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Table of contents

Lesson 2: Ruby and the Command Line



In this tutorial we are going to look at:

  • what a command line is
  • how to use the command line to navigate the file system and run programs
  • what Ruby is
  • numbers and strings
  • variables
  • calling and defining methods
  • getting user input
  • doing things multiple times using loops
  • building simple programs


By the end of this tutorial you will have built a couple of small Ruby programs, including one like this times tables game.

Introduction to the command line

What is the command line?

The command line is a text interface for your computer. Just like Windows Explorer on Windows or Finder on a Mac it lets you navigate through the files and folders of your computer, but it is completely text based. The command line works by typing commands against a prompt, which then gets passed to the operating system of the computer that runs these commands.

How do I access the command line?

On a Mac you can access the command line by opening the Terminal application from the Applications > Utilities folder (or use cmd (⌘) + space and search for “Terminal”).

The command line can seem unfamiliar and scary, but it’s really just a different way of interacting with your computer. This tutorial only covers safe commands that won’t do anything bad to your computer, even if you get them wrong.

Once you opened up your terminal you should see a window like this:

Screenshot of the Terminal app. The text says Last login: Sat May 18 11:08:55 on ttys002 ~$

Don’t worry if the text in yours is a little different, it doesn’t matter.

Basic commands are written on a single line, and run when you press the Enter button on your keyboard.

Try typing, then pressing Enter:


pwd or print working directory

The pwd command prints to the command line the current directory (another name for folder) you are in. If you just opened up your terminal, you are probably in your “home” directory, and should get an output similar to this:


So your current “working directory” is /Users/your-username.

Getting things wrong on the command line

If you type a command that the command line doesn’t understand, it will show you an error message. Don’t panic, if you see one of these - everything is fine! Just have a look at the command you wrote and see if you can work out what was wrong.

Try this for example:


You should see an error message like -bash: whargleblargle: command not found.

If you want to cancel your current entry, you can either delete the command using the backspace button, or press Ctrl + C to get a brand new line. Your mouse won’t work for navigating around the command line commands, but you can use the arrow keys on your keyboard to move the cursor left and right.

ls or list

You might wonder “how I do know which files are in a directory?”, the ls command can do this:


This should print a list of the files and folders inside the working directory. You’ll probably see directories like Applications, Desktop, Documents and Downloads.

cd or change directory

The cd command allows you to move between directories. You tell cd which directory to move to by putting the path after the cd, like this:

cd Desktop

Lots of commands need parameters like this (e.g. cd needs to know the directory to move to, while pwd does not). We call the parameters “arguments”.

Task 1: change to the directory containing your code

In Lesson 1 we created a folder to keep our source code (lesson-1-html-and-css). Create a new folder for lesson 2 called lesson-2-ruby and change the working directory in your command line using cd.


In Finder you can copy the path to a directory by right clicking, holding down the option key (), and choosing Copy “Directory” as Pathname. Alternatively, you can drag the folder onto the terminal and it will type the path for you.

If your directory includes spaces (or other weird characters) you might need to put it in quotes so the command line doesn’t get confused. So:

$ cd '/Users/your-username/My Directory With Spaces/lesson-2-ruby'

instead of

$ cd /Users/your-username/My Directory With Spaces/lesson-2-ruby

Confirm you’re in the right place by running:


and check what files there are in this directory by running:


There probably won’t be any files yet, because we haven’t created any.

Introduction to Ruby

The Ruby logo

Ruby is a programming language. For a beginner, Ruby is very similar to other languages like Python, JavaScript and PHP. The basic skills you learn in Ruby will translate directly into many other languages.

As well as being a wonderful language, Ruby has a vibrant, friendly, and sometimes weird community of people. That means there’s lots of great resources for learning, and lots of places where you can get help.

Running Ruby interactively with irb

There are two ways to run Ruby from the command line:

  • Interactively with irb (Interactive Ruby)
  • Running a file with ruby itself

To start with, let’s look at irb. We’ll get on to how to run a file later.

irb is lets us write bits of Ruby and quickly see what they evaluate to.

Start irb by running:


Your command line prompt will change from $ to irb(main):001:0>. You can then write Ruby code into the command line.

When you want to leave irb and go back to your command prompt, just type exit and press enter.


We can use Ruby as a kind of calculator. Try typing 1 + 1 into irb and pressing enter. Do you get the right answer?

In Ruby, the + operator adds numbers together. Other operators include:

  • - - subtract
  • * - multiply
  • / - divide
  • ** - raise to the power of

You can also use brackets (()) to group things, e.g. (2 + 2) / 2 which would evaluate to 2, rather than 2 + 2 / 2 which would evaluate to 3.

Task 2: Maths challenge

Use irb to work out the answer to “191 multiplied by 7”.


irb(main):016:0> 191 * 7
=> 1337


In the real world strings tie things up. Programming strings have nothing to do with real-world strings.

Programming strings are used to store collections of letters and numbers. That could be a single letter like "a", a word like "hi", or a sentence like "Hello my friends.".

A Ruby string is written as a quote (") followed by some letters, numbers, or symbols and ended with a closing quote ("). The shortest possible string is called the empty string: "". It’s not uncommon for a single string to contain paragraphs or even pages of text.

If you type a string into irb it will return it back at you:

irb(main):017:0> "Hello IRB"
=> "Hello IRB"

Getting things wrong in IRB

Just like in the command line, if you give irb a command it doesn’t understand it will show you an error message. Again, don’t panic! Everything is fine.

irb will do its best to explain why it didn’t understand your Ruby, but sometimes the error messages can be hard for a beginner to understand.

For example:

irb(main):018:0> "Hello IRB" + 7
TypeError: no implicit conversion of Fixnum into String
        from (irb):1:in `+'
        from (irb):1
        from /usr/bin/irb:11:in `<main>'

Here, irb is telling us that you’re not allowed to add strings and numbers together in Ruby.


Programming is all about creating abstractions, and in order to create an abstraction we must be able to assign names to things. Variables are a way of creating a name for a piece of data.

Creating and using variables in Ruby is super simple, it’s just name = value. Some examples:

name = "Fido"
age_human_years = 4
age_dog_years = age_human_years * 7

This would give three variables: name with a value of "Fido", age_human_years with a value of 4 and age_dog_years with a value of 28.

Variable names in Ruby have to start with a letter, and they can’t contain spaces or “special” characters like -, $, @ and &.

As a style convention, Ruby variables use underscores to separate the bits of the name - this is called snake_case (as opposed to camelCase or PascalCase which are used elsewhere).

Task 3: Set and use a variable

Use irb to set a variable called answer to the value of 7 multiplied by 6. Multiply the answer variable by 10 in irb to see what happens.


irb(main):020:0> answer = 7 * 6
=> 42
irb(main):021:0> answer * 10
=> 420
irb(main):022:0> answer
=> 42

String interpolation

We often need to build a string out of other strings and bits of Ruby. In Ruby we can do this with “string interpolation”.

Within the string we use the interpolation marker #{}. Inside those brackets we can put any variables or Ruby code which will be evaluated, converted to a string, and output in that spot of the outer string.

irb(main):019:0> "The dog #{name} is #{age_human_years} human years old, which is #{age_dog_years} in dog years."
=> "The dog Fido is 4 human years old, which is 28 in dog years."

Note: on a Mac you can enter the # key with option () + 3

Task 4: Use string interpolation

Using string interpolation in irb, create a string with the text:

“The answer to life, the universe and everything is …”

Where ... is the value of the answer variable you set in Task 3.


irb(main):023:0> answer = 7 * 6
=> 42
irb(main):024:0> "The answer to life, the universe and everything is #{answer}"
=> "The answer to life, the universe and everything is 42"

Calling methods

While programming, we often find ourselves doing the same thing over and over again. It would be nice if we could give a particular task a name, and run it just by calling its name.

In Ruby we do this with methods, and there are lots of them already built into the language. One example is rand, which generates a random number:

irb(main):025:0> rand
=> 0.11487911496307956

If we want to generate a random number less than a particular number, we can provide our maximum to rand as an “argument”:

irb(main):026:0> rand(100)
=> 7
irb(main):027:0> rand(100)
=> 43

You can also call some methods on things by using a . like this:

irb(main):028:0> "make me louder".upcase
irb(main):029:0> "em esrever".reverse
=> "reverse me"

Task 5: Decode a secret message

Decode this top secret, encrypted message:

!gniog peeK !llew repus gniod er'uoY

(Hint: it’s not Welsh - it’s just been reversed)


irb(main):030:0> "!gniog peeK !llew repus gniod er'uoY".reverse
=> "You're doing super well! Keep going!"

Running a file with Ruby

We’ve been having fun with irb - it’s a great tool for exploring and learning about Ruby. Ideally though we’d like to be able to save our programs for later, and run them whenever we want.

To do this, we can write Ruby script in a text editor (like you did with HTML in lesson 1), then run a the script from the command line with ruby name-of-your-script.rb.

Ruby files end with the extension .rb.

Task 6: My first Ruby script

  • Exit irb by running the exit command
  • Create a file in your text editor (we recommended Atom in lesson 1) called hello.rb
  • Confirm you can see the file in the command line by running ls
  • From the command line, use Ruby to run your file by typing ruby hello.rb and pressing enter
  • You should see nothing returned (because hello.rb is empty at the moment)

Interacting with the user

Now that we’re running our Ruby from a file, we can’t rely on irb to handle all of our user input and output. When we want to show something to the user we need to explicitly say so, and likewise, when we want some input from the user we have to explicitly ask for it.

Ruby provides a puts method (put string) and a gets method (get string).

  • puts - prints a string to the command line
  • gets - waits for the user to type one line of text into the command line, when they press enter, returns whatever the user typed (so you can store it in a variable)

Task 7: Hello world!

Add the following to your hello.rb file, save it, and run it with ruby hello.rb:

puts("What is your name?")
name = gets
puts("Hello #{name}")

We can use puts and gets to create a very simple text based user interface.

Getting things wrong in Ruby scripts

Just like irb, if you get something wrong in a Ruby script Ruby will tell you by printing an error message. Nothing to worry about - programmers make mistakes all the time.

Imagine a script called oops.rb containing:

puts("time for a deliberate error")
oops("this won't work")

When you run this with ruby oops.rb you’ll get an error like this:

time for a deliberate error
oops.rb:2:in `<main>': undefined method `oops' for main:Object (NoMethodError)

The oops.rb:2 bit at the start tells us what file the error was in, and what line (2 in this case) the error was on. There’s no method called oops so Ruby doesn’t know what to do.

Task 8: Create a script that reverses the input

Create a new script called reverse.rb in your text editor.

Use gets, .reverse and puts to get a string from the user, reverse it, and print it.


puts("Enter a string to reverse: ")
input = gets
puts("Your string reversed is: ")

Task 9: Create a script that multiplies two numbers

Create a new script called multiply.rb in your text editor.

Note that gets gets the user input as a string. In Ruby strings and numbers are not the same ("42" is not equal to 42).

When you need to ask the user for a number you must ask them for a string and then convert it into a number using .to_i like so:

user_input_as_string = gets
user_input_as_number = user_input_as_string.to_i

The i in .to_i stands for “integer” which is fancy way of saying “whole number”.

Use gets, .to_i and puts to get two strings from the user, converts them to numbers, multiplies them together and prints the result.


puts("Enter the first number:")
x_string = gets
x_number = x_string.to_i
puts("Enter the second number:")
y_string = gets
y_number = y_string.to_i
puts("#{x_number} * #{y_number} = #{x_number * y_number}")

Defining methods

The Ruby authors didn’t manage to write every method we’ll ever need into Ruby itself, so sometimes we need to define our own.

This means we can write abstractions that describe what we’re doing and we don’t have to repeat ourselves.

Earlier, we had this line of code:

age_dog_years = age_human_years * 7

This is okay, but it’s a little surprising to see that 7 in there - what’s it doing? It would be nicer if we could write:

age_dog_years = convert_to_dog_years(age_human_years)

and hide away the grubby implementation details inside a method.

In Ruby a method definition looks like this:

def method_name(argument)
  result = "some value"
  return result

You can name a method anything you could name a variable (they’re snake_cased by convention too). They can have as many lines of code inside them as you want.

Task 10: Create a script that converts human years to dog years

Create a script called dog-years.rb. Use puts, gets and .to_i to ask the user how old their dog is.

Assuming that dogs age faster than humans, and that there are seven “dog years” to one “human year”, define a method which:

  • takes an age in human years as a argument
  • multiplies the age in human years by seven
  • returns the result of the multiplication

Call the method in your script, and print the result.


puts("How old is your dog in human years?")
age_human_years_string = gets
age_human_years_number = age_human_years_string.to_i

def convert_to_dog_years(age)
  return age * 7

age_dog_years = convert_to_dog_years(age_human_years_number)
puts("Your dog is #{age_dog_years} in dog years")

Repeating things

Meme where a cat looks at a bowl of fruit loops, and says 'brøether, may i have the lööps'

So far, each line of code we’ve written has run exactly once (each time a script runs). Ruby goes through each line in the script, runs it, then moves on to the next line. When it reaches the end of the file, the script exits.

One of the things that computers are really good at is repeating themselves. In Ruby we can use methods like loop and .times to make the computer repeat things.

You might want to try some of the following out in irb to get a feel for how they work. (If you get stuck in a loop in irb you can escape it by pressing ctrl + c):

5.times do
10.times do |i|
  puts("#{10 - i} green bottles, hanging on the wall,")
  puts("#{10 - i} green bottles, hanging on the wall,")
  puts("And if one green bottle should accidentally fall,")
  puts("There'll be #{9 - i} green bottles hanging on the wall.")
loop do
  puts("What's your name?")
  name = gets
  puts("Hello #{name} (to exit this infinite loop, press ctrl+c)")

The bit between the do and the end is called a block, which is run by the method repeatedly. Blocks can take parameters after the do, which are wrapped in vertical pipes like |i|.

Task 11: Update one of your scripts to work in a loop

Earlier, you wrote a few scripts that ask the user for some input, do something with the input then print the result and exit. Change one of these scripts (e.g. multiply.rb) so they do this in an infinite loop (using loop).

If you get stuck in an infinite loop (a loop which never ends) in the command line you can break out by pressing ctrl + c.


loop do
  puts("Enter the first number:")
  x_string = gets
  x_number = x_string.to_i
  puts("Enter the second number:")
  y_string = gets
  y_number = y_string.to_i
  puts("#{x_number} * #{y_number} = #{x_number * y_number}")


One last thing to learn is how to get the computer to do something based on some condition. To do this, we can use the if statement, like so:

puts("What is your name?")
name = gets
if name.include?("r")
  puts("Hi #{name} - you sound like a pretty cool person")
  puts("Hi nerd")

An if statement takes a “condition” (something that evaluates to true or false). You can use things like == (equals), < (less than), or methods like .include? (like we did in the example).

Note that we use == instead of = to compare two values. This is because == tests if two values are equal, whereas = assigns the value on the right to the variable on the left.

If the condition is true Ruby will run the lines of code between the if and the else, otherwise Ruby will run the lines of code between the else and the end.

Tying it all together

Phew! That was a lot to learn. To recap, we’ve learned:

  • how to use irb
  • how to use numbers
  • what “strings” are and how to use them
  • how to use variables to name things
  • how to “interpolate” strings
  • how to call methods
  • how to run Ruby from a file
  • how to interact with a user
  • how to define methods
  • how to repeat things
  • how to do things conditionally

If you’ve understood all of that, well done! You now know the basics of programming. There’s a lot more to learn, but it all builds on these fundamentals.

To tie everything you’ve learned together, here’s one final “hard” task.

Task 12: Build a times tables game

Right at the beginning, we said you’d build this times tables game.

To do this you’ll need to use everything you’ve just learned. It’s a pretty difficult problem for a beginner, so if you get stuck on something don’t be afraid to ask for help.

Here’s a rough plan of how to implement this:

  • create a score variable to keep track of the users score
  • in a loop that happens 4 times
    • generate two numbers between 1 and 12 using rand and assign them to variables
    • print a message to the user asking them to multiply the numbers together
    • get the user’s answer and covert it to a number (using .to_i)
    • compare the user’s answer to the answer Ruby thinks is correct
    • show the user a message saying whether they’re correct (and add one to their score)
  • if the user scored 4 then show them a message saying they got them all right
  • otherwise, show the user a message commiserating them for their failure

For bonus points, consider whether you could define a method to do some of the work to make the loop easier to read.


def multiplication_challenge
  x = rand(12) + 1
  y = rand(12) + 1

  print("What is #{x} multiplied by #{y} ? ")
  user_input = gets
  user_input_as_a_number = user_input.to_i

  correct_answer = x * y
  if user_input_as_a_number == correct_answer
    puts(":) correct!")
    return true
    puts(":( oops!")
    puts("The answer was #{correct_answer}")
    return false

number_of_rounds = 4
score = 0

puts("Times tables challenge")
puts("You will be asked #{number_of_rounds} questions.")
puts("Press enter to start...")

number_of_rounds.times do |i|
  if multiplication_challenge
    score = score + 1

if score == number_of_rounds
  puts("You scored #{score}/#{number_of_rounds}. Well done!")
  puts("You scored #{score}/#{number_of_rounds}. Better luck next time!")

Further reading

If you learn best from weird books then _why’s (poignant) guide is excellent (see cartoon below). If you’d prefer something less silly, have a look at:

Which are all great.

A comic strip from _why's (poignant) guide showing Dr. Cham lost in some tunnels under a mysterious castle