nc -z hostname 22 > /dev/null
echo $?

nc -z localhost 22

-z      Specifies that nc should just scan for listening daemons,
         without sending any data to them.

nc -vzw 1 hostname 22

MACHINE=192.168.0.8
exec 3>/dev/tcp/${MACHINE}/22
if [ $? -eq 0 ]
then
    echo "SSH up"
else
    echo "SSH down"
fi