- It stands for Tool Command Language
- Tcl is interpreter based
- To interpreter a Tcl script you will require a Tcl Shell - tclsh
- Tcl interpreter is a library of C procedures that can be incorporated in the applications.
- Tk is one of the extensions of Tcl which is a ToolKit for creating GUI(Graphical User Interface) based applications
- Tk extends the core Tcl facilities with commands for building user interfaces
- Tk like Tcl is also implemented as a library of C procedures so it can be used in many different applications
- Individual applications can also extend the base Tk features with new user-interface widgets (GUI Components like button, labels, textbox etc) and geometry managers written in C
- Tcl gives the benefit of rapid development
- Many interesting GUI applications can be written entirely as Tcl scripts using a windowing shell called 'wish'.
- When wish is used one can generate and execute new scripts on the fly without recompiling the application.
- Easy and fast programming.
Features of TCL
- It provides generic programming facilities such as variables, loops and procedures
- Tcl Working: Tcl just does grouping, substitutions and command invocations
- Write once, run anywhere. It runs on Windows, Mac OS X, and almost on every Unix platform
- It's an open source, free, and can be used for commercial applications without any limit
A sample TCL Script
- A script file is a list of commands which a shell interpreter read and execute.
Example: set a 19
put $a
Comments
- # is the character used to comment a statement
- # and all the characters following it up through the next newline are treated as a comment and discarded
- # mark can occur in a position where Tcl is expecting the first character of a command
Example: A script with a comment
#This is my first tcl script
set a 19;
puts $a
Executing a Tcl Script
There are two ways of how you can execute a Tcl Script
Method 1:
- Invoke the tclsh command on the command prompt to see the tclsh prompt (%)
- Type your Tcl commands that you want to execute.
- Type 'exit' command on the prompt to exit from the shell.
Method 2:
- Type your tcl program in a notepad/text editor and save it. (Conventionally tcl scripts will have a .tcl extension). Eg: script1.tcl
- Execute your script by invoking the Tcl Shell (tclsh) C:\ tclsh script1.tclP
Part2: Commands and Variables
Script, commands and Words
- The Tcl script comprises of a set of commands
- Each command in turn consists of one or more words
- The first word is the name of the command and additional words are arguments to that command
- Words are separated by spaces or tabs
Evaluating a command
- Tcl interpreter evaluates a command in two steps - parsing and execution.
First Step: Parsing
- First step during command evaluation
- Chops commands into words
- Perform substitutions
- Does not interpret values of words.
- Certain rules are applied while performing substitutions.
Second Step: Execution
- In the execution step, meaning is applied to the words of the command
- Tcl treats the first word as the command name
- The rest of the words are treated as the arguments to the command
- A command procedure is located and invoked to carry out the command's functionality
Substitution
Tcl provides three forms of substitution they are :
- Variable substitution
- Command substitution
- Backslash substitution
Substitutions can occur in any word of the command
There may be any number of substitutions within a single word
Variable substitution
- Variable substitution is achieved by a preceding the variable name with a dollar sign character. Example: $x
- It causes the value of the Tcl variable to be inserted into a word
Example:
Command substitution
- Command substitution causes the result of a Tcl command to be substituted
- Command substitution is invoked by enclosing the command in brackets (Square brackets [])
Example:
World
Backslash substitution
- It is used to substitute the values of certain special characters preceded by a '\' character
- Example: \n (newline), \t (tab)
puts "Hello\nWorld"
- When the above code is executed, it produces the following result −
World
Grouping
- Space is a word separator hence a space, tab or a newline cannot be part of the argument
- However it can be made as part of the argument by grouping it together with the arguments with the help of quotes
- There are 2 ways of grouping words in tcl,
- Curly braces - { }
Grouping with double quotes
- Double quotes disable word and command separator
- If a word is enclosed in double quotes, then spaces, tabs, newlines and semicolons are treated as ordinary characters within the word
- Variable, command, backslash substitutions all occur as usual inside double quotes
Example:
Grouping with curly braces
Grouping with curly braces
- Within curly braces all the special characters lose their meaning
- Spaces, newlines and semicolons are treated as ordinary characters
- No substitutions are performed on the word
Example:
Variables
- Variables are containers to hold values
- In Tcl the following are some pointers on the variables:
-Length of the variable name is not limited
-Variable names are case sensitive
-They need not be pre - declared as Tcl is a weakly typed language
-No special character can be a part of the variable name
-In Tcl every value is treated as a string
-Both the name and value of the variable can be arbitrary strings of characters
The set command
- The set command is used for assigning value to a variable. The syntax for set command is,
set variableName value
- Variables may be created, read and modified with the set command
- The set commands takes either two arguments or one argument
Example: 
- Some important pre-defined variables in Tcl are:
-argv0 simple variable that holds the name of the script being executed
- argv: simple variable that holds the command line arguments
- argc: simple variable that holds the count of the command line arguments
-env: array variable that holds the process's environment variables
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