The set command is used to assign a value to a variable. It takes two arguments: The first is the name of the variable, and the second is the value. Variable names can be any length, and case is significant. In fact, you can use any character in a variable name.
It is not necessary to declare Tcl variables before you use them.
The interpreter will create the variable when it is first assigned a value. The value of a variable is obtained later with the dollar-sign syntax, illustrated in Example 1-2:
Example 1-2 Tcl variables.
set var 5
set b $var
The second set command assigns to variable b the value of variable var. The use of the dollar sign is our first example of substitution. You can imagine that the second set command gets rewritten by substituting the value of var for $var to obtain a new command.
set b 5
The actual implementation of substitution is more efficient, which is important when the value is large.