compact.1

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)compact.1	8.1	9/11/90
.TH compact 1
.SH Name
compact, uncompact, ccat \- compress and uncompress files, and cat them
.SH Syntax
.B compact
[\|\fIname...\fR\|]
.br
.B uncompact
[\|\fIname...\fR\|]
.br
.B ccat
[\|\fIfile...\fR\|]
.SH Description
.NXR "compact command"
.NXR "uncompact command"
.NXR "ccat command"
.NXR "file" "compressing"
.NXR "file" "displaying
The
.PN compact
command
compresses the named files using an adaptive Huffman code.  If no file
names are given, the standard input is compacted to the standard output.
The
.PN compact
command
operates as an on-line algorithm.  Each time a byte is read,
it is encoded immediately according to the current prefix code.
This code is an optimal Huffman code for the set of frequencies seen so far.
It is unnecessary to prepend a decoding tree to the compressed file
since the encoder and the decoder start in the same state and stay
synchronized.  Furthermore,
.PN compact
and
.PN uncompact
can operate as filters.  In particular,
.EX
	... | compact | uncompact | ...
.EE
operates as a (very slow) no-op.
.PP
When an argument
.I file
is given, it is compacted and the resulting file is placed in
.I file.C;
.I file
is unlinked.  The first two bytes of the compacted file code the
fact that the file is compacted.  This
code is used to prohibit recompaction.
.PP
The amount of compression to be
expected depends on the type of file being
compressed.  Typical values of compression are:
Text (38%), Pascal Source (43%), C Source (36%) and Binary (19%).
These values are the percentages of file bytes reduced.
.PP
The
.PN uncompact
command
restores the original file from a file compressed by
.PN compact .
If no file names are given, the standard input is uncompacted to
the standard output.
.PP
The
.PN ccat
command
cats the original file from a file compressed by
.PN compact ,
without uncompressing the file.
.PP
The 
.PN compact
command is present only for compatibility.  In general, the
.MS compress 1
command runs faster and gives better compression.
.SH Restrictions
The last segment of the file name
must contain fewer than thirteen characters
to allow space for the appended '.C'.
.SH Files
.TP 7
.PN *.C
compacted file created by compact, removed by uncompact
.SH See Also
compress(1)