begin / rescue / ensure / raise

# Basic exception handling syntax.
begin
  # Code that may raise an exception
rescue ExceptionClass => e
  # Code to run when an exception is raised
ensure
  # Code that always runs, with or without an exception
end

# Handling multiple exception types.
begin
  # Code
rescue TypeError => e
  # Handle type error
rescue ArgumentError => e
  # Handle argument error
rescue => e
  # Handle any other error (all subclasses of StandardError)
end

# Raise an exception intentionally.
raise "Error message"
raise RuntimeError, "Error message"

# Basic exception handling.
begin
  puts 10 / 0
rescue ZeroDivisionError => e
  puts "Error: #{e.message}"
end

# Use ensure to reliably release resources.
file = nil
begin
  file = File.open("nonexistent_file.txt", "r")
rescue Errno::ENOENT => e
  puts "File not found: #{e.message}"
ensure
  file&.close
  puts "Cleanup complete."
end

# Implement retry logic with a limit.
attempts = 0
begin
  attempts += 1
  puts "Attempt: #{attempts}"
  raise "Temporary error" if attempts < 3
  puts "Succeeded."
rescue RuntimeError => e
  puts "Error: #{e.message}"
  retry if attempts < 3
  puts "Retry limit reached."
end

Ruby exception handling follows the begin/rescue/ensure/end structure. You can specify multiple rescue clauses; they are evaluated top to bottom and the first matching class is applied. Omitting the exception class catches all subclasses of StandardError.

Use ensure to reliably close resources such as database connections, files, and network connections. When using retry, always set a maximum retry count to prevent an infinite loop. Inside a method definition, you can omit begin and write rescue directly.