How to Use the eval Method in Ruby



In this article, you will learn how to use the eval method in Ruby.

The eval Method

The eval method in Ruby allows you to execute a string of code as if it were part of your program. It is a powerful method, but also potentially dangerous, as it can execute any arbitrary code, including malicious code.

To use the eval method in Ruby, you simply pass a string of code to it as an argument. Here’s a basic example:

x = 2
eval("x + 2") #=> 4

In this example, the string “x + 2” is evaluated as if it were part of the program, and the result is 4. The eval method can also take a block, which can be used to capture output or handle errors.

x = 2
eval("x + 2") { |e| puts "Error: #{e}" } #=> 4

# with error handling
eval("x + 'a'") { |e| puts "Error: #{e}" } #=> Error: no implicit conversion of String into Integer

As noted earlier, it is important to exercise caution when using eval, as it can execute arbitrary code. Make sure that any strings passed to eval come from a trusted source, and avoid using it with user input whenever possible.


Here are some additional details about using the eval method in Ruby:

  • The eval method can access variables in the current scope, but it can also create new variables or modify existing ones. This can have unintended consequences if you’re not careful.

    x = 2
    eval("x = x + 2")
    puts x #=> 4

    In this example, the eval method modifies the value of the x variable in the current scope, so the output of the program is 4 instead of 2.

  • The eval method can also execute entire blocks of code, including loops and conditionals.

      x = 2
      if x > 1
        puts 'x is greater than 1'
        puts 'x is less than or equal to 1'
    ") #=> "x is greater than 1"

    In this example, the entire block of code inside the string is executed by eval, and the output is “x is greater than 1”.

  • As mentioned earlier, the eval method can be dangerous if used improperly. It can execute any arbitrary code, including code that can potentially harm your system. It is important to validate any input that is passed to eval, and to only use it with trusted code.

    user_input = gets.chomp
    eval(user_input) # Potentially dangerous!

    In this example, eval is used with user input, which could potentially contain malicious code. This is generally not a good idea, and should be avoided whenever possible.

  • If you need to execute code that is not trusted, or if you want to limit the scope of the eval method, you can use a binding object to control the environment in which the code is executed.

    x = 2
    binding_obj = binding
    eval("x = x + 2", binding_obj)
    puts x #=> 2
    y = 2
    binding_obj = binding
    eval("y = y + 2", binding_obj)
    puts y #=> 4

    In this example, the eval method is executed with a binding object, which limits the scope of the code to the context of the binding object. This allows you to control the environment in which the code is executed, and can help to prevent unintended side effects.