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interactive ruby


irb [options]

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irb stands for ’interactive ruby’. irb is a tool to execute interactively ruby expressions read from stdin. Use of irb is easy if you know ruby. Executing irb, prompts are displayed as follows. Then, enter expression of ruby. A input is executed when it is syntacticaly completed.

$ irb1.8
irb(main):001:0> 1+2
irb(main):002:0> class Foo
irb(main):003:1> def foo
irb(main):004:2> print 1
irb(main):005:2> end
irb(main):006:1> end

And, Readline extesion module can be used with irb. Using Readline is the standard default action if Readline is installed.



suppress read ~/.irbrc


bc mode (fraction or matrix are available)


set $DEBUG to true (same as ’ruby -d’)

-r load-module

same as ’ruby -r’


uses ’inspect’ for output (the default except bc mode)


doesn’t uses inspect for output


uses Readline extension module


doesn’t use Readline extension module

--prompt prompt-mode
--prompt-mode prompt-mode

switches prompt mode. Pre-defined prompt modes are ’default’, ’simple’, ’xmp’ and ’inf-ruby’


uses prompt appreciate for inf-ruby-mode on emacs. Suppresses --readline.


simple prompt mode


no prompt


display trace for each execution of commands.

--back-trace-limit n

displayes backtrace top n and tail n. The default value is 16.

--irb_debug n

sets internal debug level to n (It shouldn’t be used)

-v, --version

prints the version of irb


irb reads ’~/.irbrc’ when it is invoked. If ’~/.irbrb’ doesn’t exist irb try to read in the order ’.irbrc’, ’irb.rc’, ’_irbrc’ then ’$irbrc’. The following is altanative to the command line option. To use them type as follows in an irb session.

IRB.conf[:IRB_RC] = nil
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] = {...}


For irb commands, both simple name and ’irb_’-prefixed name are prepared.
exit, quit, irb_exit

Quits (sub)irb. if you’ve done cb (see below), exit from the binding mode.

conf, irb_context

Displays current configuration. Modifing the configuration is achieved by sending message to ’conf’.


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

Whether ^D (control-d) will be ignored or not. If false is set, ^D means quit.

conf.ignore_sigint= true/false

Whether ^C (control-c) will be ignored or not. If false is set, ^D means quit. If true,
during input: cancel inputing then return to top level.
during execute: abondon current execution.

conf.inf_ruby_mode = true/false

Whether inf-ruby-mode or not. The default value is false.

conf.inspect_mode = true/false/nil

Specifies inspect mode. true: display inspect false: display to_s nil: inspect mode in non math mode,
non inspect mode in math mode.


The level of cb.


Whether bc mode or not.

conf.use_loader = true/false

Whether irb’s own file reader method is used when load/require or not. This mode is globaly affected (irb wide).


prompt for a continuating statement (e.g, immediately after of ’if’)


standard prompt


prompt for a continuating string


Whether ~/.irbrc is read or not.

conf.use_prompt = true/false

Prompting or not.

conf.use_readline = true/false/nil

Whether readline is used or not. true: uses false: doen’t use nil: intends to use readline except for inf-reuby-mode (default)


Whether verbose messages are display or not.

cb, irb_change_binding [obj]

Enter new binding which has a distinct scope of local variables. If obj is given, obj will be self.

irb [obj]

Invoke subirb. If obj is given, obj will be self.

jobs, irb_jobs

List of subirb

fg n, irb_fg n

Switch into specified subirb. The following is candidates of n:
irb number
irb object
self(obj which is specified of irb obj)

kill n, irb_kill n

Kill subirb. The means of n is as same as the case of irb_fg.

configurating subirb

The command line option or IRB.conf specify the default behavior of (sub)irb. On the other hand, each conf of in the next sction ’6. Command’ is used to individually configurate (sub)irb. If proc is set to IRB.conf[:IRB_RC], its subirb will be invoked after execution of that proc under giving the context of irb as its aregument. By this mechanism each subirb can be configurated.

customizing prompt

To costomize the prompt you set a variable


For example, describe as follows in ’.irbrc’.

IRB.conf[:PROMPT][:MY_PROMPT] = { # name of prompt mode

:PROMPT_I => nil,

# normal prompt
:PROMPT_S => nil,

# prompt for continuated strings
:PROMPT_C => nil,

# prompt for continuated statement
:RETURN => " ==>%s\n"

# format to return value


Then, invoke irb with the above prompt mode by

$ irb1.8 --prompt my-prompt

Or add the following in ’.irbrc’.


Constants PROMPT_I, PROMPT_S and PROMPT_C specifies the format. In the prompt specification, some special strings are available.

%N command name which is running


to_s of main object (self)

inspect of main object (self)

type of string(", ’, /, ]), ’]’ is inner %w[...]

indent level. NN is degits and means as same as printf("%NNd").

It can be ommited


line number.

%% %
For instance, the default prompt mode is defined as follows: IRB.conf[:PROMPT_MODE][:DEFAULT] = {
PROMPT_I => "%N(%m):%03n:%i> ",
PROMPT_S => "%N(%m):%03n:%i%l ",
PROMPT_C => "%N(%m):%03n:%i* ",
RETURN => "%s\n"}

RETURN is used to printf.

declaration of the local variable

The following causes an error in ruby:

eval "foo = 0"
-:2: undefined local variable or method ’foo’ for #<Object:0x40283118> (NameError)

Though, the above will successfully done by irb.

>> eval "foo = 0"
=> 0
>> foo
=> 0

Ruby evaluates a code after reading entire of code and determination of the scope of local variables. On the other hand, irb do immediately. More precisely, irb evaluate at first

evel "foo = 0"

then foo is defined on this timing. It is because of this incompatibility. If you’d like to detect those differences, begin...end can be used:

>> begin
?> eval "foo = 0"
>> foo
>> end
NameError: undefined local variable or method ’foo’ for #<Object:0x4013d0f0>
(irb_local_binding):1:in ’eval’

here document

Implementation of Here-document is incomplete.


Because irb evaluates the inputs immediately after the imput is syntactically completed, irb gives slight different result than directly use ruby. Known difference is pointed out here.

session example

$ irb1.8
irb(main):001:0> irb # invoke subirb
irb#1(main):001:0> jobs # list of subirbs
#0->irb on main (#<Thread:0x400fb7e4> : stop)
#1->irb#1 on main (#<Thread:0x40125d64> : running)
irb#1(main):002:0> fg 0 # switch job
irb(main):002:0> class Foo;end
irb(main):003:0> irb Foo # invoke subirb which has the
# context of Foo
irb#2(Foo):001:0> def foo # define Foo#foo
irb#2(Foo):002:1> print 1
irb#2(Foo):003:1> end
irb#2(Foo):004:0> fg 0 # switch job
irb(main):004:0> jobs # list of job
#0->irb on main (#<Thread:0x400fb7e4> : running)
#1->irb#1 on main (#<Thread:0x40125d64> : stop)
#2->irb#2 on Foo (#<Thread:0x4011d54c> : stop)
irb(main):005:0> Foo.instance_methods # Foo#foo is defined asurely
irb(main):006:0> fg 2 # switch job
irb#2(Foo):005:0> def bar # define Foo#bar
irb#2(Foo):006:1> print "bar"
irb#2(Foo):007:1> end
irb#2(Foo):010:0> Foo.instance_methods
["bar", "foo"]
irb#2(Foo):011:0> fg 0
irb(main):007:0> f =
irb(main):008:0> irb f # invoke subirb which has the
# context of f (instance of Foo)
irb#3(#<Foo:0x4010af3c>):001:0> jobs
#0->irb on main (#<Thread:0x400fb7e4> : stop)
#1->irb#1 on main (#<Thread:0x40125d64> : stop)
#2->irb#2 on Foo (#<Thread:0x4011d54c> : stop)
#3->irb#3 on #<Foo:0x4010af3c> (#<Thread:0x4010a1e0> : running)
irb#3(#<Foo:0x4010af3c>):002:0> foo # evaluate
irb#3(#<Foo:0x4010af3c>):003:0> bar # evaluate
irb#3(#<Foo:0x4010af3c>):004:0> kill 1, 2, 3# kill job
irb(main):009:0> jobs
#0->irb on main (#<Thread:0x400fb7e4> : running)
irb(main):010:0> exit # exit


Irb can not always recognize a symbol as to be Symbol. Concretely, an expression have completed, however Irb regard it as continuation line.

system variable


The latest value of evaluation (it is local)

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