Version 11 of How should I then code if I want my program to run on most platforms

Purpose: gather together tips on writing portable Tcl and Tk programs that will more likely run on many platforms.

Stay away from platform specific extensions, including DDE, Registry and AppleScript.
See Microsoft Windows for some information about windows.
Use file rather than exec or self parsing file names, etc.
Avoid exec, because different OSs support widely different commands.
Avoid depending on console, which currently is not available on Unix.
Avoid puts to stdout and stderr (especially in library code), which might not be available for graphical applications on Microsoft Windows and MacOS.
Make certain your code can accomodate file paths with spaces in them.