Example — python3

Python is a programming language, but your computer doesn’t natively understand Python. Instead it has a program (called the Python interpreter) that does. So when you want to execute instructions in Python, you need to type the name of the interpreter.

For example, if you type python3 on the command line two things might happen:

• if your computer can’t find the python3 program, it will fail (and return to the prompt)

• if you computer can find it, it will run the python3 program:

This looks like this:

>>>

That program is now waiting for you to type Python. (See how the prompt is different from the command line’s prompt?).

You can’t issue commands on your command line until you exit that program, and get back to the command prompt. Anything you type now is interpreted as Python.

For example, if you type pwd (that was the “print working directory” command you used before), it won’t work:

>>> pwd
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'pwd' is not defined
>>> 

What’s happened there is Python — not your command interpreter — has tried to understand your command. Python thinks you’re referring to something with the name pwd but in Python you must declare things before you use their name.

One way to exit that situation is to call the Python function quit():

>>> quit()

The python3 program finishes, and then control passes back to your command line interpreter, and the prompt re-appears.