Script to dynamically choose fd for a flock

This is a sample script which automatically chooses a unused fd, opens the requested lock file with that fd and acquires a lock on that file

#!/bin/bash

# function to acquire to flock on a local file
# Usage
# flock [-e|-x|-w ] 
# Return
# non-zero in case of errors
# flock_fd variable will contain the fd
function flock
{
 [ -z "$1" ] && return 

 local file="" timeout=20 mode="-x"
 while ! [ -z "$1" ]; do
  case "$1" in
  -x)
   mode="-x"
   shift
   ;;
  -s)
   mode="-s"
   shift
   ;;
  -w)
   timeout=$2
   shift
   shift
   ;;
  *)
   file=$1
   ;;
  esac
  ! [ -z "$file" ] && break
 done
 [ -z "$file" ] && return 1

 ! [ -f "$file" ] && touch $file
 ! [ -f "$file" ] && return 1

 local freefd=`ls /proc/$$/fd | sort -n | awk 'BEGIN{count=0} {if($1 != count) {print count; exit} else {count++}}'` 
 let freefd=$freefd+1

 eval "exec $freefd>>$file"
 [ $? -ne 0 ] && return 1
 flock $mode -w $timeout $freefd
 local ret=$?
 if [ $ret -eq 0 ]; then
  flock_fd=$freefd
 else
  eval "exec $freefd>&-"
 fi

 return $ret
}

# function to unlock a local flock
# Usage
# flock_unlock 
# Return
# non-zero in case of errors
function flock_unlock
{
 # close the file, it would remove the locks
 [ -z "$1" ] && return 1
 eval "exec $1>&-"
 return 0
}