Tcl (originally from "Tool Command Language", but nonetheless conventionally rendered as "Tcl" rather than "TCL"; and pronounced like "tickle") is a scripting language created by John Ousterhout that is generally thought to be easy to learn, but powerful in the right hands. It is most commonly used for rapid prototyping, scripted applications, GUIs and testing.

Features

Tcl's features include:

• Everything is a command, including language structures. They are in Polish notation.
• 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.
• Dynamic scope
• Readily extensible (with C, C++, Java and Tcl)
• Interpreted language, code can be created and modified dynamically
• Full Unicode support (first released in 1999)
• Platform independent (Win32, UNIX, Mac, etc.)
• Close integration with windowing (GUI) interface Tk
• Code is easy to maintain. Tcl scripts are often more compact and readable than functionally equivalent code in other languages

While Tcl itself does not provide an object oriented framework, the language itself can be extended to provide new features as required. Indeed, many C extensions have been written to provide OO functionality, including the XOTcl and incr Tcl packages. Other OO extensions, including Snit, are written entirely in Tcl.

Syntax

Very simple and consistent syntax

Tcl has a very simple syntax which is applied in a consistent way. 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 

Instead of an argument, you may put another command in square brackets. The subcommand is evaluated first and the result is substituted as the argument. If you put something in curly braces as an argument, it is not evaluated but handed directly to the command as the argument.

To summarize: there is one basic construct and only the block, the curly braces and the backslash have a special meaning besides the quotes. The single equality sign (=) for example is not used at all, and the double equality sign (==) is the test for equality.

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 or strings.

Symbols with a special meaning

 $ variable substitution e.g. $argv0 might be replaced by /usr/bin/tclsh [] subcommand substitution e.g [pwd] might be replaced by /home/joe "" word grouping e.g. "you are $user" is one word; substitution still occurs {} word grouping with defered substitution e.g. {you are $user} is one word, where "$user" is not replaced \ line continuation # comment (only at the beginning of a line) 

Some examples of commands

Assignments are made with the command set, no equality sign.

 set variable value 

While loops are implemented by the command while which takes two arguments. The arguments are Tcl scripts. They are in curly braces to avoid execution on the first level of interpretation. Within the execution of the while command the scripts are executed.

 while { aTCLcommandWhichEvalutesToAnInteger } { aTCLcommand anotherTclCommand .... } 

If command

 if {$x < 0} { set x 0 } 

Commands may have no arguments

 pwd 

gives back the current working directory. With

 set wdir [pwd] 

you store the string describing the working directory in the variable wdir.

A command may give back as a result a list

 glob aPattern 

gives back a list of file names in the working directory whose names match aPattern.

Procedures

Procedures are defined as follows

 proc nameOfProc { argumentList } { .... .... } 

Associative arrays

The following code snippet creates and initializes an associative array.

 set capital(France) Paris set capital(Italy) Rome set capital(Germany) Berlin set capital(Poland) Warsaw set capital(Russia) Moscow set capital(Spain) Madrid 

To query it use and put the result on standard output use

 puts $capital(Italy) 

To get a list of all countries for which a capital is defined use

 array names capital 

The result is an unsorted

 Poland Spain Russia Germany Italy France 

If you like to have it sorted use

 lsort [array names capital] 

GUI and Expect

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.

Another popular extension is Expect, which allows automated driving of terminal-based programs (such as passwd, ftp, telnet and command driven shells).

Examples

Echo server

A simple working example, demonstrating event-based handling of a socket, follows.

 #!/bin/sh # next line restarts using tclsh in path \ exec tclsh $0 ${1+"$@"} # echo server that can handle multiple # simultaneous connections. proc newConnection { sock addr port } { # client connections will be handled in # line-buffered, non-blocking mode fconfigure $sock -blocking no -buffering line # call handleData when socket is readable fileevent $sock readable [ list handleData $sock ] } proc handleData { sock } { puts $sock [ gets $sock ] if { [ eof $sock ] } { close $sock } } # handle all connections to port given # as argument when server was invoked # by calling newConnection set port [ lindex $argv 0 ] socket -server newConnection $port # enter the event loop by waiting # on a dummy variable that is otherwise # unused. vwait forever 

Digital clock

Another example using Tk (from A simple A/D clock (http://mini.net/tcl/2563.html)) and timer events, a digital clock in three lines of code:

 proc every {ms body} {eval $body; after $ms [info level 0]} pack [label .clock -textvar time] every 1000 {set ::time [clock format [clock sec] -format %H:%M:%S]} ;# RS 

Explainer: the first line defines a command, "every", which re-schedules an action ('body') every 'ms' milliseconds; the second creates a label whose content is bound to the variable 'time'; the third line arranges so that the variable 'time' is updated to formatted local time every second.

List of content of associative array

In an array tcl_platform, platform-specific properties are kept. A list of the names of the properties is obtained by

 array names tcl_platform 

The following snippet lists them together with their values

 foreach i [array names tcl_platform] { puts [ concat $i= $tcl_platform($i) ] } 

If the properties should be sorted

 foreach i [lsort [array names tcl_platform]] { puts [ concat $i= $tcl_platform($i) ] } 

This demonstrates how commands may be nested. In fact they may be nested to any depth.

External links

• Tcl Documentation (http://www.tcl.tk/doc/)
• Main Tcl developer site (http://www.tcl.tk/)
• ActiveState's Tcl distribution with extensions (http://www.activestate.com/Products/ActiveTcl/)
• Tcl FAQ (http://www.purl.org/NET/Tcl-FAQ/)
• Tcl'ers Wiki (http://wiki.tcl.tk/)
• A Tcl (Tutorial for Cool Languages) for Tcl/Tk (http://www.geocities.com/binnyva/code/tcl/tutorial/index.html)
• Tcl Contributed Sources Archive (http://www.neosoft.com/tcl/) (seems to be broken)
• Citations from CiteSeer (http://citeseer.org/cs?q=Tcl+Tk)
• Tcl Tutorial (http://www.tcl.tk/man/tcl8.5/tutorial/tcltutorial.html)
• A Short TCL Tutorial (http://jan.netcomp.monash.edu.au/ProgrammingUnix/tcl/tcl_tut.html)
• Tclwise, a book on Tcl programming with online chapters (http://www.invece.org/tclwise/) by Salvatore Sanfilippo (http://www.invece.org).