How to portably start in interactive mode for keyboard traps

[dnewhall]

I ran into this the other day and it took me forever to figure out how to do it, so I'm putting this here for anyone who needs this.

Let's say you decide to use KornShell to write a game or a text editor. You have a script, but you need to run it in interactive mode for the KEYBD traps. However, you cannot pass arguments or redirect input to it or Ksh thinks it's supposed to run a script and falls out of interactive mode.

The only way I've found to get this to work in a platform-independent manner is the following:

#!/bin/sh
if [ -z "$_ARG_1" ]
then
    /usr/bin/env -i TERM="$TERM" PS1= HISTFILE= _ARG_1="$1" ENV="$0" ksh -i -oignoreeof
    exit
fi

# Place the ksh-specific code after here
...

This runs the script using /bin/sh, checks to see if an internal variable is defined, and if not then runs the correct ksh in interactive mode with the current script passed as its environment/.kshrc file.

All other attempts I tried using things like /usr/bin/env -S or abusing exec didn't work, or weren't portable.

The arguments need to be passed as special variables (_ARG_1, etc.), because if arguments are passed to an interactive shell, it thinks it's a script and exits when done. We don't want that; we want it to stay running for the keyboard input. Therefore, we define the arguments as _ARG_1, etc. so the code we shoehorn in through the ".kshrc file" can access them.

Only the environment variables specified with the call to /usr/bin/env -i are passed to the script. TERM is needed if you are doing any ANSI escape code shenanigans (which you probably are for an application requiring the keyboard like this). We zero out PS1 so it's not displayed and the same with HISTFILE so you can't accidentally scroll through and run commands from the history.

The only potential snafu I can identify is if your platform's /bin/sh parses the entire file before running and rejects the KornShell-specific syntax.

And the KEYBD traps for the curious:

handle_keybd_event () {
    # Do stuff using ${.sh.edchar}
    ...
    # You MUST reset this so the prompt never receives the character
    .sh.edchar=
}

insert_ctrl_c() {
    .sh.edchar=$'\003'
    handle_keybd_event
}

quit() {
  trap - KEYBD INT
  exit
}

trap handle_keybd_event KEYBD
# Pass ^C to handle_keybd_event
trap insert_ctrl_c INT

And, remember, you NEED to call exit to quit the script if you set ignoreeof, so only something like the provided quit should be used.