not exist, but a similarly-named one with (for
+.Nm compress )
+or without (for
+.Nm uncompress )
+a
+.Pa .Z
+extension does exist, the utility will waste the user's time by not
+immediately emitting an error message about the missing file and
+continuing.
+Instead, it first asks for confirmation to overwrite
+the existing file and then does not overwrite it.
projects / apple / file_cmds.git / blobdiff