For loop with variable increment

Question

I have a small shell script and what I want to do when it is run is to display each N'th parameter entered by the user, where N is the first parameter. Ex: for input 2 3 4 5 6 I should display 3 5. What I have until now:

N=$1
for ((i=2;i<$#;i+=$N))
do
    echo -n ${i}" "
done

However, I get a syntax error: operand expected at this part: i+=$N. How can I solve this? If I replace i+=$N with, say, i+=2, it works. But i want N there...

Answer 1

So in order to get your code work as you wish, you should do something like:

N=$1
for ((i=2;i<$#;((i=i+$N))))
do
  echo -n ${!i}" "
done

Note the ! before i.

Good luck :)