• Define "Command Line Interface"
• Describe the purpose of CLIs
• Identify differences between command-line interface, terminal emulator, and shell

Most computer users these days are familiar with performing actions and executing tasks using a Graphical User Interface (GUI). When you use things like MacOSX's "Finder" or Windows' "File Explorer", you're using a GUI.

Prior to the early 1990's, however, the only way to interact with a computer was by using a command-line interface (CLI). Accomplishing tasks in those days (e.g., creating, viewing or manipulating files; launching software) required the user to type commands into a terminal.

Given how prevalent GUIs are at this point, the average computer user will probably rarely or never need to use a command line to accomplish their tasks. The same is not true for developers, however. Learning to use a CLI will allow you to accomplish many tasks more quickly and reliably than you can using a GUI. Furthermore, there are some tools you will need as a developer that can only be used through the command-line interface.

Lots of people worry that using the CLI will get them in trouble, that they'll break their computer. We encourage you to think about the CLI like a high-quality, sharp kitchen knife: if you pay attention when you use it, you're going to have a valuable ally on your side.

WARNING: It's true, it is easier to run dangerous commands through the CLI than through a GUI. Commands that list files or create directories are not likely to break anything. Commands that remove files or directories, however, should be used with caution.

A CLI is a program that allows us to have a text-based conversation with the computer in which we type the commands for accomplishing tasks. With a CLI, users have wide control over the file system and operating system, and the tasks become simple. For example, you can ask, through the CLI, which files are located on the Desktop. You can create a new folder or delete it. But on top of file-management kinds of activities, you can also find out how busy your CPU is, how full your hard drive is, and whether your computer can find a network path to flatironschool.com. On top of this, you don't have to click through several menus to get there!

Experienced developers would say "the CLI gives you more control" or that it's "more powerful." With a GUI you use the mouse and the keyboard to control the file system or the operating system, which is going to be slower than using the command line (once you become familiar with the commands). In a CLI, users only use the keyboard and may need to execute only a few short commands to complete their equivalent GUI tasks. CLI users' fingers never leave the "home row" (assuming they can touch type) which adds to their speed.

While some tasks may seem "easier" to do with a GUI, development-like tasks are often much more easily completed in the CLI. If you had a task such as renaming 100+ files in a folder according to a formula based on their file size, you might well spend hours on it in the GUI. Doing the same with the CLI on your side could be completed in seconds. And if you do that sort of thing often, you can save that process and run it again whenever you wish! This is called "scripting."

In the early days of computers, users didn't work on standalone, "personal" computers. Instead, their workstation consisted of only a keyboard and monitor — no mouse and no graphical user interface. This monitor + keyboard device was called a "terminal," and multiple terminals were connected to a shared computer called a "mainframe."

Nowadays, the "terminal" is emulated in software. It's virtual. You launch the emulator by opening a program. And instead of being connected to a remote computer by a cable, your "terminal emulator" talks to the computer you're actually typing on. For Mac users, the default terminal emulator program is called Terminal. For WSL users, the Ubuntu application will act as your "terminal."

When you launch the "terminal emulator" program, it will immediately start a program called a shell program. The shell program is what actually prompts you for input and returns the output. bash and zsh are specific shells used by Unix systems (like Mac OSX and Linux).

To summarize:

• When discussing the terminal, we mean the "terminal emulation" program, i.e. the thing that handles raw input and output.

• The shell is what handles interpreting the commands you type in to the terminal. It takes input, thinks, prints things out. It knows when a command doesn't exist or make sense, and it knows how to ask the CPU to do work.

• The phrase "command-line" is roughly the same as shell. It's a style of interaction with the shell.

Although using a command line interface might seem intimidating at first as it requires learning dozens of different commands, it can be a valuable resource that makes using a computer easier. Using a command line, you can perform almost all of the same tasks that can be done with a GUI. However, many tasks can be performed quicker and can be much easier to automate.

Ultimately though, many programming languages and programming tools assume that you're comfortable with the CLI. You must have this comfort in order to be a successful programmer.

• Lifehacker on the Command Line
• Difference between CLI and GUI