Simple date math in bash script

To catch up with my blog, I needed to extract data from a twitter feed. Usually I update the blog on a weekly basis. Since I didn’t find the time recently, I fell behind.

I was looking for a quick solution to download all the feed data at once. A bash script should be good enough.

The script that I use to extract the data from the twitter feed, expects a date argument in the form mm/dd/yyyy. The script will extract all data for the range [date - 7days, date]

Create the series of dates

I need to create a series of dates for several weeks. Given a start date, I’m going to produce new dates with a 7 day interval. The starting date is included in the series. Here is what I ended up with:

#!/usr/bin/env bash

set -e

# startdate in the form mm/dd/yyyy
startDate=$1
weeks=$2

for ((i=0;i<${weeks};i++)); do
    offsetDays=$((${i}*7))
    newDate=`date -j -v+${offsetDays}d -f "%m/%d/%Y" ${startDate} "+%m/%d/%Y"`

    startDateFilename=`echo ${newDate} | sed "s/\//-/g"`

    echo ${startDateFilename}
    # download the twitter data and redirect all output to that file
    exec &> weeklyTwitterData-${startDateFilename}.txt
    # call the script to extract the data from twitter stream
    coffee src/WeeklyStats.coffee -n ${startDate}
done

Since I didn’t want to do all the calendar math myself, I rely on the magic of the date utility. The line defining newDate is where the magic happens. Parsing a date ${startDate} in a given format %m/%d/%Y, adding $offsetDays to the parsed date and print out the new date in the format %m/%d/%Y.

That’s it.