Linux中内置账户共有很多个(cat /etc/passwd )，除了有2个可登陆账户：root和自己设置的账户外。还有些其它的账户。
在知道密码的情况下，远程电脑可以使用账户：root等登陆本机！
1 root:x:0:0:root:/root:/bin/bash
2 daemon:x:1:1:daemon:/usr/sbin:/bin/sh
3 bin:x:2:2:bin:/bin:/bin/sh
4 sys:x:3:3:sys:/dev:/bin/sh
5 sync:x:4:65534:sync:/bin:/bin/sync
6 games:x:5:60:games:/usr/games:/bin/sh
7 man:x:6:12:man:/var/cache/man:/bin/sh
8 lp:x:7:7:lp:/var/spool/lpd:/bin/sh
9 mail:x:8:8:mail:/var/mail:/bin/sh
10 news:x:9:9:news:/var/spool/news:/bin/sh
11 uucp:x:10:10:uucp:/var/spool/uucp:/bin/sh
12 proxy:x:13:13:proxy:/bin:/bin/sh
13 www-data:x:33:33:www-data:/var/www:/bin/sh
14 backup:x:34:34:backup:/var/backups:/bin/sh
15 list:x:38:38:Mailing List Manager:/var/list:/bin/sh
16 irc:x:39:39:ircd:/var/run/ircd:/bin/sh
17 gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/bin/sh
18 nobody:x:65534:65534:nobody:/nonexistent:/bin/sh
19 libuuid:x:100:101::/var/lib/libuuid:/bin/sh
20 syslog:x:101:103::/home/syslog:/bin/false
21 messagebus:x:102:105::/var/run/dbus:/bin/false
22 colord:x:103:108:colord colour management daemon,,,:/var/lib/colord:/bin/false
23 lightdm:x:104:111:Light Display Manager:/var/lib/lightdm:/bin/false
24 whoopsie:x:105:114::/nonexistent:/bin/false
25 avahi-autoipd:x:106:117:Avahi autoip daemon,,,:/var/lib/avahi-autoipd:/bin/false
26 avahi:x:107:118:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/bin/false
27 usbmux:x:108:46:usbmux daemon,,,:/home/usbmux:/bin/false
28 kernoops:x:109:65534:Kernel Oops Tracking Daemon,,,:/:/bin/false
29 pulse:x:110:119:PulseAudio daemon,,,:/var/run/pulse:/bin/false
30 rtkit:x:111:122:RealtimeKit,,,:/proc:/bin/false
31 speech-dispatcher:x:112:29:Speech Dispatcher,,,:/var/run/speech-dispatcher:/bin/sh
32 hplip:x:113:7:HPLIP system user,,,:/var/run/hplip:/bin/false
33 saned:x:114:123::/home/saned:/bin/false
34 ubuntu:x:999:999:Live session user,,,:/home/ubuntu:/bin/bash
每一行最后一项就是控制是否能登录的，/bin/false一定不能登录，/bin/bash或者/bin/sh应该可登录。
改完之后，修改密码，重启reboot，在用ssh登陆上了Linux，su - root，进入了根目录：
Last login: Sat Jun 8 18:45:31 2015 from 130.251.80.103
-bash-3.2$ ls
ls: .: Permission denied
-bash-3.2$ su - root
Password:
[root@cnaps2-oracle ~]# ls
anaconda-ks.cfg giflib-utils-4.1.3-7.1.el5_3.1.i386.rpm