The best solution for this is likely to add a line to your crontab. Accessing the crontab file may differ between implementations of cron, so I've provided commands for the two cron implementations in the official Arch repos. If you want a solution that does not require a specific cron implementation, I've written another answer that uses systemd/Timers instead.
crontab -e or variants of it use the EDITOR environment variable (defaults to vi). If you wish to use a different editor, export it to the EDITOR variable like so:
where vim is replaced with the editor of your choice.
Editing crontab with cronie:
Editing crontab with fcron:
Add your cron command to the file and save it:
*/5 * * * * cd /mnt/internal-storage/coindata && shell/command coins update
The format for lines in this file is
minute hour day_of_month month day_of_week command
If cron is not running, start its daemon.
systemctl start cronie.service
systemctl start fcron.service
If you want the commands in the crontab to continue to run after rebooting, make sure that the cron daemon is enabled:
systemctl enable cronie.service or
systemctl enable fcron.service