How can I use Ruby to colorize the text output to a terminal?

As String class methods (Unix only):

class String
def black;          "\e[30m#{self}\e[0m" end
def red;            "\e[31m#{self}\e[0m" end
def green;          "\e[32m#{self}\e[0m" end
def brown;          "\e[33m#{self}\e[0m" end
def blue;           "\e[34m#{self}\e[0m" end
def magenta;        "\e[35m#{self}\e[0m" end
def cyan;           "\e[36m#{self}\e[0m" end
def gray;           "\e[37m#{self}\e[0m" end

def bg_black;       "\e[40m#{self}\e[0m" end
def bg_red;         "\e[41m#{self}\e[0m" end
def bg_green;       "\e[42m#{self}\e[0m" end
def bg_brown;       "\e[43m#{self}\e[0m" end
def bg_blue;        "\e[44m#{self}\e[0m" end
def bg_magenta;     "\e[45m#{self}\e[0m" end
def bg_cyan;        "\e[46m#{self}\e[0m" end
def bg_gray;        "\e[47m#{self}\e[0m" end

def bold;           "\e[1m#{self}\e[22m" end
def italic;         "\e[3m#{self}\e[23m" end
def underline;      "\e[4m#{self}\e[24m" end
def blink;          "\e[5m#{self}\e[25m" end
def reverse_color;  "\e[7m#{self}\e[27m" end
end

And usage:

puts "I'm back green".bg_green
puts "I'm red and back cyan".red.bg_cyan
puts "I'm bold and green and backround red".bold.green.bg_red

In my console:

Additionally,

def no_colors
  self.gsub /\e\[\d+m/, ""
end

removes formatting characters.

Note

puts "\e[31m" # set format (red foreground)
puts "\e[0m"   # clear format
puts "green-#{"red".red}-green".green # will be green-red-normal, because of \e[0

Colorize is my favorite gem! :-)

Check it out:

https://github.com/fazibear/colorize

Installation:

gem install colorize

Usage:

require 'colorize'

puts "I am now red".red
puts "I am now blue".blue
puts "Testing".yellow

I wrote a little method to test out the basic color modes, based on answers by Erik Skoglund and others.

#outputs color table to console, regular and bold modes
def colortable
  names = %w(black red green yellow blue pink cyan white default)
  fgcodes = (30..39).to_a - [38]

  s = ''
  reg  = "\e[%d;%dm%s\e[0m"
  bold = "\e[1;%d;%dm%s\e[0m"
  puts '                       color table with these background codes:'
  puts '          40       41       42       43       44       45       46       47       49'
  names.zip(fgcodes).each {|name,fg|
    s = "#{fg}"
    puts "%7s "%name + "#{reg}  #{bold}   "*9 % [fg,40,s,fg,40,s,  fg,41,s,fg,41,s,  fg,42,s,fg,42,s,  fg,43,s,fg,43,s,  
      fg,44,s,fg,44,s,  fg,45,s,fg,45,s,  fg,46,s,fg,46,s,  fg,47,s,fg,47,s,  fg,49,s,fg,49,s ]
  }
end

example output:

Combining the answers above, you can implement something that works like the gem colorize without needing another dependency.

class String
  # colorization
  def colorize(color_code)
    "\e[#{color_code}m#{self}\e[0m"
  end

  def red
    colorize(31)
  end

  def green
    colorize(32)
  end

  def yellow
    colorize(33)
  end

  def blue
    colorize(34)
  end

  def pink
    colorize(35)
  end

  def light_blue
    colorize(36)
  end
end