How to get the date of the 3. wednesday in the month

I need a code sample getting the date of the 3. wednesday of the month. I simply can't wrap my head around how to do that - the last wednesday is simple:

cal | awk '/^ *[0-9]/ { d=$4 } END { print d }'

Ultimately I need the script to return the "next 3. wednesday" - as in if we have passed the 3. wednesday of this month, return the 3. wednesday of the next month.

Solution

Here's a simple approach which would work properly in most regions:

#!/bin/bash

export LC_TIME=C
thismonth=$(date +%m)       # this month like "10"
thisyear=$(date +%Y)        # this year like "2018"
firstdayofweek=$(date -d "${thisyear}-${thismonth}-1" +%w)
# calculates the day of week of the 1st day of the month
# returns the number between 0 and 6, where 0=Sun, 1=Mon, ...

wed1=$(( (10 - $firstdayofweek) % 7 + 1 ))
# calculates the day of month of the 1st Wednesday
wed3=$(( $wed1 + 14 ))
# the day of the 3rd Wednsday

echo $wed3

which yields "17" as of today.
It would be easy to modify the code above to achieve your next goal.