exec的用法

#!/bin/sh
# Scriptname: speller
# Purpose: Check and fix spelling errors in a file
exec < tmp                   # Opens the tmp file
while read line              # Read from the tmp file
do
 echo $line
 echo  -n  "Is this word correct? [Y/N] "
 read answer < /dev/tty   # Read from the terminal
 case  "$answer" in
 [Yy]*)
      continue;;
 *)
      echo  "What is the correct spelling? "
      read word < /dev/tty
      sed  "s/$line/$word/g" tmp > error
      mv error tmp
      echo $line has been changed to $word.
 esac
done