How to start a mongodb service on mac OS X?

Try the following steps in Terminal:

which mongod

This will output the path to your mongod, but if it is not in your $PATH the command output will be empty. So you need to find your executable:

find / -name 'mongod'

In the output of this command, you will see many lines, one of which will be like bin/mongod, e.g. /usr/local/mongodb/bin/mongod. In that case take the whole absolute path and do the following:

echo "PATH=/usr/local/mongodb/bin/:$PATH" >> ~/.bash_profile
. ~/.bash_profile

Then try again:

mongod --dbpath /your/path

If you used homebrew to install MongoDB on macOS you type the following in terminal. Should do the trick in most cases.

$ brew services start mongodb