Programming Ruby

The Pragmatic Programmer's Guide

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Interactive Ruby Shell

Back on page 124 we introduced irb, a Ruby module that lets you enter Ruby programs interactively and see the results immediately. This appendix goes into more detail on using and customizing irb.

Command Line

irb is run from the command line.

irb [
            ] [
            ] [

The command-line options for irb are listed in Table B.1 on page 518. Typically, you'll run irb with no options, but if you want to run a script and watch the blow-by-blow description as it runs, you can provide the name of the Ruby script and any options for that script.
irb command-line options
Option Description
-f Suppress reading ~/.irbrc.
-m Math mode (fraction and matrix support is available).
-d Set $DEBUG to true (same as ``ruby -d'').
-r load-module Same as ``ruby -r''.
--inspect Use ``inspect'' for output (the default, unless in math mode).
--noinspect Do not use inspect for output.
--readline Use Readline extension module.
--noreadline Do not use Readline extension module.
--prompt prompt-mode Switch prompt mode. Predefined prompt modes are ``default'', ``simple'', ``xmp'', and ``inf-ruby''.
--prompt-mode prompt-mode Same as --prompt.
--inf-ruby-mode Sets up irb to run in inf-ruby-mode under Emacs. Changes the prompt and suppresses --readline.
--simple-prompt Simple prompt mode.
--noprompt Do not display a prompt.
--tracer Display trace for execution of commands.
--back-trace-limit n Display backtrace information using the top n and last n entries. The default value is 16.
--irb_debug n Set internal debug level to n (only for irb development).
-v, --version Print the version of irb.

Initialization File

irb uses an initialization file in which you can set commonly used options or execute any required Ruby statements. When irb is run, it will try to load an initialization file from one of the following sources in order: ~/.irbrc, .irbrc, irb.rc, _irbrc, and $irbrc.

Within the initialization file you may run any arbitrary Ruby code. You can also set any of the configuration values that correspond to command-line arguments as shown in Table B.2 on page 518.
irb configuration values

IRB.conf[:IRB_NAME] = "irb" IRB.conf[:MATH_MODE] = false
IRB.conf[:USE_TRACER] = false IRB.conf[:USE_LOADER] = false
IRB.conf[:IGNORE_SIGINT] = true IRB.conf[:IGNORE_EOF] = false
IRB.conf[:INSPECT_MODE] = nil IRB.conf[:IRB_RC] = nil
IRB.conf[:BACK_TRACE_LIMIT] = 16 IRB.conf[:USE_LOADER] = false
IRB.conf[:USE_READLINE] = nil IRB.conf[:USE_TRACER] = false
IRB.conf[:IGNORE_SIGINT] = true IRB.conf[:IGNORE_EOF] = false
IRB.conf[:PROMPT_MODE] = :DEFAULT IRB.conf[:PROMPT] = { ... }
IRB.conf[:DEBUG_LEVEL] = 0 IRB.conf[:VERBOSE] = true

As an interesting twist on configuring irb, you can set IRB.conf[:IRB_RC] to a Proc object. This proc will be invoked whenever the irb context is changed, and will receive that new context as a parameter. You can use this facility to change the configuration dynamically based on the context.


At the irb prompt, you can enter any valid Ruby expression and see the results. You can also use any of the following commands to control the irb session.

exit, quit, irb_exit
Quits this irb session or subsession. If you've used cb to change bindings (see below), exits from this binding mode.

conf, irb_context
Displays current configuration. Modifying the configuration is achieved by invoking methods of conf.

conf.back_trace_limit n
Sets display lines of backtrace as top n and tail n. The default value is 16.

conf.debug_level = N
Sets debug level of irb.

conf.ignore_eof = true/false
Specifies the behavior of an end of file received on input. If true, it will be ignored; otherwise, it will quit irb.

conf.ignore_sigint= true/false
Specifies the behavior of ^C (control-c). If false, ^C will quit irb. If true, ^C during input will cancel input and return to the top level; during execution, ^C will abort the current operation.

conf.inf_ruby_mode = true/false
If true, changes the prompt and disables readline support, allowing irb to work with inf-ruby-mode. [inf-ruby-mode allows Emacs users to interact with Ruby while editing programs. See the file inf_ruby.el in the misc directory of the distribution for more details.] The default value is false.

conf.inspect_mode = true/false/nil
Specifies inspect mode according to the following values:

true Display inspect (default).
false Display to_s.
nil Inspect mode in non-math mode, non-inspect mode in math mode.

Displays the current binding level (see cb).

Displays whether or not Ruby is in math mode.

conf.use_loader = true/false
Specifies whether or not irb's own file reader method is used with load/require.

The prompt for a continuing statement (for example, immediately after an ``if'').

The standard, top-level prompt.

The prompt for a continuing string.

conf.rc = true/false
Specifies whether or not to use the initialization file ~/.irbrc.

conf.use_prompt = true/false
Specifies whether or not to display prompts.

conf.use_readline = true/false/nil
Specifies whether or not to use Readline according to the following values:

true Use Readline.
false Do not use Readline.
nil Use Readline except for inf-ruby-mode (default).

Specifies whether or not verbose messages are displayed.

cb, irb_change_binding [ obj ]
Creates and enters a new binding that has its own scope for local variables. If obj is given, it will be used as self in the new binding.

irb [obj]
Starts an irb subsession. If obj is given, it will be used as self.

jobs, irb_jobs
Lists irb subsessions.

fg n, irb_fg n
Switches into the specified irb subsession. n may be any of the following values:

irb subsession number
thread id
irb object
self (the obj that launched a particular subsession)

kill n, irb_kill n
Kills an irb subsession. n may be any of the values as described for irb_fg.

Configuring the Prompt

You have a lot of flexibility in configuring the prompts that irb uses. Sets of prompts are stored in the prompt hash:


For example, to establish a new prompt mode called ``MY_PROMPT'', you might enter the following (either directly at an irb prompt or in the .irbrc file):

IRB.conf[:PROMPT][:MY_PROMPT] = { # name of prompt mode
  :PROMPT_I => "...",             # normal prompt
  :PROMPT_S => "...",             # prompt for continuing strings
  :PROMPT_C => "...",             # prompt for continuing statement
  :RETURN => "    ==>%s\n"        # format to return value

Then, invoke irb with the prompt mode above by

% irb --prompt my-prompt

Or set the following configuration value:


The constants PROMPT_I, PROMPT_S, and PROMPT_C specify the format for each of the prompt strings. Within the prompt format, the following flags are available and will expand to the given text:

Flag Description
%N Current command.
%m to_s of the main object (self).
%M inspect of the main object (self).
%l Delimiter type. In strings that are continued across a line break, %l will display the type of delimiter used to begin the string, so you'll know how to end it. The delimiter will be one of ", ', /, ], or `.
%ni Indent level. The optional number n is used as a width specification to printf, as printf("%nd").
%nn Current line number (n used as with the indent level).
%% A literal percent sign.

For instance, the default prompt mode is defined as follows:

      :PROMPT_I => "%N(%m):%03n:%i> ",
      :PROMPT_S => "%N(%m):%03n:%i%l ",
      :PROMPT_C => "%N(%m):%03n:%i* ",
      :RETURN => "%s\n"


Because of the way irb works, there is a minor incompatibility between it and the standard Ruby interpreter. The problem lies in the determination of local variables.

Normally, Ruby looks for an assignment statement to determine if something is a variable---if a name hasn't been assigned to, then Ruby assumes that name is a method call.

eval "a = 0"
prog.rb:2: undefined local variable or method `a'
for #<Object:0x401c2ce0> (NameError)

In this case, the assignment is there, but it's within a string, so Ruby doesn't take it into account.

irb, on the other hand, executes statements as they are entered.

  irb(main):001:0> eval "a = 0"
  irb(main):002:0> a

In irb, the assignment was executed before the second line was encountered, so ``a'' is correctly identified as a local variable.

If you need to match the Ruby behavior more closely, you can place these statements within a begin/end pair.

  irb(main):001:0> begin
  irb(main):002:1*   eval "a = 0"
  irb(main):003:1>   a
  irb(main):004:1> end
  NameError: undefined local variable or method `a'
  (irb):3:in `irb_binding'

rtags, xmp, and the Frame Class

The base version of irb is installed along with Ruby itself. But there is an extended version of irb in the archives containing a few extra goodies that need mentioning.


rtags is a command used to create a TAGS file for use with either the emacs or vi editor.

rtags [
            ] [

By default, rtags makes a TAGS file suitable for emacs (see etags.el). The -vi option makes a TAGS file for use with vi.

rtags needs to be installed in the same manner as irb (that is, you need to install irb in the library path and make a link from irb/rtags.rb to bin/rtags).


irb's xmp is an ``example printer''---that is, a pretty-printer that shows the value of each expression as it is run (much like the script we wrote to format the examples in this book). There is also another stand-alone xmp in the archives.

xmp can be used as follows:

require "irb/xmp"

xmp <<END artist = "Doc Severinsen" artist END
artist = "Doc Severinsen"
    ==>"Doc Severinsen"
    ==>"Doc Severinsen"

Or, it can be used as an object instance. Used in this fashion, the object maintains context between invocations:

require "irb/xmp"

x = x.puts <<END artist = "Louis Prima" END

x.puts <<END artist END
artist = "Louis Prima"
    ==>"Louis Prima"
    ==>"Louis Prima"

You can explicitly provide a binding with either form; otherwise, xmp uses the caller's environment.

xmp code_string, abinding

Note that xmp does not work with multithreading.

The Frame Class

The IRB::Frame class represents the interpreter's stack and allows easy access to the Binding environment in effect at different stack levels. = 0) Returns a Binding for the nth context from the top. The 0th context is topmost, most recent frame.
IRB::Frame.bottom(n = 0) Returns a Binding for the nth context from the bottom. The 0th context is the bottommost, initial frame.
IRB::Frame.sender Returns the object (the sender) that invoked the current method.

You can use this facility, for instance, to examine local variables from the method that called the current method:

require 'irb/frame'

def outie   b =   eval "p my_local", b end

def innie   my_local = 102.7   outie end


Note that this doesn't work with multithreaded programs.

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Extracted from the book "Programming Ruby - The Pragmatic Programmer's Guide"
Copyright © 2001 by Addison Wesley Longman, Inc. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at

Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.

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