Variables

The command in the previous example set the variable myvar to the string "27". The only data type in Tcl is a string. However, some commands will interpret arguments as numbers, for example the expr command (expr will be explained later).

To use the value of a variable, you prepend it with $$:

% puts myvar
27

String values are normally delimited with double quotes, but a string of a single word doesn't need them:

% set myvar "hello world!"
hello world!
% puts $$myvar
hello world!
% set myvar hello
hello
% puts myvar
hello

The value of a variable can be used when defining a string by prepending it with a $$:

% set myvar "world!"
world!
% set myvar2 "hello myvar"
hello world!
% puts myvar2
hello world!

To include a literal '$$' in a string, use $:

% set myvar "hello$world!"
helloworld!

Variables are not declared, but come into existence as soon as they get a value:

% puts $$unknownvar
can't read "unknownvar": no such variable
% set unknownvar "known"
known
% puts unknownvar
known

% puts $$unknownvar
known
% unset unknownvar
% puts unknownvar
can't read "unknownvar": no such variable