incr man page
incr — Increment the value of a variable
incr varName ?increment?
Increments the value stored in the variable whose name is varName. The value of the variable must be an integer. If increment is supplied then its value (which must be an integer) is added to the value of variable varName; otherwise 1 is added to varName. The new value is stored as a decimal string in variable varName and also returned as result.
Starting with the Tcl 8.5 release, the variable varName passed to incr may be unset, and in that case, it will be set to the value increment or to the default increment value of 1.
Add one to the contents of the variable x:
Add 42 to the contents of the variable x:
incr x 42
Add the contents of the variable y to the contents of the variable x:
incr x $y
Add nothing at all to the variable x (often useful for checking whether an argument to a procedure is actually integral and generating an error if it is not):
incr x 0
add, increment, variable, value