Am learning linux, so when i tried to execute the if script, but am getting the error saying that “unexpected error”

num=$5

if [ $num -gt 10 ]

then

    echo "greater than 10"

else
echo “lesser than 10”
fi

Its displaying output as :
:~# sh if.sh 11
if.sh: 4: [: -gt: unexpected operator
lesser than 10

Could anyone please help me with this issue?

//Thanks