Setting default database for MongoDB shell

It's entirely possible to set a default, albiet in a slightly strange way. Here's what I do for using automatic authentication to an admin in .mongoshrc.js:

// Persist the database selected
var selectedDB = db

// Authenticate
db = db.getSiblingDB("admin")
db.auth('admin','adminpass')

// Switch back to selected DB
db = selectedDB

// Universally allow read queries on secondaries from shell. 
rs.slaveOk()

To directly answer the question, I'd think the way to accomplish this is simply by executing a check to see if the current database loaded up is "test" and alter only if that's the case, so.

if (db.getName() === 'test') {
  db = db.getSiblingDB('yourdefaultdb')
}

This allows both selecting a database at the command line and setting a default. Naturally it'll prevent you from overriding and using the "test" db from the command line, but I think that's a bit of a strange use case given the question. Cheers.

At the moment (looking 2.6.4) there is no way to universally set the default DB for the client. It seems hardcoded to test. In my anger I downvoted the Stennie's answer [1] because it doesn't work if your user does not have permissions to access the test database. But if your case is not such, then it may work good enough.

[1] and I can't reverse my vote now unless answer is edited