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

Question

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.

Answer 1

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.

Answer 2

if you have ncal
$ ncal | awk '/We/{print $4}'
ncal format is transpose of cal and working on rows is easier. 3rd Wednesday is the fourth field.
$ ncal
October 2018
Su 7 14 21 28
Mo 1 8 15 22 29
Tu 2 9 16 23 30
We 3 10 17 24 31
Th 4 11 18 25
Fr 5 12 19 26
Sa 6 13 20 27
for example for November
$ ncal -m 11
November 2018
Su 4 11 18 25
Mo 5 12 19 26
Tu 6 13 20 27
We 7 14 21 28
Th 1 8 15 22 29
Fr 2 9 16 23 30
Sa 3 10 17 24
$ ncal -m 11 | awk '/We/{print $4}'
21

Answer 3

It's not pretty, but on a GNU/Linux system you could loop over each day of the month and count:
#!/bin/bash
month="$1" # e.g. 2018-10
number="$2" # e.g. 3
weekday="$3" # e.g "Wednesday"
export LC_ALL=C TZ=UTC
date -d today > /dev/null 2>&1 || {
echo "You are not using GNU date"
exit 1
}
n="$number"
for day in {1..31}
do
wd=$(date -d "$month-$day" +"%A" 2>&1) || continue
if [[ $wd == "$weekday" ]]
then
(( --n )) || break
fi
done
if ! (( n ))
then
echo "The $weekday number $number in $month is $month-$day"
else
echo "$month does not have a $weekday number $number"
fi
However, note that dates are hard, and this simple approach may not correctly account for various curiosities like Samoa skipping Friday 2011-12-30 due to a time zone shift.
Here are some examples on a GNU system:
$ ./finddate 2018-10 3 Wednesday
The Wednesday number 3 in 2018-10 is 2018-10-17
$ ./finddate 2018-10 5 Thursday
2018-10 does not have a Thursday number 5
Here is an example from a non-GNU system like MacOS:
$ ./finddate 2018-10 3 Wednesday
You are not using GNU date

Answer 4

Here's what I came up with:
#!/bin/bash -xv
CURRENTDAY=$(date +%e)
CURRENTMONTH=$(date +%m)
NEXTMONTH=$(( ${CURRENTMONTH}+1 ))
CURRENTYEAR=$(date +%Y)
WEDNESDAYSFORTHEMONTH=$(cal ${CURRENTMONTH} ${CURRENTYEAR} | awk 'NF <= 7 { print $4 }' | grep -v "^$"| grep -v We)
# I would normally start at 0
COUNTER=1
#Get this months 3rd Wednesday
for WEDNESDAY in ${WEDNESDAYSFORTHEMONTH}
do
if [[ ${COUNTER} -eq 3 ]]
then
echo "This months third wednesday is: ${WEDNESDAY}"
THISMONTHSTHIRDWEDNESDAY=${WEDNESDAY}
fi
COUNTER=$(( ${COUNTER}+1 ))
done
# Get the 3rd Wednesday for the next month
WEDNESDAYSFORTHEMONTH=$(cal ${NEXTMONTH} ${CURRENTYEAR} | awk 'NF <= 7 { print $4 }' | grep -v "^$"| grep -v We)
COUNTER=1
for WEDNESDAY in ${WEDNESDAYSFORTHEMONTH}
do
if [[ ${COUNTER} -eq 3 ]]
then
echo "Next months third wednesday is: ${WEDNESDAY}"
NEXTMONTHSTHIRDWEDNESDAY=${WEDNESDAY}
fi
COUNTER=$(( ${COUNTER}+1 ))
done
#Compare the current date with this months 3rd Wednesday
if [[ ${CURRENTDAY} -le ${THISMONTHSTHIRDWEDNESDAY} ]]
then
echo "Wednesday to use in your script is: ${THISMONTHSTHIRDWEDNESDAY} ${CURRENTMONTH} ${CURRENTYEAR}"
else
echo "Wednesday to use in your script is: ${NEXTMONTHSTHIRDWEDNESDAY} ${NEXTMONTH} ${CURRENTYEAR}"
fi

Answer 5

# Third Wednesday of this month
thirdWednesday=$( cal | cut -c 10-12 | sed '/^\s*$/d' | sed -n '5p' )
# Today
today=$( date +%d )
# If already passed
if [ $today -gt $thirdWednesday ]; then
# Next month
nextMonth=$( date +"%m %Y" -d "next month" )
# Get third Wednesday of next month
thirdWednesday=$( cal $nextMonth | cut -c 10-12 | sed '/^\s*$/d' | sed -n '5p' )
fi
# Result
echo $thirdWednesday