Using variable to set http file we test in a probe

In a standard HTTP monitor, no.

In an external monitor, using cURL, absolutely. Take a look at the BASH-based sample_monitor in /config/monitors. The external monitor profile sends the IP of the pool member as $1 and the port as $2, and any arguments (potentially your receive string if not hard coded) as $3. So something like this:

!/bin/sh

 these arguments supplied automatically for all external pingers:
 $1 = IP (::ffff:nnn.nnn.nnn.nnn notation or hostname)
 $2 = port (decimal, host byte order)
 $3 and higher = additional arguments

pidfile="/var/run/$MONITOR_NAME.$1..$2.pid"

if [ -f $pidfile ]
then
    kill -9 -`cat $pidfile` > /dev/null 2>&1
fi

echo "$$" > $pidfile

node_ip=`echo $1 | sed 's/::ffff://'`

curl -fNs http://$node_ip:$2 |grep $3 1> /dev/null

status=$?
if [ $status -eq 0 ]
then
    echo "up"
fi

rm -f $pidfile