root@openwrt:~# ls -al `which ash`
lrwxrwxrwx    1 root     root            7 Oct 29 13:13 /bin/ash -> busybox
root@openwrt:~# echo $SHELL
/bin/bash
root@openwrt:~# ash


BusyBox v1.4.2 (2007-10-12 17:51:00 CDT) Built-in shell (ash)
Enter 'help' for a list of built-in commands.

root@openwrt:~# sleep 4000
[1] + Stopped                    sleep 4000
root@openwrt:~# fg
sleep 4000

root@openwrt:~# echo $SHELL
/bin/bash