Ruby Cheatsheet

Language

Language

Syntax Rules

Comments start with a pound/sharp (#) character and go to EOL.
Ruby programs are sequence of expressions.
Each expression is delimited by semicolons(;) or newlines unless obviously incomplete (e.g. trailing ‘+’).
Backslashes at the end of line does not terminate expression.

Reserved Words

  alias   and     BEGIN   begin   break   case    class   def     defined 
  do      else    elsif   END     end     ensure  false   for     if 
  in      module  next    nil     not     or      redo    rescue  retry 
  return  self    super   then    true    undef   unless  until   when  
  while   yield

Array

  [1, 2, 3]
  %w(foo bar baz)
  %W(foo bar baz #{var})

Numbers

  123 1_234      # integers
  123.45 1.2e-3  # floating point
  0xffff         # hex
  0b01011        # binary 
  0377           # octal
  ?a             # ASCII character
  ?\C-a          # Control-a
  ?\M-a          # Meta-a
  ?\M-\C-a       # Meta-Control-a

String

In all of the %() cases below, you may use any matching characters or any single character for delimiters. %[], %!!, %@@, etc.

  ### string representation ###
  'no interpolation'
  "#{interpolation}, and backslashes\n"
  %q(no interpolation)
  %Q(interpolation and backslashes)
  %(interpolation and backslashes)
  `echo command interpretation with interpolation and backslashes`
  %x(echo command interpretation with interpolation and backslashes)

  ### backslash escape ###
  "\t"       # (tab) 
  "\n"       # (newline) 
  "\r"       # (carriage eturn) 
  "\f"       # (form feed)
  "\b"       # (backspace) 
  "\a"       # (bell) 
  "\e"       # (escape) 
  "\s"       # (whitespace) 
  "\nnn"     # (octal)
  "\xnn"     # (hexadecimal) 
  "\cx"      # (control) 
  "\C-x"     # (control x)
  "\M-x"     # (meta x)
  "\M-\C-x"  # (meta control x)

Variables

  $global_variable
  @@class_variable
  @instance_variable
  OtherClass::CONSTANT
  local_variable

Pseudo variables

  self       # the receiver of the current method
  nil        # the sole instance of the Class NilClass(represents false)
  true       # the sole instance of the Class TrueClass(typical true value)
  false      # the sole instance of the Class FalseClass(represents false)
  __FILE__   # the current source file name.
  __LINE__   # the current line number in the source file.

Pre-defined variables

  $!         # The exception information message set by 'raise'.
  $@         # Array of backtrace of the last exception thrown.
  $+         # The last bracket matched by the last successful match.
  $1         # The Nth group of the last successful match. May be > 1.
  $~         # The information about the last match in the current scope.
  $=         # The flag for case insensitive, nil by default.
  $/         # The input record separator, newline by default.
  $\         # The output record separator for the print and IO#write. Default is nil.
  $,         # The output field separator for the print and Array#join.
  $;         # The default separator for String#split.
  $.         # The current input line number of the last file that was read.
  $<         # The virtual concatenation file of the files given on command line.
  $>         # The default output for print, printf. $stdout by default.
  $_         # The last input line of string by gets or readline.
  $0         # Contains the name of the script being executed. May be assignable.
  $*         # Command line arguments given for the script sans args.
  $$         # The process number of the Ruby running this script.
  $?         # The status of the last executed child process.
  $:         # Load path for scripts and binary modules by load or require.
  $DEBUG     # The status of the -d switch.
  $FILENAME  # Current input file from $<. Same as $<.filename.
  $LOAD_PATH # The alias to the $:.
  $stderr    # The current standard error output.
  $stdin     # The current standard input.
  $stdout    # The current standard output.
  $VERBOSE   # The verbose flag, which is set by the -v switch.
  $-0        # The alias to $/.
  $-a        # True if option -a is set. Read-only variable.
  $-d        # The alias to $DEBUG.
  $-F        # The alias to $;.
  $-i        # In in-place-edit mode, this variable holds the extension, otherwise nil.
  $-I        # The alias to $:.
  $-l        # True if option -l is set. Read-only variable.
  $-p        # True if option -p is set. Read-only variable.
  $-v        # The alias to $VERBOSE.
  $-w        # True if option -w is set.
  $&         # The string matched by the last successful pattern match in this scope.
  $"         # The array contains the module names loaded by require.
  $`         # The string to the left  of the last successful match.
  $'         # The string to the right of the last successful match.