Did you try installing [the `seq` utility](https://en.wikipedia.org/wiki/Seq_(Unix))? It's part of GNU Coreutils, and seems to also [be available on BSDs](https://man.openbsd.org/FreeBSD-11.1/seq.1). The problem with using special shell-builtins/syntax is that it may not be compatible with other shells (ex. POSIX `sh`), whereas the `seq` utility should work with all shells.