Handle exit command executed by embedded Tcl runtime

Question

I have a small shell application that embeds Tcl to execute some set of Tcl code. The Tcl interpreter is initialized using Tcl_CreateInterp. Everything is very simple:

1. user types Tcl command
2. the command gets passed to Tcl_Eval for evaluation
3. repeat

But if a user types 'exit', which is a valid Tcl command, the whole thing - Tcl interpreter and my shell application - exit automatically.

Q: is there any way I can catch this exit signal coming from Tcl interpreter. I really would like not to check every user command. I tried Tcl_CreateExitHandler, but it didn't work.

Thanks so much.

Answer 1

Get rid of the command

rename exit ""

Or redefine it to let the user know it's disabled:

proc exit {args} { error "The exit command is not available in this context" }

Also worth considering is running the user's code in a safe interp instead of in the main shell. Doing so would allow you to control exactly what the user has access to.

You might also be able to create a child interp (non-safe) and just disable the exit command for that interp.

Lastly, you could just rename exit to something else, if you're only trying to avoid users typing it by mistake:

namespace eval ::hidden {}
rename exit ::hidden::exit

Answer 2

Rename the exit command:

rename exit __exit

proc exit {args} {
    puts -nonewline "Do you really want to exit? (y/n) "
    flush stdout
    gets stdin answer
    if {$answer == "y"} {
        __exit [lindex $args 0]
    }
}

This way, when the user type exit, he/she will execute your custom exit command, in which you can do anything you like.