As you know, Tcl is Tool command language, commands are the most vital part of the language. Tcl commands are built in-to the language with each having its own predefined function. These commands form the reserved words of the language and cannot be used for other variable namings. The advantage with these Tcl commands is that, you can define your own implementation for any of these commands to replace the original built-in functionality.
Each of the Tcl commands validates the input and it reduces the work of the interpreter.
Tcl command is actually a list of words, with the first word representing the command to be executed. The next words represent the arguments. In order to group words into a single argument, we enclose multiple words with "" or {}.
The syntax of Tcl command is as follows.