zkServer.sh uses sh's builtin echo on BSD, behaves incorrectly.

zkServer.sh output the PID of the zookeeper process with:
echo -n $! > "$ZOOPIDFILE"

This uses -n which sh's builtin echo does not support. From echo's manpage.
<snip>
Some shells may provide a builtin echo command which is similar or identical to this utility. Most notably, the builtin echo in sh(1) does not accept
the -n option. Consult the builtin(1) manual page.
</snip>

This means that echo -n PID > ZOOPIDFILE will mean the contents of ZOOPIDFILE will be "-n PID". This stops zkServer.sh stop from working correctly.