Tool Command Language: 4 Syntax and Structure

Tcl supports only one type of data: strings . All commands, all arguments to commands, all command results, and all variable values are strings. Where commands require numeric arguments or return numeric results, the arguments and results are passed as strings. Many commands expect their string arguments to have certain formats, but this interpretation is up to the individual commands. For example, arguments often contain Tcl command strings, which may get executed as part of the commands. The easiest way to understand the Tcl interpreter is to remember that everything is just an operation on a string. In many cases Tcl constructs will look similar to more structured constructs from other languages. However, the Tcl constructs are not structured at all; they are just strings of characters, and this gives them a different behavior than the structures they may look like.

Although the exact interpretation of a Tcl string depends on who is doing the interpretation, there are three common forms that strings take: commands , expressions , and lists . This section will have the following main parts:

Basic Command Syntax
Description of the syntax common to all Tcl code: comments, argument grouping, command grouping, variable substitution, backslash substitution.
Expressions
Details on interpretation of expressions by Tcl.
Lists
Details on interpretation of lists by Tcl.
Command Results
What type of results a command can return.
Procedures
The structure and building of procedures in Tcl.
Variables
Variable declaration and description.

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5 Commands