From http://learnxinyminutes.com/docs/ruby/

learnruby.rb

# This is a comment

=begin
This is a multiline comment
No-one uses them
You shouldn't either
=end

# First and foremost: Everything is an object.

# Numbers are objects

3.class #=> Fixnum

3.to_s #=> "3"


# Some basic arithmetic
1 + 1 #=> 2
8 - 1 #=> 7
10 * 2 #=> 20
35 / 5 #=> 7

# Arithmetic is just syntactic sugar
# for calling a method on an object
1.+(3) #=> 4
10.* 5 #=> 50 

# Special values are objects
nil # Nothing to see here
true # truth
false # falsehood

nil.class #=> NilClass
true.class #=> TrueClass
false.class #=> FalseClass

# Equality
1 == 1 #=> true
2 == 1 #=> false

# Inequality
1 != 1 #=> false
2 != 1 #=> true
!true  #=> false
!false #=> true

# apart from false itself, nil is the only other 'falsey' value

!nil   #=> true
!false #=> true
!0     #=> false

# More comparisons
1 < 10 #=> true
1 > 10 #=> false
2 <= 2 #=> true
2 >= 2 #=> true

# Strings are objects

'I am a string'.class #=> String
"I am a string too".class #=> String

placeholder = "use string interpolation"
"I can #{placeholder} when using double quoted strings"
#=> "I can use string interpolation when using double quoted strings"


# print to the output
puts "I'm printing!"

# Variables
x = 25 #=> 25
x #=> 25

# Note that assignment returns the value assigned
# This means you can do multiple assignment:

x = y = 10 #=> 10
x #=> 10
y #=> 10

# By convention, use snake_case for variable names
snake_case = true

# Use descriptive variable names
path_to_project_root = '/good/name/'
path = '/bad/name/'

# Symbols (are objects)
# Symbols are immutable, reusable constants represented internally by an
# integer value. They're often used instead of strings to efficiently convey
# specific, meaningful values

:pending.class #=> Symbol

status = :pending

status == :pending #=> true

status == 'pending' #=> false

status == :approved #=> false

# Arrays

# This is an array
[1, 2, 3, 4, 5] #=> [1, 2, 3, 4, 5]

# Arrays can contain different types of items

array = [1, "hello", false] #=> => [1, "hello", false]

# Arrays can be indexed
# From the front
array[0] #=> 1
array[12] #=> nil

# Like arithmetic, [var] access
# is just syntactic sugar
# for calling a method [] on an object
array.[] 0 #=> 1
array.[] 12 #=> nil

# From the end
array[-1] #=> 5

# With a start and end index
array[2, 4] #=> [3, 4, 5]

# Or with a range
array[1..3] #=> [2, 3, 4]

# Add to an array like this
array << 6 #=> [1, 2, 3, 4, 5, 6]

# Hashes are Ruby's primary dictionary with keys/value pairs.
# Hashes are denoted with curly braces:
hash = {'color' => 'green', 'number' => 5}

hash.keys #=> ['color', 'number']

# Hashes can be quickly looked up by key:
hash['color'] #=> 'green'
hash['number'] #=> 5

# Asking a hash for a key that doesn't exist returns nil:
hash['nothing here'] #=> nil

# Iterate over hashes with the #each method:
hash.each do |k, v|
  puts "#{k} is #{v}"
end

# Since Ruby 1.9, there's a special syntax when using symbols as keys:

new_hash = { defcon: 3, action: true}

new_hash.keys #=> [:defcon, :action]

# Tip: Both Arrays and Hashes are Enumerable
# They share a lot of useful methods such as each, map, count, and more

# Control structures

if true
  "if statement"
elsif false
 "else if, optional"
else
 "else, also optional"
end

for counter in 1..5
  puts "iteration #{counter}"
end
#=> iteration 1
#=> iteration 2
#=> iteration 3
#=> iteration 4
#=> iteration 5

# HOWEVER
# No-one uses for loops
# Use `each` instead, like this:

(1..5).each do |counter|
  puts "iteration #{counter}"
end
#=> iteration 1
#=> iteration 2
#=> iteration 3
#=> iteration 4
#=> iteration 5

counter = 1
while counter <= 5 do
  puts "iteration #{counter}"
  counter += 1
end
#=> iteration 1
#=> iteration 2
#=> iteration 3
#=> iteration 4
#=> iteration 5

grade = 'B'

case grade
when 'A'
  puts "Way to go kiddo"
when 'B'
  puts "Better luck next time"
when 'C'
  puts "You can do better"
when 'D'
  puts "Scraping through"
when 'F'
  puts "You failed!"
else 
  puts "Alternative grading system, eh?"
end

# Functions

def double(x)
  x * 2
end

# Functions (and all blocks) implcitly return the value of the last statement
double(2) #=> 4

# Parentheses are optional where the result is unambiguous
double 3 #=> 6

double double 3 #=> 12

def sum(x,y)
  x + y
end

# Method arguments are separated by a comma
sum 3, 4 #=> 7

sum sum(3,4), 5 #=> 12

# yield
# All methods have an implicit, optional block parameter
# it can be called with the 'yield' keyword

def surround
  puts "{"
  yield
  puts "}"
end

surround { puts 'hello world' }

# {
# hello world
# }


# Define a class with the class keyword
class Human

     # A class variable. It is shared by all instances of this class.
    @@species = "H. sapiens"

    # Basic initializer
    def initialize(name, age=0)
        # Assign the argument to the "name" instance variable for the instance
        @name = name
        # If no age given, we will fall back to the default in the arguments list.
        @age = age
    end

    # Basic setter method
    def name=(name)
        @name = name
    end

    # Basic getter method
    def name
        @name
    end

    # A class method uses self to distinguish from instance methods.
    # It can only be called on the class, not an instance.
    def self.say(msg)
       puts "#{msg}"
    end

    def species
        @@species
    end

end


# Instantiate a class
jim = Human.new("Jim Halpert")

dwight = Human.new("Dwight K. Schrute")

# Let's call a couple of methods
jim.species #=> "H. sapiens"
jim.name #=> "Jim Halpert"
jim.name = "Jim Halpert II" #=> "Jim Halpert II"
jim.name #=> "Jim Halpert II"
dwight.species #=> "H. sapiens"
dwight.name #=> "Dwight K. Schrute"

# Call the class method
Human.say("Hi") #=> "Hi"