我在系统中添加了一个新用户：
$ useradd new_user
当我尝试从 切换root到 时new_user，出现此错误：
$ su new_user
permission denied /bin/sh
$grep new_user /etc/passwd
new_user:x:2001:4001::/home/new_user:/bin/sh
$/bin/ls -ld /bin
dr-xr-xr-x 2 root root 4096 Jan 1 00:07 /bin
$/bin/ls -ld /bin/sh
lrwxrwxrwx 1 root root 20 Jan 1 00:07 /bin/sh -> /etc/alternatives/sh
$/bin/ls -ld /etc/alternatives/sh
lrwxrwxrwx 1 root root 9 Jan 1 00:07 /etc/alternatives/sh -> /bin/bash
$/bin/ls -ld /bin/bash
-r-xr-x--- 1 root shell 700076 May 27 2014 /bin/bash