http://busybox.net/FAQ.html#job_control
Job control will be turned off since your shell can not obtain a controlling terminal. This typically happens when you run your shell on /dev/console. The kernel will not provide a controlling terminal on the /dev/console device. Your should run your shell on a normal tty such as tty1 or ttyS0 and everything will work perfectly. If you REALLY want your shell to run on /dev/console, then you can hack your kernel (if you are into that sortof thing) by changing drivers/char/tty_io.c to change the lines where it sets "noctty = 1;" to instead set it to "0". I recommend you instead run your shell on a real console...
用 busybox 的init时，可以在 inittab 中用：
tty1::askfirst:-/bin/sh
格式不太确定，确认一下先 ，主要是在前面加的 ttyS0 用来指明实际终端。如果用的是 uClinux 里的 simpleinit.c ( user/init/sampleinit.c )，格式为：
tty1::/bin/sh