eval and exec in bash

eval: execute all shell replacement before issue the command.

ivan@Maintenance_Center:~$ set a b c d
ivan@Maintenance_Center:~$ eval echo The last argument is \$$#
The last argument is d
ivan@Maintenance_Center:~$ echo The last argument is \$$#     
The last argument is $4
ivan@Maintenance_Center:~$


exec: open or close the standard input or output.

#!/bin/bash
exec < /tmp/file #open the /tmp/file
while read line;do
    echo ${line}
done

#will print file /tmp/file

if you want read from terminal after you close standard input you should specify the optional input

read answer < /dev/tty