dayofyear2date

#!/bin/bash -e
# ----------------------------------------------------------------------------
OPTS=$(getopt -o f:hv -l format:,help,version \
       -n "${0##*/}" -- "$@") || { echo "Terminating..." >&2; exit 1; }
eval set -- "$OPTS"

version(){
    printf "%-s\n" "${0##*/} 2021.03

    Copyright (C) 2019 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

    Written by Yaswant Pradhan." | sed 's/^  //g'
}

usage(){
    printf "%-s\n" "Get date from serial day of year (1..366).

    Usage: ${0##*/} [OPTION] DOY [Year]

    Options:
        -f, --format[=VALUE]  specify output date format (default: %Y%m%d)
        -h, --help            print this help and exit
        -v, --version         print version and exit

    DOY: Input day of year (001..366) - must be valid for Year.
    Year (optional: Year corresponding to DOY (default: current year)

    Examples:
    $ dayofyear2date 16
    20210116

    $ dayofyear2date 57 2031
    20310226

    $ dayofyear2date 16 -f '%F'
    2021-01-16

    $ dayofyear2date 366 2020
    20201231

    $ dayofyear2date 366 2021
    dayofyear2date: invalid day of year '366'

    Report bugs to <yaswant.pradhan@metoffice.gov.uk>" | sed 's/^  //g'
}
# ----------------------------------------------------------------------------

fmt="%Y%m%d"
while true; do
  case "$1" in
    -f |--format)  fmt="$2"; shift 2 ;;
    -h |--help )  usage; exit 0 ;;
    -v |--version ) version; exit 0 ;;
    -- )  shift; break ;;
    * )   break ;;
  esac
done


[[ $# -eq 0 || $# -gt 3 ]] && usage && exit
ddd=$(printf "%03d" "${1#0}")

# Parse One parameter argument:
if [[ $# -eq 1 ]]; then
    if [[ "${#1}" -eq 7 ]]; then
        yyyy=${1%???}
        ddd=${1#$yyyy}
    else
        yyyy="$(date +%Y)"
    fi
elif [[ $# -eq 2 ]]; then
    yyyy="$2"
else
    usage && exit 1
fi

maxd=$(date -d "$yyyy-12-31" +%j)
if (( 10#$ddd > maxd )) || (( 10#$ddd < 1 )); then
    echo "${0##*/}: invalid day of year '${ddd}'"
    exit 1
fi


date -d "$yyyy-01-01 +$ddd days -1 day" +"$fmt"