For a scripting language, Tcl has a simple syntax. A Tcl command is formed by words separated by white space. The first word is the name of the command, and the remaining words are arguments to the command. In this example, the variable name is foo .
Square brackets execute a nested command.
For example, if you want to pass the result of one command as the argument to another, you use this syntax. In this example, the nested command is clock seconds, which gives the current time in seconds. Dollar signs and square brackets are interpreted inside double quotation marks. Curly braces also group words into a single argument. In this case, however, elements within the braces are not interpreted.