From Rosetta Code
Tclsh is an implementation of Tcl. Other implementations of Tcl.

tclsh is a shell for the Tcl programming language. It is the primary implementation of Tcl as it is distributed with, built with, and a thin wrapper around libtcl, and it is normally used in command-line environments. In graphical environments, it is more common to use wish instead, though this is a historical hold-over except on Windows.

Argument Handling

The tclsh shell only supports a single option, -encoding, which is used to specify the character encoding used for the Tcl script that it will evaluate. The first argument after that, or the first argument if no -encoding option is present, is the name of a Tcl script that will be executed; when the script finishes, the process will exit. All subsequent arguments are stored as a list in the global argv variable, and the length of the list is (redundantly) in the global argc variable; the name of the script itself is in the global argv0 variable, and the full name of the tclsh program itself is retreivable with the info nameofexecutable command. (Note that most other Tcl-based shells adopt the same naming standards for command line arguments.)

When tclsh is started without any arguments, it sources the file ~/.tclshrc (or ~/tclshrc.tcl on Windows) and subsequently runs in interactive mode, allowing you to type in Tcl commands and have them executed immediately.

Idiomatic Demonstration

One trick that is used when providing a file that can either be a sourced script or run by passing it directly to tclsh as its first argument is to compare the contents of the global argv0 variable with the result of the info script command; when these are the same, the file being evaluated is the main script of the whole process and can perform any additional steps necessary (e.g., by running a demonstration). This might be done like this:

<lang tcl>proc foo ... proc bar ...

if {$argv0 eq [info script]} {

   puts "foo x = [foo x]"
   puts "bar y = [bar y]"