Byebug

Debugging in Ruby 2

Byebug is a simple to use, feature rich debugger for Ruby 2. It uses the new TracePoint API for execution control and the new Debug Inspector API for call stack navigation, so it doesn't depend on internal core sources. It's developed as a C extension, so it's fast. And it has a full test suite so it's reliable.

It allows you to see what is going on inside a Ruby program while it executes and can do four main kinds of things to help you catch bugs in the act:

Start your program or attach to it, specifying anything that might affect its behavior.
Make your program stop on specified conditions.
Examine what has happened when your program has stopped.
Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.

Install

$ gem install byebug

Please upgrade your ruby to 2.0.0-p247 or higher - a bug in ruby core was directly affecting byebug and a fix for it has been released with this patchlevel (see #5 for more information)

Usage

Simply drop

byebug

wherever you want to start debugging and the execution will stop there. If you are debugging rails, start the server and once the execution gets to your byebug command you will get a debugging prompt.

Former debugger or ruby-debug users, notice:

Some gems (rails, rspec) implement debugging flags (-d, --debugger) that early require and start the debugger. These flags are a performance penalty and Byebug doesn't need them anymore so my recommendation is not to use them.
Stopping execution using the word debugger doesn't work anymore unless you explicitly alias it. Similarly, the startup configuration file is now called .byebugrc instead of .rdebugrc.
autoreload, autoeval and autolist are default options in Byebug so you no longer need to set them in the startup file.

What's different from debugger

Works on Ruby 2.x and it doesn't on 1.9.x.
Has no MRI internal source code dependencies, just a clean API.
Fixes all of debugger's open bugs in its issue tracker and provides some enhancements, such as a markdown guide or the fact that byebug can now be placed at the end of a block or method call.
Very actively mantained.
Editor agnostic: no external editor built-in support.
Pry command is built-in. No need of external gem like debugger-pry.

Semantic Versioning

Byebug tries to follow semantic versioning. Backwards compatibility doesn't seem like a critic issue for a debugger because it's not supposed to be used permanently by any program, let alone in production environments. However, I still like the idea of giving some meaning to version changes.

Byebug's public API is determined by its set of commands

Command     | Aliases      | Subcommands
----------- |:------------ |:-----------
`backtrace` | `bt` `where` |
`break`     |              |
`catch`     |              |
`condition` |              |
`continue`  |              |
`delete`    |              |
`disable`   |              | `breakpoints` `display`
`display`   |              |
`down`      |              |
`edit`      |              |
`enable`    |              | `breakpoints` `display`
`finish`    |              |
`frame`     |              |
`help`      |              |
`info`      |              | `args` `breakpoints` `catch` `display` `file` `files` `global_variables` `instance_variables` `line` `locals` `program` `stack` `variables`
`irb`       |              |
`kill`      |              |
`list`      |              |
`method`    |              | `instance` `iv`
`next`      |              |
`p`         | `eval`       |
`pp`        |              |
`pry`       |              |
`ps`        |              |
`putl`      |              |
`quit`      | `exit`       |
`reload`    |              |
`restart`   |              |
`save`      |              |
`set`       |              | `args` `autoeval` `autoirb` `autolist` `autoreload` `basename` `callstyle` `callstyle` `forcestep` `fullpath` `history` `linetrace` `linetrace_plus` `listsize` `post_mortem` `stack_on_error` `testing` `verbose` `width`
`show`      |              | `args` `autoeval` `autoirb` `autolist` `autoreload` `basename` `callstyle` `callstyle` `commands` `forcestep` `fullpath` `history` `linetrace` `linetrace_plus` `listsize` `post_mortem` `stack_on_error` `verbose` `width`
`skip`      |              |
`source`    |              |
`step`      |              |
`thread`    |              | `current` `list` `resume` `stop` `switch`
`trace`     |              |
`undisplay` |              |
`up`        |              |
`var`       |              | `class` `constant` `global` `instance` `local` `ct`

Getting Started

Read byebug's markdown guide to get started. Proper documentation will be eventually written.

Related projects

pry-byebug adds next, step, finish, continue and break commands to pry using byebug.
ruby-debug-passenger adds a rake task that restarts Passenger with byebug connected.
minitest-byebug starts a byebug session on minitest failures.

Future (possible) directions

JRuby support.
Libify and test byebug's executable.
Add printers support.

Credits

Everybody who has ever contributed to this forked and reforked piece of software, specially:

Kent Sibilev and Mark Moseley, original authors of ruby-debug.
Gabriel Horner, debugger's mantainer.
Koichi Sasada, author of the new C debugging API for Ruby.
Dennis Ushakov, author of debase, the starting point of this.
Logo by Ivlichev Victor Petrovich
@kevjames3 for testing, bug reports and the interest in the project.

mrkn/byebug