A global variable has a name beginning with $. It can be referred to from anywhere in a program. Before initialization, a global variable has the special value nil.

Global variables should be used sparingly. They are dangerous because they can be written to from anywhere. Overuse of globals can make isolating bugs difficult; it also tends to indicate that the design of a program has not been carefully thought out. Whenever you do find it necessary to use a global variable, be sure to give it a descriptive name that is unlikely to be inadvertently used for something else later (calling it something like $foo as above is probably a bad idea).

One nice feature of a global variable is that it can be traced; you can specify a procedure which is invoked whenever the value of the variable is changed.

When a global variable has been rigged to work as a trigger to invoke a procedure whenever changed, we sometimes call it an active variable. For instance, it might be useful for keeping a GUI display up to date.

There is a collection of special variables whose names consist of a dollar sign ($) followed by a single character. For example, $$ contains the process id of the ruby interpreter, and is read-only. Here are the major system variables:

In the above, $_ and $~ have local scope. Their names suggest they should be global, but they are much more useful this way, and there are historical reasons for using these names.