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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.

ruby> $foo
   nil
ruby> $foo = 5
   5
ruby> $foo
   5

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.

ruby> trace_var :$x, proc{print "$x is now ", $x, "\n"}
   nil
ruby> $x = 5
$x is now 5
   5

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 is 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 and their meanings (see the ruby reference manual for details):

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.

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