How to Use the mruby Debugger

copyright (c) 2014 Specified Non-Profit Corporation mruby Forum

1. Summary

This file documents the mruby debugger (‘mrdb’) methods.

2 Debugging with mrdb

2.1 Building mrdb

The trunk of the mruby source tree, with the most recent mrdb, can be checked out with the following command:

bash $ git clone

To run the make command:

bash $ cd mruby $ make

By default, the make command will install the debugger files into mruby/bin.

You can add the path for mrdb on your host environment with the following command:

bash $ echo "export PATH=\$PATH:MRUBY_ROOT/bin" >> ~/.bashrc $ source ~/.bashrc

*MRUBY_ROOT is the directory in which mruby source code will be installed.

To confirm mrdb was installed properly, run mrdb with the --version option:

bash $ mrdb --version mruby 2.1.0 (2019-11-19)

2.2 Basic Operation

2.2.1 Debugging mruby Script Files (rb file) with mrdb

To invoke the mruby debugger, just type mrdb.

To specify the script file:

bash $ mrdb [option] file name

For example: Debugging sample.rb

bash $ mrdb sample.rb

You can execute the shell commands listed below:

command description
run execute programs
step execute stepping
continue execute continuing program
break configure the breaking point
delete deleting the breaking points
disable disabling the breaking points
enable enabling the breaking points
info breakpoints showing list of the breaking points
print evaluating and printing the values of the mruby expressions in the script
list displaying the source cords
help showing help
quit terminating the mruby debugger

2.2.2 Debugging mruby Binary Files (mrb file) with mrdb

You can debug the mruby binary files. Debugging the binary files

  • notice To debug mruby binary files, you need to compile mruby files with option -g.

bash $ mrbc -g sample.rb

You can debug the mruby binary files with following command and the option -b.

bash $ mrdb -b sample.mrb

Then you can execute all debugger shell commands.

Break Command

You can use any breakpoint to stop the program by specifying the line number and method name. The breakpoint list will be displayed after you have set the breakpoint successfully.


break [file:]linenum b [file:]linenum break [class:]method b [class:]method

The breakpoint will be ordered in serial from 1. The number, which was given to the deleted breakpoint, will never be given to another breakpoint again.

You can give multiple breakpoints to specified the line number and method. Be ware that breakpoint command will not check the validity of the class name and method name.

You can get the current breakpoint information by the following options.

breakpoint breakpoint number : file name. line number

breakpoint breakpoint number : [class name,] method name

Continue Command


continue [N] c [N]

N: the next breakpoint number

When resuming the program, it will stop at breakpoint N (N-1 breakpoint will be ignored).

When you run the continue command without specifying N, the program will be stopped at the next breakpoint.


(foo.rb:1) continue 3

This will resume the program and stop it at the third breakpoint.

Delete Command

This will delete the specified breakpoint.


delete [breakpoint-no] d [breakpoint-no]

breakpoint-no: breakpoint number


(foo.rb:1) delete

This will delete all of the breakpoints.

(foo.rb:1) delete 1 3

This will delete the breakpoint at 1 and 3.

Disable Command

This will disable the specified breakpoint.


disable [breakpoint-no] dis [breakpoint-no]

reappointing: breakpoint number


(foo.rb:1) disable

Use disable if you would like to disable all of the breakpoints.

(foo.rb:1) disable 1 3

This will disable the breakpoints at 1 and 3.

Enable Command

This will enable the specified breakpoints.


enable [breakpoint-no] e [breakpoint-no]

breakpoint-no: breakpoint number


(foo.rb:1) enable

Enabling all breakpoints (foo.rb:1) enable 1 3

Enabling the breakpoint 1 and 3

eval command

Evaluating the string as source code and printing the value.

Same as print command, please see print command.

help command

Displaying the help message.


help [command] h [command]

Typing help without any options will display the command list.

Info Breakpoints Command

Displaying the specified breakpoint information.


info breakpoints [breakpoint-no] i b [breakpoint-no]

breakpoint-no: breakpoint number

Typing “info breakpoints” without ant option will display all breakpoint information. Example:

(sample.rb:1) info breakpoints Num Type Enb What 1 breakpoint y at sample.rb:3 -> file name,line number 2 breakpoint n in Sample_class:sample_class_method -> [class:]method name 3 breakpoint y in sample_global_method

Displaying the specified breakpoint number:

(foo.rb:1) info breakpoints 1 3 Num Type Enb What 1 breakpoint y at sample.rb:3 3 breakpoint y in sample_global_method

List Command

To display the code of the source file.


list [filename:]first[,last] l [filename]:first[,last]

first: the opening row number last : the closing row number

When you specify the first, but not the last option, you will receive 10 rows. When you do not specify both the first and last options, you will receive the next 10 rows.


Specifying file name and first row number sample.rb:1) list sample2.rb:5

Specifying the file name and the first and last row number:

(sample.rb:1) list sample2.rb:6,7

Evaluating the string as source code and printing the value.


print [expr] p [expr]

expr: expression

The expression is mandatory. The displayed expressions will be serially ordered from 1. If an exception occurs, the exception information will be displayed and the debugging will be continued.


(sample.rb:1) print 1+2 $1 = 3 (sample.rb:1) print self $2 = main

Below is the case of the exception:

(sample.rb:1) print (1+2 $1 = SyntaxError: line 1: syntax error, unexpected $end, expecting ')'

Quit Command

Quitting the debugger.


quit q

Run Command

Running the program and stopping at the first breakpoint.


run r

Step Command

This will run the program step by step. When the method and the block are invoked, the program will be stop at the first row. The program, which is developed in C, will be ignored.