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== Argument Handling ==
The tclsh shell only supports a single option, <code>-encoding</code>, 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 <code>-encoding</code> 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 <code>argv</code> variable, and the length of the list is (redundantly) in the global <code>argc</code> variable; the name of the script itself is in the global <code>argv0</code> variable, and the full name of the tclsh program itself is retreivable with the <code>info nameofexecutable</code> 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 <code>~/.tclshrc</code> (or <code>~/tclshrc.tcl</code> 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 <code>source</code>d script or run by passing it directly to tclsh as its first argument is to compare the contents of the global <code>argv0</code> variable with the result of the <code>info script</code> 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]"
}</lang>
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