用root用户安装MySQL后，会自动创建一个名为mysql的组和一个名为mysql的系统用户，但是无法从root用户切换到mysql用户：
[root@hhu MySQL]# su - mysql
[root@hhu MySQL]# su - mysql
原因是刚刚安装的mysql用户的Shell设置问题，可以从/etc/passwd文件看出
[root@hhu home]# cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
gopher:x:13:30:gopher:/var/gopher:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
usbmuxd:x:113:113:usbmuxd user:/:/sbin/nologin
rpc:x:32:32:Rpcbind Daemon:/var/cache/rpcbind:/sbin/nologin
vcsa:x:69:69:virtual console memory owner:/dev:/sbin/nologin
rtkit:x:499:497:RealtimeKit:/proc:/sbin/nologin
avahi-autoipd:x:170:170:Avahi IPv4LL Stack:/var/lib/avahi-autoipd:/sbin/nologin
abrt:x:173:173::/etc/abrt:/sbin/nologin
saslauth:x:498:76:"Saslauthd user":/var/empty/saslauth:/sbin/nologin
postfix:x:89:89::/var/spool/postfix:/sbin/nologin
rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/sbin/nologin
nfsnobody:x:65534:65534:Anonymous NFS User:/var/lib/nfs:/sbin/nologin
ntp:x:38:38::/etc/ntp:/sbin/nologin
apache:x:48:48:Apache:/var/www:/sbin/nologin
haldaemon:x:68:68:HAL daemon:/:/sbin/nologin
gdm:x:42:42::/var/lib/gdm:/sbin/nologin
pulse:x:497:495:PulseAudio System Daemon:/var/run/pulse:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tcpdump:x:72:72::/:/sbin/nologin
oprofile:x:16:16:Special user account to be used by OProfile:/home/oprofile:/sbin/nologin
hhu:x:500:500:hhu:/home/hhu:/bin/bash
vboxadd:x:496:1::/var/run/vboxadd:/bin/false
oracle:x:501:502::/home/oracle:/bin/bash
mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/false
可以看到，mysql用户的Shell属性是/bin/false，这意味着：false - do nothing, unsuccessfully。可用usermod命令来修改mysql用户的Shell属性，解锁Shell。
[root@hhu home]# usermod -s /bin/bash mysql
[root@hhu home]# su - mysql
-bash-4.1$ cd
-bash-4.1$ ls
auto.cnf ib_logfile0 performance_schema
ca-key.pem ib_logfile1 private_key.pem
ca.pem ibtmp1 public_key.pem
client-cert.pem mysql server-cert.pem
client-key.pem mysqld_safe.pid server-key.pem
ib_buffer_pool mysql.sock sys
ibdata1 mysql.sock.lock
-bash-4.1$ pwd
/var/lib/mysql
-bash-4.1$ echo $HOME
/var/lib/mysql