Why does the mongo shell work without mongod server being explicitly run?

Question

Running MongoDB Shell v-3.2.8

I've noticed that articles and tutorials always mention to run the mongod server before running the mongo shell.

However, when I skip the first step and simply type mongo into my terminal, the mongo shell works without any errors / interruptions.

MacBook:Desktop user$ mongo
MongoDB shell version: 3.2.8
connecting to: test

Why does this work? Does mongo call mongod?

Answer 1

The mongod is being ran as a service or daemon, which means that there is always a mongod process running listening to a port. I use ubuntu, and when I install mongodb through the package manager, it immediately starts up a mongod process and begins listening on the standard port.

Running mongo is simply a small utility that attempts to connect to the localhost at the standard ip. The data reading, writing, and querying is done by the mongod process while mongo is a small program that sends the the commands to mongod.

If mongod wasn't running, you would see an error stating "Unable to connect to mongodb server"