在系统中使用man useradd查看帮助：
>USERADD(8) System Management Commands USERADD(8)
NAME
useradd – create a new user or 便宜美国vps update default new user information
SYNOPSIS
useradd [options] LOGIN
useradd -D
useradd -D [options]
DESCRIPTION
useradd is a low level utility for adding users. On Debian, administrators should usually use adduser(8) instead.
从文档中可以看出推荐在Debian系统上使用adduser，而不是useradd。
使用man adduser查看文档：
>ADDUSER(8) System Manager’s Manual ADDUSER(8)
NAME
adduser, addgroup – add a user or group to the system
SYNOPSIS
adduser [options] [–home DIR] [–shell SHELL] [–no-create-home] [–uid ID] [–firstuid ID] [–lastuid ID] [–ingroup GROUP | –gid ID] [–dis‐
abled-password] [–disabled-login] [–gecos GECOS] [–add_extra_groups] [–encrypt-home] user
adduser –system [options] [–home DIR] [–shell SHELL] [–no-create-home] [–uid ID] [–group | –ingroup GROUP | –gid ID] [–disabled-password]
[–disabled-login] [–gecos GECOS] user
addgroup [options] [–gid ID] group
addgroup –system [options] [–gid ID] group
adduser [options] user group
COMMON OPTIONS
[–quiet] [–debug] [–force-badname] [–help|-h] [–version] [–conf FILE]
DESCRIPTION
adduser and addgroup add users and groups to the system according to command line options and configuration information in /etc/adduser.conf.
They are friendlier front ends to the low level tools like useradd, groupadd and usermod programs, by default choosing Debian policy conformant
UID and GID values, creating a home directory with skeletal configuration, running a custom script, and other features. adduser and addgroup can
be run in one of five modes:
可以看出adduser的参数更全面，更好用。
顺便提一下，如果想为一个系统服务创建专门的用户，比如说为hadoop服务创建一个专门的hadoop用户用来运行相关程序，这时候可以使用如下命令：
adduser –system hadoop
使用–system创建的是系统用户，系统用户默认的shell是/usr/sbin/nologin，这个shell禁止登录，只能运行程序，所以非常安全。
本地用户想要切换到系统用户可以使用如下命令：
su – hadoop -s /bin/bash
现在就切换到bash，可以正常运行了。
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