How does `rescue $!` work?

Question

I know that the global variable $! holds the most recent exception object, but I am having confusion with the syntax below. Can anyone help me understand the following syntax?

 rescue $!

Answer 1

This construct prevents exception from stopping your program and bubbling up the stack trace. It also returns that exception as a value, which can be useful.

a = get_me_data rescue $!

After this line, a will hold either requested data or an exception. You can then analyze that exception and act accordingly.

def get_me_data
  raise 'No data for you'
end

a = get_me_data rescue $!
puts "Execution carries on"

p a
# >> Execution carries on
# >> #<RuntimeError: No data for you>

More realistic example

lines = File.readlines(filename) rescue $!

You either get the lines or the error (if file doesn't exist, you don't have permissions, etc). In any case, execution doesn't stop.