useradd
useradd功能说明：建立用户帐号。
语 法：useradd [-mMnr][-c <备注>][-d <登入目录>][-e <有效期限>][-f <缓冲天数>][-g <群组>][-G <群组>][-s <shell>][-u <uid>][用户帐号] 或 useradd -D [-b][-e <有效期限>][-f <缓冲天数>][-g <群组>][-G <群组>][-s <shell>]
补充说明：useradd可用来建立用户帐号。帐号建好之后，再用passwd设定帐号的密码．而可用userdel删除帐号。使用useradd指令所建立的帐号，实际上是保存在/etc/passwd文本文件中。
参 数：
-c<备注> 加上备注文字。备注文字会保存在passwd的备注栏位中。
-d<登入目录> 指定用户登入时的启始目录。
-D 变更预设值．
-e<有效期限> 指定帐号的有效期限。
-f<缓冲天数> 指定在密码过期后多少天即关闭该帐号。
-g<群组> 指定用户所属的群组。
-G<群组> 指定用户所属的附加群组。
-m 自动建立用户的登入目录。
-M 不要自动建立用户的登入目录。
-n 取消建立以用户名称为名的群组．
-r 建立系统帐号。
-s<shell> 指定用户登入后所使用的shell。
-u<uid> 指定用户ID。
useradd
useradd [options] [user]
System administration command. Create new user accounts or update default account information. Unless invoked with the -D option, user must be given. useradd will create new entries in system files. Home directories and initial files may also be created as needed.
Options
Comment field.
Home directory. The default is to use user as the directory name under the home directory specified with the -D option.
Account expiration date. Use the format MM/DD/YYYY. Two-digit year fields are also accepted. The value is stored as the number of days since January 1, 1970. This option requires the use of shadow passwords.
Permanently disable account this many days after the password has expired. A value of -1 disables this feature. This option requires the use of shadow passwords.
Initial group name or ID number. If a different default group has not been specified using the -D option, the default group is 1.
Supplementary groups given by name or number in a comma-separated list with no whitespace.
Copy default files to the user's home directory. Meaningful only when used with the -m option. Default files are copied from /etc/skel/ unless an alternate dir is specified.
Make user's home directory if it does not exist. The default is not to make the home directory.
Do not create a home directory for the user, even if the system default in /etc/login.defs is to create one.
Red Hat-specific option. Turn off the Red Hat default that creates a group with the same name as the username and puts the user in that group.
Override. Accept a nonunique uid with the -u option. (Probably a bad idea.)
The encrypted password, as returned by crypt(3).
Red Hat-specific option. Create a system account with a non-expiring password and a UID lower than the minimum defined in /etc/login.defs. Do not create a home directory for the account unless -m is also specified.
Login shell.
Numerical user ID. The value must be unique unless the -o option is used. The default value is the smallest ID value greater than 99 and greater than every other uid.
Set or display defaults. If options are specified, set them. If no options are specified, display current defaults. The options are:
-b dir Home directory prefix to be used in creating home directories. If the -d option is not used when creating an account, the user name will be appended to dir.
-e date Expire date. Requires the use of shadow passwords.
-f days Number of days after a password expires to disable an account. Requires the use of shadow passwords.
-g group Initial group name or ID number.
-s shell Default login shell.