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Global variables

A word about the scope of variables. Variables used within procedures are normally created only for the duration of that procedure and have local scope.

It is possible to declare a variable as having global scope, through the global command:

global name1 ? name2 ...?

where name1, name2, ..., are the names of global variables. Any references to those names will be taken to denote global variables for the duration of the procedure call.

Global variables are those variables declared at the topmost calling context. It is possible to run a global command at anytime in a procedure call. After such a command, the variable name will refer to a global variable until the procedure exits.

An example:

set x 10

proc fred { } {
    set y 20
    global x
    puts [expr $x + $y]
}

fred

will print the result 30 where 20 comes from the local variable y and 10 comes from the global variable x.

Without the global x line, the call to fred will fail with an error because there is no variable x defined locally in the procedure for the expr to evaluate over.