Interactive Conveniences

The unknown command provides a few other conveniences. These are used only when you are typing commands directly. They are disabled once execution enters a procedure or if the Tcl shell is not being used interactively. The convenience features are automatic execution of programs, command history, and command abbreviation. These options are tried, in order, if a command implementation cannot be loaded from a script library.

Auto Execute

The unknown procedure implements a second feature: automatic execution of external programs. This makes a Tcl shell behave more like other UNIX shells that are used to execute programs. The search for external programs is done using the standard PATH environment variable that is used by other shells to find programs. If you want to disable the feature all together, set the auto_noexec variable:

set auto_noexec anything

History

The history facility described in Chapter 13 is implemented by the unknown procedure.

Abbreviations

If you type a unique prefix of a command, unknown recognizes it and executes the matching command for you. This is done after automatic program execution is attempted and history substitutions are performed.