/etc/passwd文件结构

每一行都代表一个账号，有几行就代表有几个账号在你的系统中；不过需要特别留意的是，里面很多账号本来就是系统正常运行所必须的，我们可以简称它为系统账号，例如bin，daemon，adm等等。这些账号不要随意删除。

[root@RHEL6 ~]# head -n 4 /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

每一行都使用":"作为分隔符，共有7个字段，分别是
1、账号名称
2、密码，早期密码是放在这里的，但后期为了安全，将整个字段的密码数据改放到/etc/shadow中了，所以这里看到一个“x”
3、UID
4、GID
5、用户信息说明，没什么重要用途，只是用来解释整个账号的意义而已
6、主文件夹，家目录$HOME
7、shell，此字段可以定义用户默认使用的shell，默认是/bin/bash；有一个shell可以用来替代成让账号无法取得shell环境的登陆操作，那就是/sbin/nologin。

man 5 passwd命令关于passwd文件的字段的描述如下 

The field descriptions are:
       account   the name of the user on the system. It should not contain capital letters.
       password  the encrypted user password, an asterisk (*), or the letter 'x'. (See  pwconv(8)
                 for an explanation of 'x'.)
       UID       the numerical user ID.
       GID       the numerical primary group ID for this user.
       GECOS     This field is optional and only used for informational purposes. Usually, it con-
                 tains the full username. GECOS means  General  Electric  Comprehensive  Operating
                 System, which  has been renamed to GCOS when GE's large systems division was sold
                 to Honeywell. Dennis Ritchie has reported: "Sometimes we sent printer  output  or
                 batch  jobs  to the GCOS machine.  The gcos field in the password file was a place
                 to stash the information for the $IDENTcard.  Not elegant."
       directory the user's $HOME directory.
       shell     the program to run at login (if empty, use /bin/sh). If set to a  non-existing
                executable, the user will be unable to login through login(1).