Tcl/Tk 8.5 Programming Cookbook: With over 100 recipes, this Cookbook is ideal for both beginners and advanced Tcl/Tk programmers. From the basics to creating applications, it’s full of indispensable tips and tricks to make the most of the language.

So, you've installed Tcl, written some scripts, and now you're ready to get a deeper understanding of Tcl and all that it has to offer. So, why are we starting with the shell when it is the most basic tool in the Tcl toolbox?

When I started using Tcl I needed to rapidly deliver a Graphical User Interface (GUI) to display a video from the IP-based network cameras. The solution had to run on Windows and Linux and it could not be browser-based due to the end user's security concerns. The client needed it quickly and our sales team had, as usual, committed to a delivery date without speaking to the developer in advance. So, with the requirement document in hand, I researched the open source tools available at the time and Tcl/Tk was the only language that met the challenge.

The original solution quickly evolved into a full-featured IP Video Security system with the ability to record and display historic video as well as providing the ability to attach to live video feeds from the cameras. Next search capabilities were added to review the stored video and a method to navigate to specific dates and times. The final version included configuring advanced recording settings such as resolution, color levels, frame rate, and variable speed playback. All was accomplished with Tcl.

Due to the time constraints, I was not able get a full appreciation of the capabilities of the shell. I saw it as a basic tool to interact with the interpreter to run commands and access the file system. When I had the time, I returned to the shell and realized just how valuable a tool it is and the many capabilities I had failed to make use of.

When used to its fullest, the shell provides much more that an interface to the Tcl interpreter, especially in the early stages of the development process. Need to isolate and test a procedure in a program? Need a quick debugging tool? Need real-time notification of the values stored in a variable? The Tcl shell is the place to go.

Since then, I have learned countless uses for the shell that would not only have sped up the development process, but also saved me several headaches in debugging the GUI and video collection. I relied on numerous dialog boxes to pop up values or turned to writing debugging information to error logs. While this was an excellent way to get what I needed, I could have minimized the overhead in terms of coding by simply relying on the shell to display the desired information in the early stages.

While dialog windows and error logs are irreplaceable, I now add in quick debugging by using the commands the shell has to offer. If something isn't proceeding as expected, I drop in a command to write to standard out and voila! I have my answer. The shell continues to provide me with a reliable method to isolate issues with a minimum investment of time.

The Tcl Shell (Tclsh) provides an interface to the Tcl interpreter that accepts commands from both standard input and text files. Much like the Windows Command Line or Linux Terminal, the Tcl shell allows a developer to rapidly invoke a command and observe the return value or error messages in standard output. The shell differs based on the Operating System in use. For the Unix/Linux systems, this is the standard terminal console; while on a Windows system, the shell is launched separately via an executable.

If invoked with no arguments, the shell interface runs interactively, accepting commands from the native command line. The input line is demarked with a percent sign (%) with the prompt located at the start position. If the shell is invoked from the command line (Windows DOS or Unix/Linux terminal) and arguments are passed, the interpreter will accept the first as the filename to be read. Any additional arguments are processed as variables. The shell will run until the exit command is invoked or until it has reached the end of the text file.

When invoked with arguments, the shell sets several Tcl variables that may be accessed within your program, much like the C family of languages. These variables are:

The following recipe illustrates a basic command invocation. In this example, we will use the puts command to output a "Hello World" message to the console.

% puts "Hello World"
Hello World

The expr command is used to evaluate mathematical expressions. This command can address everything from simple addition and subtraction to advanced computations, such as sine and cosine. This eliminates the need to make system calls to perform advanced mathematical functions. The expr command evaluates the input and arguments, and returns an integer or floating-point value.

A Tcl expression consists of a combination of operators, operands, and parenthetical containers (parenthesis, braces, or brackets). There are no strict typing requirements, so any white space is stripped by the command automatically. Tcl supports non-numeric and string comparisons as well as Tcl-specific operators.

Tcl operands are treated as integers, where feasible. They may be specified as decimal, binary (first two characters must be 0b), hexadecimal (first two characters must be 0x), or octal (first two characters must be 0o). Care should be taken when passing integers with a leading 0, for example 08, as the interpreter would evaluate 08 as an illegal octal value. If no integer formats are included, the command will evaluate the operand as a floating-point numeric value. For scientific notations, the character e (or E) is inserted as appropriate. If no numeric interpretation is feasible, the value will be evaluated as a string. In this case, the value must be enclosed within double quotes or braces. Please note that not all operands are accepted by all operators. To avoid inadvertent variable substitution, it is always best to enclose the operands within braces. For example, take a look at the following:

Operands may be presented in any of the following:

Tcl supports a subset of the C programming language math operators and treats them in the same manner and precedence. If a named function (such as sine) is encountered, expr automatically makes a call to the mathfunc namespace to minimize the syntax required to obtain the value.

Tcl expr operators may be specified as noted in the following table, in the descending order of precedence:

Mathematical functions (such as sine and cosine) are replaced with a call to the Tcl mathfunc namespace. This does not require any additional syntax to access the namespace as it is called automatically. These are invoked by passing the Function followed by the value or values to evaluate to the expr command. Those functions that accept multiple arguments require that the arguments be comma delimited. The default Mathematical functions are listed below in alphabetical order. These functions require a specific syntax (for example expr {function(value,value)}) to be accessed, as described in the Computing mathematical expressions section that follows: