exec - (built-in) used to fork new processes.

exec - (built-in) when used with redirection, 
  allows additional file descriptors.

< - stdin, > - stdout, and 2> - stderr 
  are standard file descriptors and are set to your display terminal.

Run tty to see what it actually is.


Use exec to assign/create additional file descriptors.

# Open file descriptor to a file.
exec 3> listing

# Redirect command's standard out to the file descriptor 3.
#   All commands' outputs will be appended to single file.
ls >&3 
ls -l >&3 

# Close file descriptor 3 by setting to null, can't use anymore.
exec 3>&- 
cat listing

# Re-initialize file descriptor 3 and file1.
exec 3> listing

#This works, but output from previous command[s] gone.
ps >&3 
exec 3>&- 
cat listing


Alternatively, you can use a file descriptor to store terminal's ID 

# initialize a new file descriptor to terminal, use stdout for source
exec 6>&1

# now redirect stdout to a target file.
exec > target-file

#command output should go to target-file
echo "mylist"
ls

#Reset file descriptor 1 to terminal (stored in 6).
exec 1>&6

#Close the temp file descriptor
exec 6>&-

#command output should go to terminal screen.
echo "I'm home"


This will also work for standard input (0).

check out :
  http://wiki.bash-hackers.org/howto/redirection_tutorial
  http://tldp.org/LDP/abs/html/x17974.html