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Catching Errors and Exceptions

Errors are child classes of StandardError class. To catch errors we surround the offending code in begin and rescue block as follows :

begin
  puts 1 / 0 # code that can generate an error
rescue
  puts "Cant divide by Zero" # catch the error here
end

Catching specific exceptions

We can catch specifying their name in the rescue bloc as follows :

begin
  file = File.open "nginx.conf"
  puts 1 / 0
rescue StandardError => error # Catch the StandardError
  puts "Cant divide by Zero"
  puts "Error was #{error.class}"
end

We can catch more errors in a block, the specific errors first and general at the end as follows :

begin
  file = File.open "nginx.conf"
  puts 1 / 0
rescue ZeroDivisionError => error # more specific
  puts "Cant divide by Zero"
  puts "Error #{error.to_s}"
rescue Errno::ENOENT => error # general
  puts "Sorry, we can't open the file specified"
  puts "Error: #{error.to_s}"
end

Raising Exceptions

We raise exceptions by using the raise keyword as follows :

begin
  raise "Testing an error" # raise and exception
  puts “Shouldn't execute this code"
rescue Exception => error
  puts "Rescued an exception: #{error.inspect}"
end

Creating Our Own Exceptions

We can create custom exceptions by extending from the StandardError class as follows :

class MyException < StandardError

end

Ensure block

We can have a block of code that always runs by using the ensure keyword as follows :

begin
  raise "Testing an error" # raise the error
  puts "Shouldnt execute this code"
rescue Exception => error # catch the error
  puts "Rescued an exception: #{error.inspect}"
ensure
  puts "This code will always run" # this code always runs
end