doy2ymd

#!/bin/bash
# Name
#   doy2ymd
#
# Purpose
#   Convert Year and serial day of year to calendar year-month-day
#
# Syntax
#   doy2ymd yyyyddd
#   doy2ymd yyyy ddd
#
# Example
#   doy2ymd 2012171
#   doy2ymd 2012 171
#   Both the commands above should return same result: 20120619
#
# yaswant.pradhan


usage(){
cat<<EOF
Usage:  doy2ymd YYYYddd [or]
        doy2ymd yyyy ddd [or]
        doy2ymd yyyy ddd fmt-specifier

This script works on GNU-based system.
Serial day 0 will return last day of the previous year.
EOF
}

[[ $# -eq 0 || $# -gt 3 ]] && usage && exit


# Parse One paramater argument:
if [[ $# -eq 1 ]]; then
    [[ "${#1}" -ne 7 ]] && usage && exit 1
    yyyy=${1%???}
    ddd=${1#$yyyy}
fi


# Parse Two paramater arguments:
if [[ $# -eq 2 || $# -eq 3 ]]; then
    [[ "${#1}" -ne 4 || "${#2}" -ne 3 ]] && usage && exit 1
    yyyy=${1}
    ddd=${2}
fi

fmt=${3-%Y%m%d}

# Return calendar date:
# date -d "$yyyy-01-01 +$ddd days -1 day" +"%Y%m%d"
date -d "$yyyy-01-01 +$ddd days -1 day" +$fmt