Ah, there were no details.  Very well.  The most general bash idiom
(also works in ksh):

exec 3<"$filename"                   # open the file
while IFS= read -r -u3 line; do
    # do stuff with "$line" here
done
exec 3<&-                                # close the file

You can also just use input redirection on the whole loop (while...
do;...done <filename), but the exec method is the most flexible.

The IFS= disables whitespace separation so that whitespace in the
lines, even leading space, is preserved.