extended tcl


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 Tk GUI toolkit is referred to as Tcl/Tk.


The Tcl programming language was created in the spring of 1988 by John Ousterhout while working at the University of California, Berkeley.

Date Event
January 1990 Tcl announced beyond Berkeley (Winter USENIX).
June 1990 Expect announced (Summer USENIX).
January 1991 First announcement of Tk (Winter USENIX).
June 1993 First Tcl/Tk conference (Berkeley). [table] geometry manager (forerunner of [grid]), [incr Tcl], TclDP and Groupkit, announced there.
September 2002 Ninth Tcl/Tk conference (Vancouver). Announcement of starkit packaging system.

Tcl conferences and workshops are held in both the United States and Europe.


Tcl's features include:

  • Everything is a command, including language structures. They are in Prefix notation.
  • Commands can be variadic.
  • Everything can be dynamically redefined and overridden.
  • All data types can be manipulated as strings, including code.
  • Extremely simple syntactic rules.
  • Event-driven interface to sockets and files. Time-based and user-defined events are also possible.
  • Flexible scope, with variable visibility restricted to lexical (static) scope by default, but uplevel and upvar allowing procs to interact with the enclosing functions' scopes.
  • Simple exception handling using exception code returned by all command executions.
  • All commands defined by Tcl itself generate informative error messages on incorrect usage.
  • Readily extensible, via C, C++, Java, and Tcl.
  • Interpreted language using bytecode for improved speed whilst maintaining dynamic modifiability
  • Full Unicode (3.1) support, first released 1999.
  • Platform independent: Win32, UNIX, Linux, Mac, etc.
  • 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:

puts "Hello, world!"

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.

Interfacing with other languages

Tcl interfaces natively with the C language.

C++ Interoperability

Java Interoperability

Extension packages

The Tcl language has always supported extension packages, which provide additional functionality (such as a GUI, terminal-based application automation, database access, etc.)


The most popular Tcl extension is the Tk toolkit, which provides a graphical user interface library for a variety of operating systems. Each GUI consists of one or more frames. Each frame has a layout manager.


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 X11 KDE 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.


A number of database extensions are available:

  • tclodbc
  • mk4tcl
  • SQLite
  • Pgtcl, pgintcl

  • mysqltcl, msqltcl
  • AdabasTcl
  • ibtcl

  • Oratcl
  • Sybtcl
  • db2tcl

and many, many others - see the Tcl/Tk Wiki, specifically the Tcl/Tk Wiki Database Category

See also


  • John K. Ousterhout, Tcl and the Tk Toolkit, Addison-Wesley, Reading, MA, USA, ISBN 0-201-63337-X, 1994.
  • Brent B. Welch, Practical Programming in Tcl and Tk, Prentice Hall, Upper Saddle River, NJ, USA, ISBN 0-13-038560-3, 2003.
  • J Adrian Zimmer, Tcl/Tk for Programmers, IEEE Computer Society, distributed by John Wiley and Sons, ISBN 0-8186-8515-8, 1998.
  • Mark Harrison and Michael McLennan, Effective Tcl/Tk Programming, Addison-Wesley, Reading, MA, USA, ISBN 0-201-63474-0, 1998
  • Mark Harrison (ed), Tcl/Tk Tools, O'Reilly Media, ISBN 1-56592-218-2, 1997


External links

Main Tcl developer sites

Tcl distributions

  • ActiveState's Tcl distribution for MS Windows, Mac OS X, Linux, Solaris, AIX, and HP-UX, with numerous libraries.
  • WinTclTk open-source MinGW-based distribution for Microsoft Windows with extensions

Tcl tutorials, books, etc.

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