Today's date, minus X days in shell script

bash sh
gcubed picture gcubed · Nov 23, 2012 · Viewed 76.7k times · Source

I need to create three variables, each for Year, Month, and Day for Today's date, minus X number of days. For this question I'll choose a random amount of days: 222.

So if:

TodayYear=`date +%Y`
TodayMonth=`date +%m`
TodayDay=`date +%d`

What I want is 222 days before this.

222days_before_TodayYear=???
222days_before_TodayMonth=???
222days_before_TodayDay=???

Edit: Need 222 working days instead 222 regular days.

Answer

sampson-chen picture sampson-chen · Nov 23, 2012

For GNU date:

date_222days_before_TodayYear=$(date --date="222 days ago" +"%Y")
date_222days_before_TodayMonth=$(date --date="222 days ago" +"%m")
date_222days_before_TodayDay=$(date --date="222 days ago" +"%d")

For BSD date::

If you are using OS X or FreeBSD, use the following instead because BSD date is different from GNU date:

date_222days_before_TodayYear=$(date -j -v-222d +"%Y")
date_222days_before_TodayMonth=$(date -j -v-222d +"%m")
date_222days_before_TodayDay=$(date -j -v-222d +"%d")

Source: BSD date manual page

Note:

In bash and many other languages, you cannot start a variable name with a numerical character, so I prefixed them with date_ for you.

Second Update: New requirement - Using 222 Working days instead of 222 Regular days:

(Assumption: Not considering statutory holidays, because that just gets far beyond the scope of what I can help you with in a shell script:)

Consider 222 working days:

  • 5 working days per week, that is floor(222/5) == 44 weeks
  • 44 weeks * 7 days per week == 308 days
  • Extra days leftover: 222 % 5 == 2
  • Therefore 222 working days == 310 regular days

But, there is a catch! If the number of regular days is 308 or some multiple of 7, then we would have been fine, because any multiple of 7-days ago from a working day is still a working day. So we need to consider whether today is a Monday or a Tuesday:

  • If today is a Monday, we'd get Saturday where we wanted Thursday
  • If today is a Tuesday, we'd get Sunday where we wanted Friday

So you see we need an additional offset of 2 more days if today is either Monday or Tuesday; so let's find that out first before we proceed:

#!/bin/bash

# Use 310 days as offset instead of 222
offset=310
# Find locale's abbreviated weekday name (e.g., Sun)
today=$(date -j +"%a")
# Check for Mon/Tue
if [[ "$today" == "Mon" ]] || [[ "$today" == "Tue" ]]; then
     offset=$((offset+2))
fi

date_222_working_days_before_TodayYear=$(date -j -v-${offset}d +"%Y")
date_222_working_days_before_TodayMonth=$(date -j -v-${offset}d +"%m")
date_222_working_days_before_TodayDay=$(date -j -v-${offset}d +"%d")

And that should do it =)

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