irb1.9.1(1) interactive ruby


irb [options]


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.

    $ irb1.9.1
    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.


-f suppress read ~/.irbrc
-m bc mode (fraction or matrix are available)
-d set $DEBUG to true (same as `ruby -d')
-r load-module same as `ruby -r'
--inspect uses `inspect' for output (the default except bc mode)
--noinspect doesn't uses inspect for output
--readline uses Readline extension module
--noreadline 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'
--inf-ruby-mode uses prompt appreciate for inf-ruby-mode on emacs. Suppresses --readline.
--simple-prompt simple prompt mode
--noprompt no prompt
--tracer 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] = {...}

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

    $ irb1.9.1 --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
    %m  to_s of main object (self)
    %M  inspect of main object (self)
    %l  type of string(", ', /, ]), `]' is inner %w[...]
    %NNi        indent level. NN is degits and means as same as printf("%NNd"). 
          It can be ommited
    %NNn        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.

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.


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'.
conf.back_trace_limit 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. 
conf.irb_level The level of cb.
conf.math_mode 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).
conf.prompt_c prompt for a continuating statement (e.g, immediately after of `if')
conf.prompt_i standard prompt
conf.prompt_s prompt for a continuating string
conf.rc 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)
conf.verbose=T/F 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.

System variable

_ The latest value of evaluation (it is local)

Session Example

    $ irb1.9.1
    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


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.

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'


Implementation of Here-document is incomplete.


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