Range arrays with custom step-size in Bourne shells

Question

Say I want to create a range array of integers in a shell such that the array:

• Starts in $START_VALUE
• Ends in $END_VALUE
• Elements are $STEP_SIZE apart, starting from $START_VALUE.

For anyone familiar with Python, I am effectively seeking the range counterpart in Bourne shells, and specifically in Zsh.

Are there any shell built-ins that help here?

---

Note: Python's range function supports increasing and decreasing range arrays (i.e. where $START_VALUE is either greater or smaller than $END_VALUE).

Answer 1

As in bash, in zsh, this works:

printf '%s ' {2..23..3}; echo

As this also work:

set   -- {2..23..3}
for   i
do    echo "$i"
done

As this:

set   -- {2..23..3}
for i; do
      echo "$i"
done

As this:

for i in {0..20..2}
do
    echo "$i"
done

What works in zsh but not in bash is ranges with variables:

a=2 b=23 c=3
set   -- {$a..$b..$c}
for   i
do    echo $i
done

Answer 2

In bash and zsh you can use {START..STOP..INCREMENT} and it works as expected when START < STOP. For example,

 for i in {0..20..2}
 do
      echo $i
 done

prints out 0, 2, 4, 6, ..., 20. In bash you can also do "backward" ranges by changing it to {20..0..-2} which would print 20, 18, 16, ..., 2, 0. This however doesn't work for zsh.

A method that works for both bash and zsh is to use seq.

for i in `seq 20 -2 0`;
do
    echo $i
done

prints out 20, 18, 16, ..., 0 (in both bash and zsh). The format here is FIRST, STEP, LAST (and the STEP is optional, if left out, it becomes 1).