Skip to content
Permalink
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
786 lines (553 sloc) 22.4 KB
---
title: "Lesson 2: Ruby and the Command Line"
weight: 40
---
# Lesson 2: Ruby and the Command Line
## Introduction
### Objectives
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
### Goal
By the end of this tutorial you will have built a couple of small Ruby
programs, including one like [this times tables game](https://timestablesgame.richardtowers.repl.run).
## 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.
### Navigating around in the terminal
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 ~$ ](/images/lesson-2/image1.png)
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`:
```shell
pwd
```
#### `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:
```
/Users/your-username
```
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:
```shell
whargleblargle
```
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:
```shell
ls
```
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:
```shell
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`.
> **Notes**
> 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:
```shell
pwd
```
and check what files there are in this directory by running:
```shell
ls
```
There probably won't be any files yet, because we haven't created any.
## Introduction to Ruby
![The Ruby logo](/images/lesson-2/ruby.png)
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` (**I**nteractive **R**u<strong>b</strong>y)
* 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:
```shell
irb
```
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.
### Numbers
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".
<details>
<summary>Answer</summary>
```
irb(main):016:0> 191 * 7
=> 1337
```
</details>
### Strings
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.
### Variables
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:
```ruby
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.
<details>
<summary>Answer</summary>
```
irb(main):020:0> answer = 7 * 6
=> 42
irb(main):021:0> answer * 10
=> 420
irb(main):022:0> answer
=> 42
```
</details>
### 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.
<details>
<summary>Answer</summary>
```
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"
```
</details>
### 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
=> "MAKE ME LOUDER"
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)
<details>
<summary>Answer</summary>
```
irb(main):030:0> "!gniog peeK !llew repus gniod er'uoY".reverse
=> "You're doing super well! Keep going!"
```
</details>
### 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`:
```ruby
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:
```ruby
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.
<details>
<summary>Answer</summary>
```ruby
puts("Enter a string to reverse: ")
input = gets
puts("Your string reversed is: ")
puts(input.reverse)
```
</details>
#### 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:
```ruby
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.
<details>
<summary>Answer</summary>
```ruby
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}")
```
</details>
### 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:
```ruby
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:
```ruby
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:
```ruby
def method_name(argument)
result = "some value"
return result
end
```
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.
<details>
<summary>Answer</summary>
```ruby
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
end
age_dog_years = convert_to_dog_years(age_human_years_number)
puts("Your dog is #{age_dog_years} in dog years")
```
</details>
### Repeating things
<img src="/images/lesson-2/brøther.jpg" alt="Meme where a cat looks at a bowl of fruit loops, and says 'brøether, may i have the lööps'" style="max-width: 300px;">
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`):
```ruby
5.times do
puts("Odelay!")
end
```
```ruby
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.")
puts
end
```
```ruby
loop do
puts("What's your name?")
name = gets
puts("Hello #{name} (to exit this infinite loop, press ctrl+c)")
end
```
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`.
<details>
<summary>Answer</summary>
```ruby
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}")
end
```
</details>
### Conditionals
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:
```ruby
puts("What is your name?")
name = gets
if name.include?("r")
puts("Hi #{name} - you sound like a pretty cool person")
else
puts("Hi nerd")
end
```
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](https://timestablesgame.richardtowers.repl.run).
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.
<details>
<summary>Answer</summary>
```ruby
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
else
puts(":( oops!")
puts("The answer was #{correct_answer}")
return false
end
end
number_of_rounds = 4
score = 0
puts("Times tables challenge")
puts("----------------------")
puts
puts("You will be asked #{number_of_rounds} questions.")
puts("Press enter to start...")
gets
number_of_rounds.times do |i|
if multiplication_challenge
score = score + 1
end
end
if score == number_of_rounds
puts("You scored #{score}/#{number_of_rounds}. Well done!")
else
puts("You scored #{score}/#{number_of_rounds}. Better luck next time!")
end
```
</details>
## Further reading
If you learn best from _weird_ books then [_why's (poignant) guide](https://poignant.guide/)
is excellent (see cartoon below). If you'd prefer something less silly, have a look at:
* [Learn to Program](https://pine.fm/LearnToProgram/)
* [Learn Ruby The Hard Way](https://learnrubythehardway.org/book/)
* [Ruby Koans](http://rubykoans.com/)
Which are all great.
![A comic strip from _why's (poignant) guide showing Dr. Cham lost in some tunnels under a mysterious castle](/images/lesson-2/poignant-guide.gif)
You can’t perform that action at this time.