Tcl (Tool Command Language) is a small language interpreter designed to link with compiled C libraries, providing scripted control of C code ( extended scripts ). Its original application was to control libraries for electronic simulators (SPICE-like applications). Tcl is also suitable for embedded scripts —that is, scripts called from within C programs and returning values to those programs. Tcl had its first general public release in 1990.

The Tk toolkit is so important that the language is often referred to as Tcl/Tk. Tk is also frequently used with Perl and Python.

The main drawback of Tcl is that the pure language has only weak facilities for namespace control and modularity, and two of them (upvar and uplevel) are rather dangerous if not used with great caution. Also, there are no data structures other than association lists. Tcl therefore scales up very poorly — it is difficult to organize and debug pure Tcl programs of even moderate size (more than a few hundred lines) without tripping over your own feet. In practice, almost all large Tcl programs use one of several OO extensions to the language.

Pure Tcl only provides access to a relatively small and commonly used part of the Unix API (essentially just file handling, process-spawning, and sockets). Indeed, Tcl has the flavor of an experiment in seeing how small a scripting language can get and still be useful. Tcl extensions (similar to Perl modules) provide a richer set of capabilities, but are (like CPAN modules) not guaranteed to be installed everywhere.

The original Tcl reference is Tcl and the Tk Toolkit [Ousterhout94], but that book has been largely superseded by Practical Programming in Tcl and Tk [Welch]. Brian Kernighan has written a description of a real-world Tcl project [Kernighan95] that summarizes Tcl's strengths and weaknesses as a rapid-prototyping and production tool; his contrast with Microsoft Visual Basic is particularly balanced and instructive.

The Tcl world doesn't have one central repository run by a core group analogous to Perl's or Python's, but several excellent websites both point to each other and cover most Tcl tool and extension development. Look at the Tcl Developer Xchange first; among other things, it offers Tcl sources of an interactive Tcl tutorial. There is also a Tcl foundry at SourceForge.

Tcl scripts have portability problems similar to those of shell scripts; the language itself is highly portable, but the components it calls may not be. Tcl implementations exist for the Microsoft family of operating systems, MacOS, and many other platforms. Tcl/Tk scripts will run on any platform with GUI capabilities.

Summing up: Tcl's best side is its spare, compact design and the extensibility of the Tcl interpreter. Its worst side is the odd positional parser and the weakness of its data structures and namespace controls; the latter defect makes it scale poorly for large projects.