[ $((i/2+7)) -ge 18 ].
[ i / 2 + 7 -ge 18 ]
[ $((i / 2 + 7)) -ge 18 ]
ShellCheck found a loose
+*/% in a test statement. This usually happens when trying to do arithmetic in a condition, but without using the arithmetic expansion construct
if (a+b == c) is perfectly fine, but in sh this must be written to first expand the arithmetic operation like
if [ $((a+b)) = c ].
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.