A Tcl script consists of a series of strings separated from each other by a newline character. Each string contains a command or series of semi-colon separated commands. A command is a series of words separated by spaces. The first word in a command is the name of the command and subsequent words are its arguments.
An example is:
set a 1 set b 2
which is a Tcl script of two commands: the first command sets the value
1, and the second command sets the value
An example of two commands on the same line separated by a semi-colon is:
set a 1; set b 2
which is equivalent to the previous example but written entirely on one line.
A command is executed in two phases. In the first phase, the command is broken down into its constituent words and various textual substitutions are performed on those words. In the second phase, the procedure to call is identified from the first word in the command, and the procedure is called with the remaining words as arguments.
There are special syntactic characters that control how the first phase, the substitution phase, is carried out. The three major substitution types are variable substitution, command substitution, and backslash substitution.