Rubocop がチェックしてくれるもの一覧表 (Style編) ref: http://qiita.com/Yinaura/items/9928e32fa9fc9092f24c

.rubocop.yml

#例：

Style/RegexpLiteral:
  EnforcedStyle: percent_r

file0.txt

# bad
class SomeClass
private

  def private_method
    # ...
  end
end

# good
class SomeClass
  private

  def private_method
    # ...
  end
end

file1.txt

# bad
def set_attribute(value) ...

# good
def attribute=(value)

# bad
def get_attribute ...

# good
def attribute ...

file10.txt

# bad
$ORIGINAL_PATH = '/path/to/user'
puts $ORIGINAL_PATH

# good
puts $LOAD_PATH

file11.txt

# bad
hash = { :kind => 'Apple' }

# good
hash = { kind: 'Apple' }

file12.txt

def test
  puts 'hello'
-   puts 'world'
+  puts 'world'
end

file13.txt

# bad
def test
 puts 'hello'
end

# good
def test
  puts 'hello'
end

file14.txt

# bad
if cond then
end

# good
if cond
end

# good
if cond then a
elsif cond then b
end

file15.txt

- expect{ subject } 
+ expect { subject } 

file16.txt

# bad
true ?
yes :
no

# good
true ? yes : no

# good
if true
  yes
else
  no
end

file17.txt

# bad
some_str = 'ala' +
           'bala'

some_str = 'ala' <<
           'bala'

# good
some_str = 'ala' \
           'bala'

file18.txt

# bad
def some_method()
 # body omitted
end

# good
def some_method
 # body omitted
end

# bad
def some_method_with_parameters param1, param2
 # body omitted
end

# good
def some_method_with_parameters(param1, param2)
 # body omitted
end

file19.txt

# bad
def ExampleMethod
end

# good
def example_method
end

file2.txt

# 悪い例
%w(one two three) * ', '
# => 'one, two, three'

# 良い例
%w(one two three).join(', ')
# => 'one, two, three'

file20.txt

# bad
if x != nil
if !x.nil?

# good
if x

file21.txt

# good
def signed_in?
  !current_user.nil?
end

file22.txt

# bad
if (a && b)
end

# good
if a && b
end

# good

if (a && b) || c
end

file23.txt

# bad
def is_even?(value) ...

# good
def even?(value)

# bad
def has_value? ...

# good
def value? ...

file24.txt

def redundant
  begin
    ala
    bala
  rescue StandardError => e
    something
  end
end

def preferred
  ala
  bala
rescue StandardError => e
  something
end

file25.txt

# bad
raise RuntimeError, 'message'

# good - signals a RuntimeError by default
raise 'message'

file26.txt

# maybe bad
def test
  return something
end

# maybe good
def test
  something
end

file27.txt

# bad
begin
  something
  rescue
  puts 'error'
end

# good
begin
  something
rescue
  puts 'error'
end

file28.txt

# EnforcedStyle の設定が slashes / mixed の場合は良い。 percent_r の場合は悪い。
snake_case = /^[\dA-Z_]+$/

# EnforcedStyle の設定が percent_r の場合は良い。 slashes / mixed の場合は悪い。
snake_case = %r{^[\dA-Z_]+$}

# EnforcedStyle の設定が slashes の場合は良い。 percent_r / mixed の場合は悪い。
regex = /
  foo
  (bar)
  (baz)
/x

# EnforcedStyle の設定が percent_r / mixed の場合は良い。 slashes の場合は悪い。
regex = %r{
  foo
  (bar)
  (baz)
}x

# スラッシュ記法の中でスラッシュを使わないこと。
# AllowInnerSlashes の設定を true にしていれば警告しない。
x =~ /home\//

file29.txt

# EnforcedStyle の設定が slashes / mixed の場合は良い。 percent_r の場合は悪い。
snake_case = /^[\dA-Z_]+$/

# EnforcedStyle の設定が percent_r の場合は良い。 slashes / mixed の場合は悪い。
snake_case = %r{^[\dA-Z_]+$}

# EnforcedStyle の設定が slashes の場合は良い。 percent_r / mixed の場合は悪い。
regex = /
  foo
  (bar)
  (baz)
/x

# EnforcedStyle の設定が percent_r / mixed の場合は良い。 slashes の場合は悪い。
regex = %r{
  foo
  (bar)
  (baz)
}x

# スラッシュ記法の中でスラッシュを使わないこと。
# AllowInnerSlashes の設定を true にしていれば警告しない。
x =~ /home\//

file3.txt

# bad
if x = 1 
end

file30.txt

# bad
x = x + 1

# good
x += 1

file31.txt

# bad
def method; nil; end

# good
def method
  nil
end

file32.txt

# bad
x = 1;y = 2

# good
x = 1; y = 2

file33.txt

# bad
hash.map! {|k, v|v }

# bad
hash.map { | k, v | v }

# good
hash.map { |k, v| v }

file34.txt

#bad
['Apple','Banana','Strawberry']

#good
['Apple', 'Banana', 'Strawberry']

file35.txt

# bad
if(a && b) || c then result end

# good_
if (a && b) || c then result end

file36.txt

# bad
def func (x)
end

# good
def func(x)
end

file37.txt

# bad
! something

# good
!something

file38.txt

# bad
x = 1;y = 2;

# good
x = 1; y = 2;

file39.txt

# bad
def example(value='')
end

# good
def example(value = '')
end

file4.txt

# 例
attr_accessor :getter_with_setter
attr_reader :getter1, :getter2, :getter3
attr_writer :setter1, :setter2, :setter3

file40.txt

# bad
x=1+1

# bad
x  =  1  +  1

# good
x = 1 + 1

file41.txt

# bad
x = 2 ** 3

# good
x = 2**3

file42.txt

# bad
(1..10).each{ |i| i }

# good
(1..10).each { |i| i }

file43.txt

# bad
method# Comment

# good
method # Comment

file44.txt

# bad
result if(a && b) || c

# good
result if (a && b) || c

file45.txt

# bad
(1..10).each {|i| i}

# good
(1..10).each { |i| i }

file46.txt

# bad
furuits = {apple: 10, banana: 20}

# good
furuits = { apple: 10, banana: 20 }

file47.txt

# bad
some( arg ).other
[ 1, 2, 3 ].size

# good
some(arg).other
[1, 2, 3].size

file48.txt

# bad
1 .. 3

# good
1..3

# bad
'a' .. 'z'

# good
'a'..'z'

file49.txt

$: => $LOAD_PATH
$" => $LOADED_FEATURES
$0 => $PROGRAM_NAME
$! => $ERROR_INFO
$@ => $ERROR_POSITION
$; => $FIELD_SEPARATOR / $FS
$, => $OUTPUT_FIELD_SEPARATOR / $OFS
$/ => $INPUT_RECORD_SEPARATOR / $RS
$\\ => $OUTPUT_RECORD_SEPARATOR / $ORS
$. => $INPUT_LINE_NUMBER / $NR
$_ => $LAST_READ_LINE
$> => $DEFAULT_OUTPUT
$< => $DEFAULT_INPUT
$$ => $PROCESS_ID / $PID
$? => $CHILD_STATUS
$~ => $LAST_MATCH_INFO
$= => $IGNORECASE
$* => $ARGV => / => ARGV
$& => $MATCH
$` => $PREMATCH
$\' => $POSTMATCH
$+ => $LAST_PAREN_MATCH

file5.txt

# 悪い例
== begin
comment line
another comment line
== end

# 良い例
# comment line
# another comment line

file50.txt

# bad
:symbol"

# good
:symbol

file51.txt

# bad
something.map { |s| s.upcase }

# good
something.map(&:upcase)

file52.txt

# maybe bad
x_ = 1

file53.txt

# bad
class Foo
  def example=(value)
    @example = value
  end

  def example
    @example
  end
end

# good
class Foo
  attr_accessor :example
end

file54.txt

# bad
someVar = 5

# good
some_var = 5

file55.txt

# bad
when x; apple

# good
when x then apple

file56.txt

# bad
while x > 5 do
  # body omitted
end

until x > 5 do
  # body omitted
end

# good
while x > 5
  # body omitted
end

until x > 5
  # body omitted
end

file57.txt

# bad 
class Example
  def some_method(argument1, argument2, argument3, argument4, argument5, argument6)
  end
end

file6.txt

# bad
case
  when song.name == 'Misty'
    puts 'Not again!'
  when song.duration > 120
    puts 'Too long!'
  when Time.now.hour > 21
    puts "It's too late"
  else
    song.play
end

# good
case
when song.name == 'Misty'
  puts 'Not again!'
when song.duration > 120
  puts 'Too long!'
when Time.now.hour > 21
  puts "It's too late"
else
  song.play
end

file7.txt

# bad
#FIXME Comment

# good
# FIXME: Comment

file8.txt

some_array = []
some_hash = {}
some_string = ''

file9.txt

# bad
if x % 2 == 0

# good
if x.even?