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, $$ contains the process id of the ruby interpreter, and is read-only. Here are the major system variables and their meanings:
- $! - latest error message
- $@ - location of error
- $_ - string last read by gets
- $. - line number last read by interpreter
- $& - string last matched by regexp
- $~ - the last regexp match, as an array of subexpressions
- $ n - the nth subexpression in the last match (same as $~[ n ])
- $= - case-insensitivity flag
- $/ - input record separator
- $\ - output record separator
- $0 - the name of the ruby script file
- $* - the command line arguments
- $$ - interpreter's process ID
- $? - exit status of last executed child process
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.
