How can I get the ISO date for a specific week's Monday? For example, the Monday of the 32nd week of 2021?
I know that if I run date -dmonday +%Y%m%d I will get the the Monday date of the current week. How can I pass the 2021 and 32 as variables, and get the date of the Monday of that week?
This seems to work correctly.
#! /bin/bash year=$1 week=$2 read s w d < <(date -d $year-01-01T13:00 '+%s %V %u') (( s += ((week - w) * 7 - d + 1) * 24 * 60 * 60 )) date -d @$s '+%V:%w %Y-%m-%d %a' Tested with
for y in {1970..2022} ; do maxw=$(date +%V -d $y-12-31) if (( max == 1 )) ; then # The last day of the year belongs to week #1. max=52 fi for ((w=1; w<=maxw; ++w)) ; do date.sh $y $w | tail -n1 | grep -q "0\?$w:1 " || echo $y $w done done where date.sh is the above script.
Wikipedia has an article on ISO week dates The "first week" is defined to be the one with the first thursday in it. The article states that this is equivalent to Jan 4th is always in week 1. GNU date allows you to give dates like date -d "Jan 4 2021 +3 weeks" which will get you a date in the 4th week, not usually a monday.
Using date we can find the day of the week, and use that to adjust the date to ask for.
#!/bin/bash # pass in $1 as the week number and $2 as the year # Make sure we are no going to be bothered by daylight saving time # Use explicit time idea from https://unix.stackexchange.com/a/688968/194382 export TZ=UTC # Jan 4th is always in week 1. ask for the day of the week that is # $1-1 weeks ahead. %u gives 1 for Monday, 2 for tuesday, ... weekday=$(date -d"13:00 Jan 4 $2 +$(($1 -1)) weeks" "+%u") # So now just go back 0 days for a monday, 1 day for tuesday ... # Could use Jan 5 and go back 1 day for monday instead. date -d"13:00 Jan 4 $2 +$(($1 -1)) weeks -$((weekday -1)) days" "+%a %F (%G %V)" So for 2021 this shows the monday of week 1 was 2021-01-04, and for 2020 it was 2019-12-30 (i.e. at the end of the previous year)
This works at least from 2000 to 2025:
Y=2021 W=32 LC_ALL=C date -d"${Y}0101 ${W} week -$(date -d"${Y}0104" +%u) day -3day" Mon Aug 9 00:00:00 CEST 2021 The LC_ALL=C prefix eliminates locale influences.
via python it is possible to do with oneliner:
echo "2021 32"| python3 -c 'import datetime as dt; print(dt.datetime.strptime(input()+" 1", "%G %V %u").strftime("%Y-%m-%d"));' 2021-08-09 you can wrap it into a BASH function and then use it clearly for any other day of week 1..7, for example for Wednesday (day=3):
> iso_year_week_day() { echo "$1 $2 $3" | python3 -c 'import datetime as dt; print(dt.datetime.strptime(input(), "%G %V %u").strftime("%Y-%m-%d"));'; } > > iso_year_week_day 2021 32 3 2021-08-11 > iso_year_week_day 2022 52 7 2023-01-01 > q=$(iso_year_week_day 2021 32 3) > echo $q 2021-08-11 P.S. of course output format can be easily adjusted by editing strftime("%Y-%m-%d")
-dflag isn't standard, should we assume you are using Linux, or at least GNU date?