Ruby is a scripting language. This means that Ruby programs are simply lists, or scripts, of statements to be executed. By default, these statements are executed sequentially, in the order they appear. Ruby’s control structures (described in Chapter 5) alter this default execution order and allow statements to be executed conditionally or repeatedly, for example.

Programmers who are used to traditional static compiled languages like C or Java may find this slightly confusing. There is no special main method in Ruby from which execution begins. The Ruby interpreter is given a script of statements to execute, and it begins executing at the first line and continues to the last line.

(Actually, that last statement is not quite true. The Ruby interpreter first scans the file for BEGIN statements, and executes the code in their bodies. Then it goes back to line 1 and starts executing sequentially. See BEGIN and END for more on BEGIN.)

Another difference between Ruby and compiled languages has to do with module, class, and method definitions. In compiled languages, these are syntactic structures that are processed by the compiler. In Ruby, they are statements like any other. When the Ruby interpreter encounters a class definition, it executes it, causing a new class to come into existence. Similarly, when the Ruby interpreter encounters a method definition, it executes it, causing a new method to be defined. Later in the program, the interpreter will probably encounter and execute a method invocation expression for the method, and this invocation will cause the statements in the method body to be executed.

The Ruby interpreter is invoked from the command line and given a script to execute. Very simple one-line scripts are sometimes written directly on the command line. More commonly, however, the name of the file containing the script is specified. The Ruby interpreter reads the file and executes the script. It first executes any BEGIN blocks. Then it starts at the first line of the file and continues until one of the following happens:

Before it quits, the Ruby interpreter typically (unless the exit! method was called) executes the bodies of any END statements it has encountered and any other “shutdown hook” code registered with the at_exit function.