shell programming: preparation of three shell programs, respectively for, while, until required and from 4 + 2 + ... + 100.

for

#!/bin/sh
i=1
sum=0
for i in `seq 2 2 100`
do
((sum=sum+$i))
done
echo "2+4+...+100=$sum"

while

#!/bin/bash   
i=2
sum=0     
while(( i <= 100 ))  
do  
     let "sum+=i"  
     let "i += 2"     
done   
echo "2+4+...100=$sum"

until

#!/bin/bash  
i=2
until [ $i -gt 100 ]
do
sum=$((sum + i))
i=$((i + 2))
done
echo "2+4+...100=$sum"

operation result: