I/O redirection in a while loop

Question

What is the difference between the following two blocks of code in reading the content of a file?

while read i; do 
    echo $i
done < ${filename}

while read i < ${filename}; do 
    echo $i
done 

Answer 1

You can write it that way, but it always reads the first line.

Think of the while loop like this:

1. In the first case:
   1. Connect the file to stdin
   2. Read until a newline
   3. Loop (read the next line)
   4. Disconnect the file
2. In the second case:
   1. Connect the file to stdin
   2. Read until a newline
   3. Disconnect the file
   4. Loop (reconnect the file, read a line, disconnect the file)