Unit 1: The Terminal

A different way to ask

When you want a file, you point at it and click.

There is another way in. You type what you want, and the computer does it. That is a terminal, and it is how almost every serious computer in the world is driven — the ones running websites, films, telescopes and your school.

Why bother?

Clicking is lovely for one file. It is horrible for four hundred.

In a terminal, "rename all four hundred" is one line. And because it is a line, you can save it, send it to a friend, or make the computer do it every morning while you are asleep.

This is Linux

The computer you are about to type at runs Linux — the same system that runs most of the internet, every Android phone, and a Raspberry Pi.

The program listening to your typing is called a shell. Ours is called bash. You say something, it does it, and it comes back for more.

Reading the prompt

Before it lets you type, the shell tells you three things:

kid — who you are.
alguni — the name of this computer.
~ — where you are standing.

The $ at the end means "your turn". You never type the prompt; it is already there.

Terminal

kid@alguni:~$ whoami
kid

One giant folder

On Linux, every single file lives inside one enormous folder called / — just a slash. People say the root.

Inside it are folders, inside those are more folders, and that is the whole computer. There is no C drive and no D drive. Everything hangs off /.

Your own corner

Your things live in /home/kid. That is your home, and it is where the shell starts you off.

It is such a common place to be that it has a nickname: ~. That squiggle means "my home", so ~ and /home/kid are the same place written two ways.

Print working directory

You are always standing *somewhere*. The command pwd tells you where.

It stands for print working directory, which is a long way of saying "where am I?".

Terminal

kid@alguni:~$ pwd
/home/kid

Looking around

A terminal shows you nothing until you ask. ls is how you ask — it lists what is in the folder you are standing in.

It is the command you will type more than any other, which is probably why it is only two letters.

Terminal

kid@alguni:~$ ls
photos
school
shopping.txt

Somewhere else

You can also point ls at a folder you are not in. Give it the name and it looks there instead.

That is a pattern you will meet all day: the command first, then what to do it to.

Terminal

kid@alguni:~$ ls school
maths
kid@alguni:~$ ls school/maths
shapes.txt
sums.txt

The hidden ones

Files whose names start with a dot are hidden. Programs use them to keep settings out of your way.

They are not secret and they are not protected — they are just quiet. ls -a shows them.

The -a on the end is called a flag: a small instruction to the command, always after it.

Terminal

kid@alguni:~$ ls
notes.txt
kid@alguni:~$ ls -a
.
..
.settings
notes.txt

Two dots that are always there

ls -a shows two odd entries in every folder: . and ..

. means this folder.
.. means the folder above.

They are not files. They are two names that every folder gets for free, and the next lesson is built on them.

Change directory

cd moves you. Give it a folder name and that is where you are standing now.

Watch the prompt when you do it — it changes to show where you have got to. That is what the prompt is for.

Terminal

kid@alguni:~$ cd school
kid@alguni:~/school$ pwd
/home/kid/school
kid@alguni:~/school$ cd maths
kid@alguni:~/school/maths$ pwd
/home/kid/school/maths

Going back up

To go up one folder, cd .. — those two dots from the last lesson.

You can stack them. cd ../.. goes up two. And cd on its own, with nothing after it, takes you all the way home from anywhere.

Terminal

kid@alguni:~/school/maths/homework$ pwd
/home/kid/school/maths/homework
kid@alguni:~/school/maths/homework$ cd ..
kid@alguni:~/school/maths$ pwd
/home/kid/school/maths
kid@alguni:~/school/maths$ cd ../..
kid@alguni:~$ pwd
/home/kid
kid@alguni:~$ cd
kid@alguni:~$ pwd
/home/kid

When it does not work

Type the name of a folder that is not there and the shell tells you so, plainly.

That message is not a telling-off. It is the shell being honest about the only thing it could not do, and reading it is half of using a terminal.

Terminal

kid@alguni:~$ cd nowhere
bash: cd: nowhere: No such file or directory

From here, or from the top

There are two ways to tell the computer where something is.

Relative — from where you are standing. maths/sums.txt
Absolute — from the root, so it starts with /. /home/kid/school/maths/sums.txt

The difference is one character: a path starting with / starts at the top of the whole computer.

Same file, two names

Standing in /home/kid/school, both of these reach the same file. The short one only works from here; the long one works from anywhere.

Use the short one when you are close. Use the long one when you are lost, or when you are writing something down for later.

Terminal

kid@alguni:~/school$ cat maths/sums.txt
2 + 2 = 4
kid@alguni:~/school$ cat /home/kid/school/maths/sums.txt
2 + 2 = 4

The tour of the root

Every Linux computer keeps the same handful of folders at the top, and they mean the same thing on all of them.

/home — people
/etc — settings
/usr/bin — the programs themselves
/var/log — notes about what happened
/tmp — scribbling paper, thrown away later

Terminal

kid@alguni:~$ ls /
bin
dev
etc
home
root
tmp
usr
var

The commands are files too

Here is the thing that surprises everybody. ls is not built into the shell — it is a file, sitting in /usr/bin, exactly like your homework is a file.

When you type ls, the shell goes and finds that file and runs it. Nothing is magic; it is all just files.

Terminal

kid@alguni:~$ ls /usr/bin/ls
/usr/bin/ls
kid@alguni:~$ which ls
/usr/bin/ls
kid@alguni:~$ which grep
/usr/bin/grep

Nobody remembers all of it

There are hundreds of commands and every one has flags. Nobody has them all in their head — not your teacher, not the person who wrote Linux.

What they have instead is the habit of asking the computer.

The manual

man opens the manual for a command. man ls tells you what ls does and what its flags mean.

On a real Linux machine these pages are much longer than ours — pages and pages. Ours are short enough to fit on a card.

Terminal

kid@alguni:~$ man ls
ls — list what is in a folder.
  -l  one line each, with size and permissions
  -a  show hidden files too
  -R  go into every folder inside as well

What you already typed

Press the up arrow and your last command comes back. Press it again for the one before.

history shows the whole list at once. Between them you almost never have to type a long command twice.

Terminal

kid@alguni:~$ pwd
/home/kid
kid@alguni:~$ ls
notes.txt
kid@alguni:~$ history
    1  pwd
    2  ls
    3  history

Everything at once

Somebody has hidden a word in this machine. You have all four tools you need:

pwd — where am I
ls — what is here
ls -a — including the quiet ones
cd — go there

No new commands. Just the habit of looking before you leap.