Ellane learns the command line

Flags and options

We almost understand the syntax of a command. To recap, we've met the command, and its (often optional) arguments.

Now let's meet flags and options, which are two sides of the same coin.

Behaviour modifiers #

Flags modify the behaviour of a command. Let's try one out.

ls -l

That's a dash/hyphen, then a lowercase letter 'ell'.

Try the un-flagged ls and then ls -l and compare the output. The flagged variant gives you a longer output; more detailed.

The l stands for long. We've said to ls please exercise your ability to give us a longer output.

Let's try another.

ls -a

There's more stuff here: depending on your system you might see .DS_store, .localized, and look carefully: our friends . and .. are now shown.

By default, some system-level stuff is hidden from a basic ls. But a is short for all: you're asking ls to show you all the things.

Combining flags #

You can ask ls for both things.

ls -l -a

Note the order doesn't matter. But that's clunky. Dash-l, dash-a. It'd be nicer if we could combine them...

ls -la

This pattern -- ls -la -- is so common that many people make it their default. We'll see how to do that later. I just type it instinctively, it's seared in to my muscle-memory.

You can still specify an argument(s) #

Let's combine everything we know.

ls -la ~/Downloads

If that doesn't make you want to clean up that folder, nothing will.

(We'll go in to more detail about what ls -l is showing you later.)

Conventions #

There are well-held conventions that are used here. They aren't hard rules -- developers can and do break them -- but if you use them as a guide you won't go far wrong.

The command always comes first. That is a hard rule.

Then flags come next. A convention, very widely held. I can't think of an example where it isn't the case.

Then lastly, the arguments.

  ls -la ~/Desktop
`# │   │  └─ argument(s)`
`# │   └──── flag(s)`
`# └──────── command`

Flags-as-words #

-l isn't very expressive, and there are only 26 letters of the alphabet. So full-word flags are also totally normal.

And here's a well-held convention that I do see broken: full-word flags should start with two dashes.[1]

ls --color=always

...and this is now an option #

I just realised that we've moved from a flag to an option. Do you know why?

Flags are either there or not. On or off. I imagine the analogy is to a raised flag: up, or down.

Options have more data. What's with =always? Well, the other options are =auto or =none.

You can think of this as an argument to the option. We'll see a lot more of this as we go on so don't worry too much about this for now.

All together now #

As you might imagine, you can mash all of this together.

ls -la --color=always ~/Downloads

Everything is just a variety of this #

First the command.

Then all of the flags and options.

And finally any arguments.

That's the basic structure of every command.

Homework #

The man page for ls is online here.

Try a bunch more options and see what you can get ls to produce. Soon, I'll show you how to bring these man(ual) pages up directly in Terminal.

  1. Looking at you, Apple. ↩ī¸Ž