Shell Scripting to extract date - shell

Question

Need a logic in shell scripting where I give the start and end date,
say
startDate=20140101 &
endDate=20160130
I should be able to extract startDate_new=20140122 and endDate_new=20140221 in first loop.
In next loop will need startDate_new=20140222 and endDate_new=20140321 so on, till startDate_new=20151221 & endDate_new=20160122 and exit out. Can you provide any shell scripting logic that can be used to achieve this.
test.sh has,
startdate=2013-03-01
enddate=2013-04-30
curr="$startdate"
while true; do
echo "$curr"
[ "$curr" \< "$enddate" ] || break
curr=$( date +%Y-%m-%d --date "$curr +1 day" )
done
the above code prints,
2013-03-01
2013-03-02
2013-03-03
2013-03-04
2013-03-05
2013-03-06
2013-03-07
2013-03-08
.
.
.
.
2013-04-30
Can you help me how to extract the range I mentioned above ?
Came up with a new logic,
startdate=2013-03-21
enddate=2014-05-30
curr="$startdate"
while true; do
[ "$curr" \< "$enddate" ] || { echo "$curr"; break; }
echo "$curr"
curr=$( date +%Y-%m-%d --date "$curr +1 month" )
end=$( date +%Y-%m-%d --date "$curr +1 month +1day" )
done
the above logic gives
2013-03-21
2013-04-21
2013-05-21
2013-06-21
2013-07-21
2013-08-21
2013-09-21
2013-10-21
2013-11-21
2013-12-21
2014-01-21
2014-02-21
2014-03-21
2014-04-21
2014-05-21
2014-06-21
Can you help with how to retrieve end date as 22nd of each month ?

Answer 1

Make a function:
# Usage: drange startdate endate [ time_increment ]
drange ()
{
curr="$( date +%Y-%m-%d --date $1)"
enddate="$( date +%Y-%m-%d --date $2)"
shift 2
inc="${*:-+1 day}"
until [ "$curr" \> "$enddate" ]; do
echo $curr
curr=$( date +%Y-%m-%d --date "$curr $inc" )
done
}
Run it like so:
drange 20140222 20140321
drange 20151221 20160122 +1 month
s=20151221 e=20160122 i="+1 month"
drange $((s + 1)) $e $i