need to grep a string in a string

[kandarpa]

Hi I am tryibg to check for presance of a string in a big string. I tried as follows:

IS_VECTOR_TEST_CASE=`grep AIV "$PWD"`
if [ "$IS_VECTOR_TEST_CASES" = "" ]
then
   #do something
else
   #do something
fi

This always giving me as if [ "$IS_VECTOR_TEST_CASES" = "" ] is true with following error:
grep: /abc/bcd/cde/AIV/def: Invalid argument

Please help me in this issue...

[masijade]

echo "string" | grep "pattern"

[kandarpa]

Thnq Very much.........