#!/bin/sh -u
# A "while" loop using numbers
# Syntax:   $0 number increment
#
# Loop counting down from $number by $increment.
#
# 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

# Validate the number of arguments before using any of them.
# Tell the user (always on stderr) what was expected, and what was found.
#
if [ $# -ne 2 ] ; then
    echo 1>&2 "$0: expecting a number and increment, found $# ($*)"
    exit 1
fi

# Transfer the arguments to variables with a readable names.
# From this point on, use the readable names, not $1 and $2.
#
number="$1"
increment="$2"

# Test that $number and $increment are both numbers (not strings).
# This is better done with a shell function; but, we do a simple version here.
# We rely on the "test" command returning "2" for a non-numeric error.
# (Normally "test" returns only 0 or 1.)  We throw away error messages.
# Bug: Invalid "octal" numbers (e.g. leading zero 09) pass unchecked.
#
test 2>/dev/null "$number" -eq 0 -a "$increment" -eq 0
if [ $? -eq 2 ] ; then
    # a status of 2 from "test" indicates a non-numeric error
    echo 1>&2 "$0: '$number' or '$increment' is not numeric"
    exit 1
fi

# Make sure the increment is positive, non-zero.
#
if [ "$increment" -le 0 ] ; then
    echo 1>&2 "$0: increment must be > 0, found: $increment"
    exit 1
fi

# Loop counting down from $number by $increment.
# Note that variables don't need '$' in a "let" statement.
# Bug: Invalid "octal" numbers (e.g. 09) will cause an infinite loop.
#
while [ "$number" -gt 0 ] ; do
    echo "number is $number"
    let number=number-increment
done

# Bug: In a "let" statement, bash treats numbers with leading zeroes as
# octal, and it will complain and fail to execute if the octal number has
# bad form, e.g. "x=09 ; let y=x" generates an error.  This isn't true in
# "test" statements, so you can't use a "test" statement to syntax-check an
# octal number; you have to use a let statement.  (But note that you can't
# use a "let" statement to test for a non-numeric value; you have to use a
# "test" statement.)  It's messy.