Counting to 100 using a for loop

#!/bin/sh

for (( i=1 ; ((i-100)) ; i=(($i+1)) ))
do
  echo $i
done;

The 2nd expression in the for loop must evaluate to 0 so that the loop will finish. This looks a bit unnatural compared to normal programming languages. Instead, consider using a while loop.

Counting to 100 using a while loop

#!/bin/sh

i=1;
while [[ i -le 100 ]] ;
do
  echo $i;
  i=$((i+1));
done;

Running something every second

while true;  do
  sleep 1
  echo print every second
done