add in bash code example

Example 1: add bash command

a=`expr "$a" + "$num"`

a=$(($a+$num))

((a=a+num))

let a=a+num

((a+=num))

Example 2: bash calculate sum

#Just use following structure for summing up varibles:
sum=$((var1 + var2))
#For float numbers use next instruction:
sum=$(bc <<< $var1'+'$var2)