UNIX date: How to convert week number to a date range (Mon-Sun)?

Question

I have list of week numbers extracted from huge log file, they were extracted using syntax:

$ date --date="Wed Mar 20 10:19:56 2012" +%W;
12

I want to create a simple bash function which can convert these week numbers to a date range. I suppose function should accept 2 arguments: $number and $year, example:

$ week() { ......... }
$ number=12; year=2012
$ week $number $year
"Mon Mar 19 2012" - "Sun Mar 25 2012"

Answer 1

With GNU date:

$ cat weekof.sh
function weekof()
{
    local week=$1 year=$2
    local week_num_of_Jan_1 week_day_of_Jan_1
    local first_Mon
    local date_fmt="+%a %b %d %Y"
    local mon sun

    week_num_of_Jan_1=$(date -d $year-01-01 +%W)
    week_day_of_Jan_1=$(date -d $year-01-01 +%u)

    if ((week_num_of_Jan_1)); then
        first_Mon=$year-01-01
    else
        first_Mon=$year-01-$((01 + (7 - week_day_of_Jan_1 + 1) ))
    fi

    mon=$(date -d "$first_Mon +$((week - 1)) week" "$date_fmt")
    sun=$(date -d "$first_Mon +$((week - 1)) week + 6 day" "$date_fmt")
    echo "\"$mon\" - \"$sun\""
}

weekof $1 $2
$ bash weekof.sh 12 2012
"Mon Mar 19 2012" - "Sun Mar 25 2012"
$ bash weekof.sh 1 2018
"Mon Jan 01 2018" - "Sun Jan 07 2018"
$

Answer 2

Monday is the first day of week, ISO week numbers:

function week2date () {
  local year=$1
  local week=$2
  local dayofweek=$3
  date -d "$year-01-01 +$(( $week * 7 + 1 - $(date -d "$year-01-04" +%w ) - 3 )) days -2 days + $dayofweek days" +"%Y-%m-%d"
}

week2date 2017 35 1
week2date 2017 35 7

Output:

2017-08-28
2017-09-03

Answer 3

If the start of a week is Sunday, you can use this version of weekof:

function weekof()
{
    local week=$1 year=$2
    local week_num_of_Jan_1 week_day_of_Jan_1
    local first_Sun
    local date_fmt="+%Y-%m-%d"
    local sun sat

    week_num_of_Jan_1=$(date -d $year-01-01 +%U)
    week_day_of_Jan_1=$(date -d $year-01-01 +%u)

    if ((week_num_of_Jan_1)); then
        first_Sun=$year-01-01
    else
        first_Sun=$year-01-$((01 + (7 - week_day_of_Jan_1) ))
    fi

    sun=$(date -d "$first_Sun +$((week - 1)) week" "$date_fmt")
    sat=$(date -d "$first_Sun +$((week - 1)) week + 6 day" "$date_fmt")
    echo "$sun $sat"
}

Answer 4

If anybody needs it: I found an even shorter way (not sure if easier):

function weekof() {
        local year=$2
        local week=`echo $1 | sed 's/^0*//'` # Fixes random bug
        local dateFormat="+%a %b %d %Y"
        # Offset is the day of week, so we can calculate back to monday
        local offset="`date -d "$year/01/01 +$((week - 1)) week" "+%u"`"
        echo -n "`date -d "$year/01/01 +$((week - 1)) week +$((1 - $offset)) day" "$dateFormat"`" # Monday
        echo -n " - "
        echo "`date -d "$year/01/01 +$((week - 1)) week +$((7 - $offset)) day" "$dateFormat"`" # Sunday    }

I take the first day of the year and go n weeks forward to be somewhere in the right week. Then I take my weekday and go back/forward to reach monday and sunday.