Tcl Guide, Meaning , Facts, Information and Description
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:
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.
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.
Assignments are made with the command set, no equality sign.
A command may give back as a result a list
Procedures are defined as follows
The following code snippet creates and initializes an associative array.
Another popular extension is Expect, which allows automated driving of terminal-based programs (such as passwd, ftp, telnet and command driven shells).
# call handleData when socket is readable
fileevent $sock readable [ list handleData $sock ]
proc handleData { sock } {
This is an Article on Tcl. Page Contains Information, Facts Details or Explanation Guide About 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.Symbols with a special meaning
* $ for accessing the content of a variable
* [] evaluation of subcommand
* ""
* {} deferring evaluation - construction of a list
* \\ line continuation
* # commentSome examples of commands
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. glob aPattern
gives back a list of file names in the working directory whose names match aPattern.Procedures
proc nameOfProc { argumentList } {
....
....
}Associative arrays
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 useputs $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. Examples
Echo server
A simple working example, demonstrating event-based handling of a socket, follows.
exec tclsh $0 ${1+"$@"}
proc newConnection { sock addr port } {
# client connections will be handled in
# line-buffered, non-blocking mode
fconfigure $sock -blocking no -buffering line} puts $sock [ gets $sock ]
if { [ eof $sock ] } {
close $sock
}
}
set port [ lindex $argv 0 ]
socket -server newConnection $port
vwait forever
Digital clock
Another example using Tk (from A simple A/D clock) and timer events, a digital clock in three lines of code:
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.
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
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 byarray 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