#!/bin/sh
# sample-posix.sh: a portable POSIX shell script.
# Free sample from 99helpers.com/file-types/sh
# Runs in any POSIX shell: dash (Debian and Ubuntu's /bin/sh), bash, zsh in
# sh mode, ksh and BusyBox ash.
# Run:  sh sample-posix.sh          (a Celsius to Fahrenheit table)
#       sh sample-posix.sh -10 40 5 (from, to, step)
#
# The same jobs written without bash-only features:
#   bash                     POSIX sh
#   [[ $a == b* ]]           case $a in b*) ... ;; esac
#   arr=(one two)            set -- one two   (the positional parameters)
#   function name { ... }    name() { ... }
#   echo -e "a\tb"           printf 'a\tb\n'
#   $(( x ** 2 ))            $(( x * x ))
#   set -o pipefail          not available in every sh: dash rejects it

set -eu

# 1. Functions: no `local` keyword in POSIX, so prefix names to avoid clashes.
#    Shell arithmetic is integer-only: work in tenths of a degree, then format.
to_fahrenheit() {
  _tenths=$(( $1 * 18 + 320 ))
  _sign=''
  if [ "$_tenths" -lt 0 ]; then
    _sign='-'
    _tenths=$(( -_tenths ))
  fi
  printf '%s%d.%d' "$_sign" $(( _tenths / 10 )) $(( _tenths % 10 ))
}

# 2. Check that an argument is a whole number, with case and glob patterns.
is_integer() {
  case $1 in
    '' | - | *[!0-9-]* | ?*-*) return 1 ;;
    *) return 0 ;;
  esac
}

from=${1:-0}
to=${2:-100}
step=${3:-10}

for value in "$from" "$to" "$step"; do
  if ! is_integer "$value"; then
    printf '%s: "%s" is not a whole number\n' "${0##*/}" "$value" >&2
    exit 2
  fi
done
if [ "$step" -le 0 ]; then
  echo "step must be greater than 0" >&2
  exit 2
fi

# 3. A while loop with [ ] tests (the test command) and $(( )) arithmetic.
printf '%8s  %s\n' "Celsius" "Fahrenheit"
c=$from
while [ "$c" -le "$to" ]; do
  printf '%8d  %10s\n' "$c" "$(to_fahrenheit "$c")"
  c=$(( c + step ))
done