Need to get the dates of all mondays in a year

Question

I need to sort data on a weekly base and all i have are dates in a logfile. Therefore to sort out data per week i would like to create a list with the dates of all mondays for a given year. I have tried to work something out and the only idea i currently have is to use ncal with year and month as argument looping over all months and extracting all mondays. Isn't there a more efficient way?

Answer 1

To get all mondays, by getting all dates and filtering by Mondays:

for i in `seq 0 365`
    do date -d "+$i day"
done | grep Mon

Of course, you could also take a monday and keep incrementing by 7 days.

Answer 2

hope that's what you mean. Below can be changed to vary the output formats of the dates.

date command can be used for that, dunno if ncal is any more/less efficient.

I know you went for "binning" now, but here is a more readable v.

$ cat /tmp/1.sh
#!/bin/bash

test -z "$year" && {
    echo "I expect you to set \$year environment variable"
    echo "In return I will display you the Mondays of this year"
    exit 1
}

# change me if you would like the date format to be different
# man date would tell you all the combinations you can use here
DATE_FORMAT="+%Y-%m-%d"

# change me if you change the date format above. I need to be
# able to extract the year from the date I'm shoing you
GET_YEAR="s/-.*//"

# this value is a week, in milliseconds. Changing it would change
# what I'm doing.
WEEK_INC=604800

# Use another 3-digit week day name here, to see dates for other week days
DAY_OF_WEEK=Mon

# stage 1, let's find us the first day of the week in this year

d=1
# is it DAY_OF_WEEK yet?
while test "$(date -d ${year}-1-${d} +%a)" != "$DAY_OF_WEEK"; do
# no, so let's look at the next day
    d=$((d+1));
done;

# let's ask for the milliseconds for that DAY_OF_WEEK that I found above
umon=$(date -d ${year}-1-${d} +%s)

# let's loop until we break from inside
while true; do
   # ndate is the date that we testing right now
   ndate=$(date -d @$umon "$DATE_FORMAT");
   # let's extract year
   ny=$(echo $ndate|sed "$GET_YEAR");
   # did we go over this year? If yes, then break out
   test $ny -ne $year && { break; }
   # move on to next week
   umon=$((umon+WEEK_INC))
   # display the date so far
   echo "$ndate"
done