c1,c2,c3,c4,c5
c1,c2,c3,c4,c5,30,50
10,20,30,40,50,30,50
20,30,40,50,60,30,50

awk -F, '
NR == 1         {for (i=1; i<=NF; i++) if ("," MCOL "," ~ "," $i ",") COL[++CNT] = i
                 print $0,MCOL[i]
                }
FNR == 1        {next
                }

NR == FNR       {for (i=1; i<=CNT; i++) SUM[i] += $(COL[i])
                 next
                }
                {for (i=1; i<=CNT; i++) $(NF+1) = SUM[i]
                 print
                }
' OFS="," MCOL="c1,c2" file file

c1,c2,c3,c4,c5,sum_c1,sum_c3
10,20,30,40,50,30,50
20,30,40,50,60,30,50

#!/usr/bin/sh
col="c1,c3"   
idx=   
sum=0         
{
IFS=, read -ra values <<< "$col"
for v in "${values[@]}"
do
  for i in ${v//,/ } do
     while IFS=, read -r -a line; do
     sum=$(( sum + ${line[$i]} 
	 echo $sum
	 done
  done
done
}<testfile