if conditional [then]
  code...
[elsif conditional [then]
  code...]...
[else
  code...]
end

#!/usr/bin/ruby

x=1
if x > 2
  puts "x is greater than 2"
elsif x <= 2 and x!=0
  puts "x is 1"
else
  puts "I can't guess the number"
end

x is 1

#!/usr/bin/ruby

$debug=1
print "debug\n" if $debug

debug

unless conditional [then]
  code
[else
  code ]
end

#!/usr/bin/ruby

x=1
unless x>2
  puts "x is less than 2"
 else
 puts "x is greater than 2"
end

x is less than 2

#!/usr/bin/ruby

$var = 1
print "1 -- Value is set\n" if $var
print "2 -- Value is set\n" unless $var

$var = false
print "3 -- Value is set\n" unless $var

1 -- Value is set
3 -- Value is set

case expression
[when expression [, expression ...] [then]
  code ]...
[else
  code ]
end

case expr0
when expr1, expr2
  stmt1
when expr3, expr4
  stmt2
else
  stmt3
end

_tmp = expr0
if expr1 === _tmp || expr2 === _tmp
  stmt1
elsif expr3 === _tmp || expr4 === _tmp
  stmt2
else
  stmt3
end

#!/usr/bin/ruby

$age = 5
case $age
when 0 .. 2
  puts "baby"
when 3 .. 6
  puts "little child"
when 7 .. 12
  puts "child"
when 13 .. 18
  puts "youth"
else
  puts "adult"
end

little child