The short version
Before there were windows, icons, and mice, there was the command line. You type a command, press enter, and the computer does something. That's a CLI.
It might sound old-fashioned, but CLIs are everywhere in modern tech. Most developer tools, cloud platforms, and AI tools are CLI-first. If you've ever opened Terminal on a Mac or Command Prompt on Windows, you've seen one.
How it works
A CLI gives you a prompt (usually a blinking cursor) and waits for instructions. Commands follow a pattern:
command [options] [arguments]
For example:
lslists files in the current directoryls -lalists files with details and hidden filescd projectsmoves into the "projects" foldernpm installinstalls a project's dependencies
The -la part is a flag. It modifies what the command does. Think of it like adding "with extra detail" to your request.
Commands can be chained too. The output of one becomes the input of another:
cat log.txt | grep "error" | wc -l
That reads a file, filters for lines containing "error", and counts them. Three tools, one pipeline.
Why it matters
CLIs are faster than GUIs for repetitive tasks. They're scriptable, so you can save a sequence of commands and run them again. And many tools (Git, Docker, Claude Code, AWS CLI) are designed CLI-first, with graphical interfaces added later.
You don't need to memorise every command. You need to understand the pattern: command, options, arguments. The rest you look up.