Tcl workers often first encounter special keywords when confronted with a situation where the same application behaves differently for two different users/hosts/... For example, distinct auto_path-s can lead to loading of subtly different packages, which ...
[More motivation.] "tclvars" covers a different but related territory, that of particular variables (as opposed to files or commands) that influence Tcl. Let's don't list specific variables here just for listing - use the tclvars page for that. However, if there are other magic files or commands which make use of the variables, then a mention here, as well perhaps as a cross reference over on tclvars, makes sense. I just prefer not to see data scattered across the various pages haphazardly.
Tcl command auto_path [overloaded since 7.5--an understandable historical mistake]
Tcl file pkgIndex.tcl: package currently searches auto_path for files named "pkgIndex.tcl". More precisely, [... tclPkgUnknown ...]
Tclsh file .tclshrc
Note that the ~/.tclshrc file (~/tclshrc.tcl on Windows) are special values given by tclsh to the global variable tcl_rcFileName. The value of $::tcl_rcFileName is only special to applications like tclsh that call Tcl_Main(), not to Tcl itself.
.Xdefaults [... option ... ] This file would actually only be used, I suspect, by the X window system, which is used with the Unix specific portion of the Tk code (which probably also gets used on VMS and a number of other platforms). I don't THINK that Tk accesses this file directly. There are other files that X also access - I suppose that the X code in Tk searchs XFILESEARCHPATH and XUSERFILESEARCHPATH, doesn't it? What about the X Resource Database?
TCLLIBPATH
exec may use PATH, if the invocation of exec uses a relative path or simple command name to run. If a fully qualified pathname of the command is used, PATH is not referenced. Does this happen on MacOS and Windows as well?
* Yes it does. Also, on Windows, the extensions .com, .exe, and
.bat are magically applied to the executable name.(or maybe it's really auto_execok ...).
* On Unix, [auto_execok] does its search for executables in the
directories of PATH, yes, but [exec] does not call [auto_execok]
internally.
* On Windows, the search is guided by PATHEXT, PATH, and WINDIR.
See tcl/library/init.tcl for details.What about VMS and other non-Unix/Windows/MacOS operating systems?
DKF: auto_execok is part of the machinery of code that is used to parse interactively-typed commands; while it does path searching, it really does more than that (like handling the bizarreness of shell commands on DOS/Windows, etc.) It returns a list of words to substitute (in a loose sense) in place of the command name when passed to exec, or an empty string if the word does not refer to anything executable.
See also Tcl syntax help - Tk syntax help