Pry debugging

Invoking pry debugging

To invoke the debugger, place binding.pry somewhere in your code. When the Ruby interpreter hits that code, execution will stop, and you can type in commands to debug the state of the program

byebug vs binding.pry

byebug has a very similar interface as gdb, but byebug does not use the powerful Pry REPL.

binding.pry uses Pry, but lacks some of the byebug features. GitLab uses the pry-byebug gem. This gem brings some capabilities byebug to binding.pry, so using that, will give you the most debugging powers.

byebug

Check out the docs for the full list of commands.

You can start the Pry REPL with the pry command.

pry

There are a lot of features present in pry, too much to cover in this document, so for the full documentation head over to the Pry wiki.

Below are a few features definitely worth checking out, also run help in a pry session to see what else you can do.

State navigation

With the state navigation you can move around in the code to discover methods and such:

# Change context
[1] pry(main)> cd Pry
[2] pry(Pry):1>

# Print methods
[2] pry(Pry):1> ls -m

# Find a method
[3] pry(Pry):1> find-method to_yaml

Source browsing

You look at the source code from your pry session:

[1] pry(main)> $ Array#first
# The above is equivalent to
[2] pry(main)> cd Array
[3] pry(Array):1> show-source first

$ is an alias for show-source.

Documentation browsing

Similar to source browsing, is Documentation browsing.

[1] pry(main)> show-doc Array#first

? is an alias for show-doc.

Command history

With Ctrl+R</kbd> you can search your [command history](https://github.com/pry/pry/wiki/History).

Stepping

To step through the code, you can use the following commands:

  • break: Manage breakpoints.
  • step: Step execution into the next line or method. Takes an optional numeric argument to step multiple times.
  • next: Step over to the next line within the same frame. Also takes an optional numeric argument to step multiple lines.
  • finish: Execute until current stack frame returns.
  • continue: Continue program execution and end the Pry session.

Callstack navigation

You also can move around in the callstack with these commands:

  • backtrace: Shows the current stack. You can use the numbers on the left side with the frame command to navigate the stack.
  • up: Moves the stack frame up. Takes an optional numeric argument to move multiple frames.
  • down: Moves the stack frame down. Takes an optional numeric argument to move multiple frames.
  • frame <n>: Moves to a specific frame. Called without arguments will show the current frame.

Short commands

When you use binding.pry instead of byebug, the short commands like s, n, f, and c do not work. To reinstall them, add this to ~/.pryrc:

if defined?(PryByebug)
  Pry.commands.alias_command 's', 'step'
  Pry.commands.alias_command 'n', 'next'
  Pry.commands.alias_command 'f', 'finish'
  Pry.commands.alias_command 'c', 'continue'
end

Repeat last command

You can repeat the last command by just hitting the Enter key (e.g., with step ornext), if you place the following snippet in your ~/.pryrc:

Pry::Commands.command /^$/, "repeat last command" do
  _pry_.run_command Pry.history.to_a.last
end

byebug supports this out-of-the-box.