bash - how to get (current) Julian day number?

问题

How to get today's Julian day number (JDN) equivalent? or any date's, for that matter?

I've looked and looked, but only found some functions that produced "year-dayOfYear", not something like: 2457854.

回答1:

in bash date +%j returns julian date.

[root@TX-Serv-1 ~]# date +%j
108

Also Julian dates in the future are pretty easy too.

[root@es-host01 ~]# date --date='10 days' +%j
119
[root@es-host01 ~]# date --date='2 weeks' +%j
123

回答2:

I ended up porting one myself. Here is a SSCCE:

#!/bin/bash

function GetJulianDay # year, month, day
{
  year=$1
  month=$2
  day=$3

  jd=$((day - 32075 + 1461 * (year + 4800 - (14 - month) / 12) / 4 + 367 * (month - 2 + ((14 - month) / 12) * 12) / 12 - 3 * ((year + 4900 - (14 - month) / 12) / 100) / 4))

  echo $jd
}

function GetTodayJulianDay
{
  year=`date +"%Y"`
  month=`date +"%m"`
  day=`date +"%d"`

  todayJd=$(GetJulianDay $year $month $day)

  echo $todayJd
}

# samples

todayJd=$(GetTodayJulianDay)
echo "got today jd: $todayJd"

yesterdayJd=$(GetJulianDay 2017 4 9)
echo "got yesterday jd: $yesterdayJd"

Test it: http://aa.usno.navy.mil/data/docs/JulianDate.php

回答3:

You could get the Unix date with date +%s --date=YYYYMMDD and use it to compute the Julian Day from Unix date

It would end up something like:

echo $(( $(date +%s --date=YYYYMMDD) / 86400 + 2440587 ))

Edit: I don’t know how important this is, but some people like to add 0.5 to the conversion. Basically this means that if your current date/time is after noon, the conversion would put the Julian date 1 day later.

回答4:

OLD_JULIAN_VAR=$(date -u -d 1840-12-31 +%s)

TODAY_DATE=`date --date="$odate" +"%Y-%m-%d"`
TODAY_DATE_VAR=`date -u -d "$TODAY_DATE" +"%s"`
export JULIAN_DATE=$((((TODAY_DATE_VAR - OLD_JULIAN_VAR))/86400))
echo $JULIAN_DATE

the mathematically

[(date in sec)-(1840-12-31 in sec)]/86400

来源：https://stackoverflow.com/questions/43317428/bash-how-to-get-current-julian-day-number

标签

bash

julian-date