#!/bin/sh -u
# using SIGINT interrupt traps to change variables in running scripts
#
#   $0 (no arguments)
#
# Demonstrate the use of interrupt traps to change a shell variable
# value in a running script.
#
# NOTE: This script fragment is incomplete - it does not follow all
# the script guidelines set out in "script_style.txt".  It also
# contains many "Instructor-Style" comments that you must not use in
# your own scripts.  See script_style.txt for details.
#
# -IAN! idallen@idallen.ca

PATH=/bin:/usr/bin ; export PATH
LC_COLLATE=C ; export LC_COLLATE
LANG=C ; export LANG
umask 022

# The Prompt function is called when the user sends a SIGINT interrupt.
# SIGINT is usually caused by typing the CTRL-C character; but, you can
# change it using the "stty" command, e.g. to ESC via:  stty intr '^['
# Remember to reset the intr character before exit if you do change it!
#
# The interrupt trap is *disabled* when this function is running,
# meaning that if you type another interrupt character, you will kill
# your script.  If you don't want that, you have to ignore interrupts
# inside the interrupt function, as shown below.
#
# XXX All the input validation is missing here - normally you
# XXX want to check for a valid numeric value and retry as needed.
# XXX Don't set "x" until you know you have good, numeric input.
#
Prompt () {
    # Don't let a second interrupt kill the script -
    # ignore interrupts while reading user input.
    #
    trap '' 2 # disable SIGINT

    echo 1>&2 "Enter new input: "
    read x || exit

    # Restore the interrupt prompting function again and return to the
    # main script execution.
    #
    trap 'Prompt' 2 # enable SIGINT trap to Prompt function
}

# In the main program, consider carefully *where* you want to allow
# your script to be interrupted, and only enable the trap around that
# code.  If you allow interrupts anywhere, the user might interrupt some
# external command vital to the execution of the script; however, if
# you only allow interrupts in some places, the user may signal SIGINT
# at a place where interrupts are ignored.  You have to trade off
# responsiveness to SIGINT against possible corruption of script
# functioning.  If your script mostly does output and doesn't call
# many external programs, you may wish to leave SIGINT always enabled.
#
# Allowing interrupts only around a sleeping part of a script is safest.
#
trap '' 2             # disable SIGINT
let x=20
while [ $x -gt 0 ] ; do
    echo "x is $x"
    let x=x-1
    trap 'Prompt' 2   # enable SIGINT trap to Prompt function
    sleep 1           # this sleep can be interrupted
    trap '' 2         # disable SIGINT
done