how do i compare strings???

In my program,user will have to enter some input and i have to make sure that it is not blank(some input must be there)....

i tried this but it gives me error:
echo "enter publisher"
read publisher
while test $publisher == " "
do
echo "publisher cannot be blank enter again"
read publisher
done

output:
enter publisher

validate: line 26: test: ==: unary operator expected


any ideas????

This code ll give required output. but i don know why it is not possible to compare strings??

Code:

echo "enter publisher"
read publisher
cnt=`echo $publisher | wc -c`
while test $cnt -eq 1
do
echo "publisher cannot be blank enter again"
read publisher
cnt=`echo $publisher | wc -c`
done


echo "enter publisher"
read publisher
while test -z $publisher
do
echo "publisher cannot be blank enter again"
read publisher
done

OUTPUT
sh str.sh
enter publisher

publisher cannot be blank enter again
sara

Here, the -z will check if the length of the string that is tested is zero or not...

In test cmd..
-z STRING
the length of STRING is zero

@christopher
why does cnt display the value 1

even if u enter 3 letters, it ll give u 4 as ouput. bcoz wc includes the EOF character too.

@saradha n @christopher
thanks it works!!!!

test ! $publisher will work as well!