Syntax: incr varName ?increment?

Increments integer-valued-variable varName by the given increment, which defaults to 1. If the variable is unset, it is set to the increment. The command returns the incremented value.

unset a
incr a    ;# => 1
incr a    ;# => 2
incr a 3  ;# => 5

for {set a 1} {$a < 10} {incr a} {
    ...
}