I also just realized that you might have already installed the mongo Shell, you just couldn’t find it. Try typing in the terminal
which mongo. If there is any input at all, it’s usually the path where you can find the mongo program to run. If that’s the case, just try typing in
mongo in the terminal and it should run the mongo shell for you. Of course, you would still need to supply the arguments to connect to the Atlas cluster. If there is no output from the
which command above, this most likely means that you didn’t install the mongo shell correctly. If that’s the case, please refer to my previous answer on documentation about installing the mongo shell.