for line in `cat ${input_filename}`
do
  echo $line
done

while read line
do
  echo $line
done < ${input_filename}

n<file
n>file
n>>file
n<>file

<file#same as 0<file
>file  #same as 1>file
<>file#same as 0<>file

exec 7<file1
exec 8<file2

read data <&7 #使用符合是為了區(qū)分7是文件描述符，而不是文件名
read data <&8

exec 7</dev/null
exec 8</dev/null

readfiles() {
	local FD1=7
	local FD2=8
	local file1=$1
	local file2=$2
	local count1=0
	local count2=0
	local eof1=0
	local eof2=0
	local data1
	local data2
 
	# Open files.
	exec 7<$file1
	exec 8<$file2
	while [[ $eof1 -eq 0  ||  $eof2 -eq 0 ]]
	do
		if read data1<&$FD1; then
			let count1++
			printf "%s, line %d: %s\n" $file1 $count1 "$data1"
		else
			eof1=1
		fi
		if read data2 <&$FD2; then
			let count2++
			printf "%s, line %d: %s\n" $file2 $count2 "$data2"
		else
			eof2=1
		fi
	done
}
#read file1 and file2
readfiles file1 file2