Your first program

In this tutorial you write a Lua program with mcos’s editor, run it, and learn the shell session you do it in. mcos is built to behave like Linux, so what you learn here carries over to the real thing.

What you need

A running computer at its shell prompt, from Your first computer
Its screen open, so keys you type reach it

Steps

1. Look around

At the prompt, type each command and press Enter:

pwd
ls -a
ls /

You will see your working directory (/home), the hidden file .shrc in it, and then the top of the filesystem. Your home, /home, is yours: updating mcos never writes over it. See pwd and cd in the shell, ls and the filesystem.

Every command has a manual. Try:

man ls

Press q to leave the page. (man pages the text with less.)

2. Write a program

Open the editor on a new file:

edit hello.lua

You will see a title bar with the file’s name, an empty text area, and two lines of shortcuts at the bottom. The status line says [ New File ]. Type this, pressing Enter at the end of each line:

local name = ...
print("Hello, " .. (name or "world") .. "!")
for i = 1, 3 do
  print(i, i * i)
end

Now save and quit:

Press Ctrl+O. The status line asks for a name and offers hello.lua. Press Enter.
Press Ctrl+X to exit.

If you press Ctrl+X with unsaved changes, the editor asks whether to save: Y saves, N throws the changes away, Ctrl+C goes back. The editor’s page lists every key (arrows, Ctrl+K to cut a line, Ctrl+U to paste, Ctrl+W to search).

You will see the prompt return. Check the file is there:

ls
cat hello.lua

3. Run it

Every mcos program is Lua, and lua is Lua’s own interpreter:

lua hello.lua
lua hello.lua Steve

You will see:

Hello, world!
1
2
4

and then the same again, starting Hello, Steve!. Everything after the file’s name arrives in the program as ..., which is how name got its value.

4. When it goes wrong

Open the file again with edit hello.lua, delete the do on the for line, save, and run it. Lua refuses to run it and prints a line starting lua: that names the file and line, and a traceback. Fix the line and run it again. Errors always say where they are; read the first line.

5. Try Lua interactively

Run lua with nothing after it. You will see a > prompt where each line you type is a Lua expression or statement:

> 2^10
1024.0
> #"modular"
7

Ctrl+D on an empty line leaves. You can also run a line without a file: lua -e "print(1 + 1)".

6. Pipes, redirection and files

The shell works as sh does. Send a command’s output to a file with >, add to it with >>, and feed one command’s output to another with |:

lua hello.lua Alex > greeting.txt
cat greeting.txt
ls | grep hello

You will see the greeting, then hello.lua. The shell remembers what you typed: the Up arrow brings back earlier lines, and Tab completes command and file names. See the shell and grep.

7. Shut down properly

Type poweroff (page). mcos closes its files and turns the computer off, so what you saved is on the disk. Switching the power off with the button does the same; the disk writes a file when it is closed, so a program that never closes its file loses what it wrote if the power is cut.

Congratulations

You wrote and ran a program. Your file lives on the disk like any other, as a real file in the world’s save. Next, make your computer touch the world with redstone.