Purpose of <> Post: 302607368

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Top Forums UNIX for Dummies Questions & Answers Purpose of <> Post 302607368 by drl on Wednesday 14th of March 2012 08:43:07 AM

03-14-2012

Registered User

Hi.

The purpose is as noted, opens for both input and output:

Code:

Opening File Descriptors for Reading and Writing
The redirection operator

[n]<>word

causes the file whose name is the expansion of word to be opened
for both reading and writing on file descriptor n, or on file
descriptor 0 if n is not specified.  If the file does not exist,
it is created. 

-- excerpt ffom man bash

I don't recall an instance when I needed such a construct, however, it is syntactically acceptable:

Code:

#!/usr/bin/env bash

# @(#) s1	Demonstrate <> re-direction operator.

pe() { for _i;do printf "%s" "$_i";done; printf "\n"; }
pl() { pe;pe "-----" ;pe "$*"; }
db() { ( printf " db, ";for _i;do printf "%s" "$_i";done;printf "\n" ) >&2 ; }
db() { : ; }
C=$HOME/bin/context && [ -f $C ] && . $C

rm -f f
pl " Create descriptor 4 and file f, show file:"
exec 4<>f
ls -lgG f

pl " Write to f:"
echo hi >f
ls -lgG f

pl " Read from f:"
cat <f

exit 0

producing:

Code:

% ./s1

Environment: LC_ALL = C, LANG = C
(Versions displayed with local utility "version")
OS, ker|rel, machine: Linux, 2.6.26-2-amd64, x86_64
Distribution        : Debian GNU/Linux 5.0.8 (lenny) 
bash GNU bash 3.2.39

-----
 Create descriptor 4 and file f, show file:
-rw-r--r-- 1 0 Mar 14 12:43 f

-----
 Write to f:
-rw-r--r-- 1 3 Mar 14 12:43 f

-----
 Read from f:
hi

It may be useful for writing on stdin, but I don't see the point of that.

Perhaps someone will describe a useful situation ... cheers, drl

( edit 1: corrected for exec mis-typed as echo )
( edit 2: misspelling )

Last edited by drl; 03-14-2012 at 05:50 PM..

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lore

GENERATELORE(1) 														   GENERATELORE(1)

NAME

       lore - convert documentations formats

SYNOPSIS

       lore [-l linkrel] [-d docsdir] [-i input] [-o output] [--config attribute[=value] [...]] [-p] [file [...]]

       lore --help

DESCRIPTION

       The --help prints out a usage message to standard output.

       -p, --plain
	      Use non-flashy progress bar - one file per line.

       -n, --null
	      Do not report progress at all.

       -N, --number
	      Add chapter/section numbers to section headings.

       -l, --linkrel
	      Where non-document links should be relative to.

       -d, --docsdir
	      Where to look for .html files if no files are given.

       -e, --inputext <extension>
	      The extension that your Lore input files have (default: .xhtml)

       -i, --input
	      Input  format. New input formats can be dynamically registered. Lore itself comes with "lore" (the standard format), "mlore" (allows
	      LaTeX equations) and "man" (man page format). If the input format is not registered as a plugin, a module of the named input will be
	      searched. For example, --i twisted.lore.default is equivalent to using the default Lore input.

       -o, --output
	      Output  format.  Available  output  formats  depend on the input. For the core formats, lore and mlore support html, latex and lint,
	      while man allows lore.

       -x, --index <filename>
	      The base filename you want to give your index file.

       -b, --book <filename>
	      The book file to generate a book from.

       --prefixurl <prefix>
	      The prefix to stick on to relative links; only useful when processing directories.

       --version
	      Display version information and exit.

       --config
	      Add input/output-specific information.  HTML output allows for 'ext=<extension>', output allows for 'section' or 'chapter' in  Lore,
	      and  nothing  in	Math-Lore.   Lore  output allows for 'ext=<extension>'. Lint output allows nothing.  Note that disallowed --config
	      options are merely ignored, and do not cause errors.

DESCRIPTION

       If no files are given, all *.html documents in docsdir are processed.

AUTHOR

       Written by Moshe Zadka

REPORTING BUGS

       To report a bug, visit http://twistedmatrix.com/bugs/

COPYRIGHT

       Copyright (C) 2003-2008 Twisted Matrix Laboratories.
       This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICU-
       LAR PURPOSE.

								   October 2002 						   GENERATELORE(1)

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lore