Use awk to analyze csv file - combined with shell 'date' command in awk - shell

Question

I have a .csv file which has dates and the answer about enjoyable or not:
2019-04-1,enjoyable
2019-04-2,unenjoyable
2019-04-3,unenjoyable
2019-04-4,enjoyable
2019-04-5,unenjoyable
2019-04-6,unenjoyable
2019-04-7,enjoyable
2019-04-8,unenjoyable
2019-04-9,unenjoyable
2019-04-10,enjoyable
2019-04-11,enjoyable
2019-04-12,enjoyable
2019-04-13,unenjoyable
2019-04-14,enjoyable
2019-04-15,unenjoyable
2019-04-16,unenjoyable
2019-04-17,unenjoyable
2019-04-18,enjoyable
2019-04-19,unenjoyable
2019-04-20,unenjoyable
2019-04-21,unenjoyable
2019-04-22,unenjoyable
2019-04-23,unenjoyable
2019-04-24,unenjoyable
2019-04-25,unenjoyable
2019-04-26,unenjoyable
What I want to do is to print the day of the week in the third column seperate by ',' like this:
2019-04-1,enjoyable,2
2019-04-2,unenjoyable,3
I tried:
dates=$(awk '{FS=","}{print $1,$2}' weather_stat.csv')
weeks=$(
for vars in $dates[first_row]
do
echo $(date -j -f '%Y-%m-%d' $vars "+%w")
done
)
merge($dates,$weeks)
The first part of the code works without any problem, but in the second part, I am confused about how to get the data in the first row (so I use dates[first_row] to mean the first row in dates variable) from the variable "dates" so we can apply 'date' method on it
And for the third part, I want to merge these two tables together. I found the 'join' function but it seem to work on two files instead of two variables(I don't want to have any new files during the process)
Could anyone tells me how to get the rows in a variable instead of a file in shell and the way to merge two table-like variables?

Answer 1

As you're learning shell scripting, here's some code to study:
to read your csv file, and get the weekday number for each date in the file:
while IFS=, read -r date rest; do echo "$date,$(date -d "$date" +%w)"; done < file.csv
to join the output of that command with your file:
weekdays=$(while IFS=, read -r date rest; do echo "$date,$(date -d "$date" +%w)"; done < file.csv)
join -t, file.csv <(echo "$weekdays")
or, without needing to store the result in an intermediate variable
join -t, file.csv <(
while IFS=, read -r date rest; do echo "$date,$(date -d "$date" +%w)"; done < file.csv
)
The newlines within the <() are not necessary, but useful for maintainable code.
However, you can see that this is less efficient because you have to process the file twice. With awk you only have to read through the file once.

Answer 2

With GNU awk:
awk' BEGIN{FS=OFS=","}
{ split($1,a,"-")
t=sprintf("%0.4d %0.2d %0.2d 00 00 00",a[1],a[2],a[3]);
print $0,strftime("%w",mktime(t))
}' file.csv

Answer 3

With only your Bourne shell, so less efficient than awk if you have a lot of lines in your CSV file:
while IFS=, read date enjoy; do
date -d "$date" +"$date,$enjoy,%w"
done < your.csv