Operate on all databases from the mongo shell

Question

We have a system with many different mongo databases. I regularly want to write ad-hoc queries that will apply to all (or a subset) of them, without having a priori knowledge of what databases are there.

I can do show dbs, which will visually print a list, but is there a way to do something like:

var db_list = listDatabases();

for (i = 0; i < db_list.length; i++) {
     do_something(db_list[i])
}

My problem with show dbs is that it doesn't capture any return values, so I can't do anything productive with the output.

Answer 1

You can use the 'listDatabases' admin command for that:

var db_list = db.adminCommand('listDatabases');

That returns an object that looks like this:

{
    "databases" : [
        {
            "name" : "test",
            "sizeOnDisk" : 2097152000,
            "empty" : false
        },
        {
            "name" : "local",
            "sizeOnDisk" : 1,
            "empty" : true
        }
    ],
    "totalSize" : 8487174144,
    "ok" : 1
}

Answer 2

There is also getDBNames() (i like how JohnnyHK's answer gets the size.

d = db.getMongo().getDBNames()
[ "graylog2", "local", "admin", "test" ]

Then I can:

for (var x in d) { db = new Mongo().getDB(d[x]); print(db); y = print(db.getCollectionNames()); }