Linux useradd命令
0 Preface/Foreword
1 useradd介绍
该命令是系统管理命令（System management command）。
useradd - create a new user or update default new user information
1.1 描述
useradd is a low level utility for adding users. On Debian, administrators should usually use adduser instead.
When invoked without the -D option, the useradd command creates a new user account using the values specified on the command line plus the default values from the system. Depending on command line options, the useradd command will update system files and may also create the new user's home directory (主目录，根目录) and copy intial files.
1.2 --comment
-c, --comment COMMENT
Any text string. It is generally a short description of the login, and is currently used as the field for the user's full name.
1.3 --create-home
-m, --create-home
Create the user's home directory if it does not exist. The files and directories contained in the skeleton directory (which can be defined with the -k) will be copied to the home directory.
1.4 --shell
-s, --shell SHELL
The name of the user's login shell. The default is to leave the field blank, which causes the system to select the default login shell specified by the SHELL variable in /etc/default/useradd, or an empty string by defaut.