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What Is a Shell?

So if your terminal is just a program that lets you issue text-based commands and renders the output of those commands...

...What is the program that runs those commands???

That's a shell.

Shells do a lot of things, but their main job is to interpret the commands you type and execute them.

REPL

Shells are often referred to as "REPLs." REPL stands for

Read
Eval (evaluate)
Print
Loop

This is a fancy way of saying that shells are programs that:

Read the commands you type
Evaluate those commands, usually by running other programs on your computer
Print the output of those commands
Give you a new prompt to type another command and repeat

Assignment

The expr command evaluates a simple expression and prints the result. It's a tiny program, but it's a great way to watch the shell "read, eval, and print."

In the shell on the right, run:
```
expr 123456 + 7890
```
Then, using the same syntax, print the result of 420 minus 69 (using the - operator) to the shell.

Submit the shell checks.

Tip

If you're confused about prompts vs outputs, here's a quick explanation. Here's a session with my shell:

wagslane:~$ echo hello
hello
wagslane:~$

This is the "prompt": wagslane:~$

It's just telling me that the shell is ready to accept a command.

This is the command I typed: echo hello

Once I pressed Enter, the shell read my command, evaluated it, and printed the output: hello

Then, the shell printed a new prompt so I can run a different command: wagslane:~$