Chapter 5. Managing Local Linux Users and Groups.

1. Users and Groups
   
   # id
   可以查詢某人或自己的相關 UID/GID 等資訊
   
   # ps au
   觀察系統所有的程序資料
   
   The format of /etc/passwd follows：
   username:password:UID:GID:GECOS:/home/dir:shell
   
   Like users, groups have a name and a number(GID).
   Local groups are defined in /etc/group.
   The format of /etc/group follows：
   
   groupname:password:GID:list of users in this group
    
2. Gaining Superuser Access
   
   # su -
   切換成 root 使用者
   
   # su <username>
   切換成 <username> 使用者
   
   # sudo command
   Running commands as root with sudo
   
   ※ One additional benefit to using sudo is that all commands executed using sudo are logged by default to /var/log/secure.
   
   ※ In  Red Hat Enterprise Linux 7, all members of group wheel can use sudo to run commands as any user, including root.



3. Managing Local User and Group Accounts
   
   
4. Managing User Passwords
   
   Shadow passwords and password policy
   
   # cat /etc/shadow
   
   The format of /etc/shadow follows：
   name:password:lastchange:minage:maxage:warning:inactive:expire:blank
   
   Password  aging
   
   # chang <username>