how to expand date by week and by month in shell? - bash

Question

In short, I want something works like this:
When I input a date like 2012-12-27 and want to expand the date by
week(start with Monday), it
outputs:2012-12-24,2012-12-25,2012-12-26,2012-12-27,2012-12-28,2012-12-29,2012-12-30
When I input a date like 2012-12-27 and want to expand the date by month, it outputs:2012-12-01,2012-12-02 ... 2012-12-31
or, how can I group a bunch of dates by week? e.g. when I input2012-12-01,2012-12-02 ... 2012-12-31. It outputs:2012-12-01,2012-12-02|2012-12-03 ... 2012-12-09|2012-12-10 ... 2012-12-16|...|2012-12-31
I have no idea how to complete this, any clue may be helpful!

Answer 1

DAYSECS=86400 # seconds in a day
WEEKSECS=604800
echo "Expand on week:"
g_epoch=$(date +"%s" -d $1) # given date as seconds from epoch
g_dayno=$(date +"%u" -d $1) # given date as day of week
g_month=$(date +"%m" -d $1) # given month
g_year=$(date +"%Y" -d $1) # given year
s_epoch=$(($g_epoch - $DAYSECS * ($g_dayno - 1)))
e_epoch=$(($s_epoch + $WEEKSECS))
for etime in $(seq $s_epoch $DAYSECS $e_epoch); do
date +"%Y-%m-%d" -d "#$etime"
done
echo "Expand on month:"
s_epoch=$(date +"%s" -d "$g_year-$g_month-01")
e_epoch=$(($s_epoch + 4 * $WEEKSECS))
for etime in $(seq $s_epoch $DAYSECS $e_epoch); do
if [ $(date +"%m" -d "#$etime") -ne "$g_month" ]; then
break;
fi
date +"%Y-%m-%d" -d "#$etime"
done

Answer 2

The script below work out from #perreal, I leave it here because:
It shows the power of GNU date.
It makes #perreal 's idea more clear and more universal.
Thank you, perreal!
Here it is
dd="2012-12-27" # test date
DAYSECS=86400 # seconds in a day
echo "expand by week:"
s_epoch=$(date +%s -d "$dd -$(($(date +%u -d $dd) - 1)) day") # start date of the week
e_epoch=$(date +%s -d "1970-01-01 00:00:00 +0000 +${s_epoch} seconds +6 days") # end date of the week
for etime in $(seq $s_epoch $DAYSECS $e_epoch); do
date +"%Y-%m-%d" -d "#$etime";
done
echo "expand by month:"
s_epoch=$(date +%s -d "$dd -$(($(date +%d -d $dd) - 1)) day") # start date of the month
e_epoch=$(date +%s -d "1970-01-01 00:00:00 +0000 +${s_epoch} seconds +1 month -1 day") # end date of the month
for etime in $(seq $s_epoch $DAYSECS $e_epoch); do
date +"%Y-%m-%d" -d "#$etime";
done