Mark files to be ignored by dropbox based on pattern. Example with node_modules directories.

dropbox_ignore_pattern.bash

# ignore `node_modules` directories recursively.
# see https://help.dropbox.com/files-folders/restore-delete/ignored-files
# apt install attr
LC_ALL=C find . -type d -regextype posix-extended -regex '.*/node_modules' -prune 2> >(grep -v 'Permission denied' >&2) | while read -r path; do attr -s com.dropbox.ignored -V 1 "$path"; done;