convertMJD.sh

# convert functions about MJD

function date2mjd(){
    if (( $# == 0 )) ; then
        # pipe
        args=$(cat /dev/stdin)
    else
        # direct arguments
        args=$@
    fi
    date_strings=($args)
    for date_str in ${args[@]}; do
        # the format of date_str is required "(yy)yy[-/](m)m[-/](d)d" or "Mon[-/](d)d[-/](yy)yy"
        date_str_tmp=$(date -d ${date_str} +%Y-%m-%d 2>/dev/null)
        if [[ "x${date_str_tmp}" == "x" ]]; then
            continue
        fi
        date_list=(${date_str_tmp//-/ })
        y=$(echo ${date_list[0]} |sed s/^.*([0-9]+).*$/\1/)
        m=$(echo ${date_list[1]}|sed s/^.*([0-9]+).*$/\1/)
        m=$((10#$m))
        d=$(echo ${date_list[2]}|sed s/^.*([0-9]+).*$/\1/)
        if [[ $m -le 2 ]]; then
            y=$(($y - 1))
            m=$(($m + 12))
        fi
        val1=$(echo "$y * 365.25 - 0.5" | bc | printf "%.0f" $(cat))
        val2=$(( $y / 400))
        val3=$(( $y / 100))
        val4=$(echo "($m - 2) * 30.59 - 0.5"| bc | printf "%.0f" $(cat))
        MJD=$(($val1 + $val2 - $val3 + $val4 + $d - 678912))
        echo $MJD
    done
}

function mjd2date(){
    if (( $# == 0 )) ; then
        # pipe
        args=$(cat /dev/stdin)
    else
        # direct arguments
        args=$@
    fi
    date_strings=($args)
    for mjd_str in ${args[@]}; do
    mjd_tmp=$(echo ${mjd_str} | sed s/^.*([0-9]+).*$/\1/)
    val_n=$(($mjd_tmp + 678881 ))
    val_a1=$(( 4 * ($val_n + 1) /146097 + 1))
    val_a2=$((3 * $val_a1 / 4))
    val_a=$(( 4 * $val_n + 3 + 4 * $val_a2))
    val_b1=$(($val_a % 1461 / 4 ))
    val_b=$((5 * $val_b1 + 2))
    y=$(( $val_a / 1461 ))
    m=$(( $val_b / 153 + 3))
    d=$(( $val_b % 153 / 5 + 1))
    if [[ $m -ge 13 ]]; then
        y=$(($y + 1))
        m=$(($m - 12))
    fi
    echo "$y-$m-$d"
    done
}