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

#!/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

% ./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