I occasionally post so called "one liners", shell commands that can be used to filter out some data I need. The main reason I do this is that I can later find this when I try to google for it. This will be my first one liner for MongoDB. Ok, so it is actually 3 related one liners.
If you need to find out if a shard is already part of your MongoDB cluster, try this:
echo "db.shards.find()" | mongo $MONGOS/config | grep Shard4 | wc -l
The result will be either 1 or 0.
Now, if the shard exists, you might want to know a hostname and port number of one of the members of that replicaset: