Calculating days with BC bash

Question

So my script has to calculate day when someone will be back from trip. For calculating using user input for leaving day and length of trip. Everyday has certain number like Monday-1; Tuesday-2 etc but it gets complex when user input for Sunday must be 0.

Script atm what I have made:

#!/bin/bash
#Data
echo -e "\e[00;31mRemember!\e[00m Sunday-0; Monday-1; Tuesday-2; Wednesday-3; Thursday-4; Friday-5; Saturday-6;"
echo
echo -n "What day will you leave? "
read left
echo -n "How long is your trip? "
read days
#Calculating
cb=`echo "$left+$days" | bc`
comeback=`echo "$cb % 6" | bc`
#Output
if [ $comeback = 0 ]; then
echo "You will be back at Sunday"
fi
if [ $comeback = 1 ]; then
echo "You will be back at Monday"
fi
if [ $comeback = 2 ]; then
echo "You will be back at Tuesday"
fi
if [ $comeback = 3 ]; then
echo "You will be back at Wednesday"
fi
if [ $comeback = 4 ]; then
echo "You will be back at Thursday"
fi
if [ $comeback = 5 ]; then
echo "You will be back at Friday"
fi
if [ $comeback = 6 ]; then
echo "You will be back at Saturday"
fi

My problem is that It never calculates right when trip is over a week or when it should end on Sunday. I got my script working when I started counting like Sunday-1; Monday-2 etc but Sunday must be 0.

Answer 1

To correct your script:

comeback=$(echo "(${left} + ${days}) % 7" | bc)

Also, you should be using an array for output:

days_of_week=( Sunday Monday Tuesday Wednesday Thursday Friday Saturday )
echo "You will be back on ${days_of_week[${comeback}]}"

---

I also want to point out a fun answer. With date from GNU coreutils, you can do this after you read $left and $days :

echo "You will be back on $(date -d "${days_of_week[${left}]} + ${days} days" +%A)"