Tcl (originally from "Tool Command Language", but nonetheless conventionally rendered as "Tcl" rather than "TCL"; pronounced as "tickle" or "tee-cee-ell) is a scripting language created by John Ousterhout. Originally "born out of frustration—according to the author—with programmers devising their own (poor quality) languages intended to be embedded into applications, Tcl quickly gained wide acceptance on its own and is generally thought to be easy to learn, but powerful in competent hands. It is most commonly used for rapid prototyping, scripted applications, GUIs and testing. Tcl is used extensively on embedded systems platforms, both in its full form and in several other small-footprinted versions. Tcl is also used for CGI scripting and as the scripting language for eggdrop bots.
The combination of Tcl and the TkGUI toolkit is referred to as Tcl/Tk.
Close integration with windowing (GUI) interface Tk.
Easy to maintain code. Tcl scripts are often more compact and readable than functionally equivalent code in other languages.
Can be used for many purposes, and in many environments: as a text-only scripted language, as a GUI-capable language for applications, as an embedded language in: web pages (server-side; or client-side, as Tclets), and databases (server-side, in PostgreSQL).
Exists as development version (e. g. ActiveState Tcl), as tclkit (kind of runtime version, only about 1 megabyte in size), as starpack (single-file executable of a script/program), as BSD licensed freely distributable source
Tcl did not originally support object oriented syntax, being a functional language, but recent versions do support extensions which provide OO functionality, such as the XOTcl extension to Tcl. Other OO extensions also exist, such as incr Tcl, Snit, and STOOOP (simple tcl-only object-oriented programming).
A Tcl script consists of several commands. A command is a list of words separated by whitespace.
word0 word1 word2 ... wordN
The first word is the name of a command, which is not built into the language, but which is in the library. The following words are arguments. So we have:
commandName argument1 argument2 ... argumentN
Practical example, using the puts command which outputs a string, adding a trailing newline, by default to the stdout channel:
Any argument may be replaced by another command in square brackets. The subcommand is evaluated first and the result is substituted as the argument. Alternatively, any argument placed in curly braces will not be evaluated, but rather will be handed directly back to the original command as an argument.
To summarize: there is one basic construct (the command) and a set of simple substitution rules, and only the square brackets, the curly braces, quotes, and the backslash have special meaning. The single equality sign (=) for example is not used at all, and the double equality sign (==) is the test for equality, and even then only in expression contexts such as the expr command or the first argument to if.
All commands have the same structure: a keyword which is followed by several parameters. A command is terminated by a newline or a semicolon. Even comments are just commands which happen to do nothing.
Tcl is not statically typed: each variable may contain integers, floats, strings, lists or any other value.
Tile/Ttk is a styles and theming widget collection which can replace most of the widgets in Tk with variants which are truly platform native through calls to an operating system's API. Themes covered in this way are Windows XP, Windows Classic, Qt (which hooks into the X11KDE environment libraries) and Aqua (Mac OS X). A theme can also be constructed without these calls using widget definitions supplemented with image pixmaps. Themes created this way include Classic Tk, Step, Alt/Revitalized, Plastik and Keramik.
Under Tcl 8.4, this package is known as Tile, while in Tcl 8.5 it is included in the core distribution as Ttk.
Itcl is an object system for Tcl, and is normally named as [incr Tcl] (that being the way to increment in Tcl, similar in fashion to the name C++).
Tcllib is a set of scripted packages for Tcl that can be used with no compilation steps.