grep options and fgrep

$grep -V
GNU grep 2.6.3

Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

$

GREP -F or fgrep

$grep -F H   hello.txt
Hello
$grep -F helloKello   hello.txt
$grep -F helloello   hello.txt
$grep -F hell   hello.txt
hello
helloo
hello
$time grep -F hell   hello.txt
hello
helloo
hello

real    0m0.009s
user    0m0.000s
sys     0m0.008s
$time grep  hell   hello.txt
hello
helloo
hello

real    0m0.009s
user    0m0.004s
sys     0m0.004s
$