useradd - Create a new user or update default new user information

useradd [-c comment] [-d home_dir]
[-e expire_date] [-f inactive_days]
[-g initial_group] [-G group[,...]]
[-m [-k skeleton_dir]] [-o] [-p passwd]
[-s shell] [-u uid] login

useradd -D [-g default_group] [-b default_home]
[-e default_expire_date] [-f default_inactive]
[-s default_shell]

Creating New Users
When invoked without the -D option, the useradd command creates a new user account using the values specified on the command line and the default
values from the system. The new user account will be entered into the system files as needed, the home directory will be created, and initial
files copied, depending on the command line options. The options which apply to the useradd command are:

-c comment
The new user's password file comment field.

-d home_dir
The new user will be created using home_dir as the value for the user's login directory. The default is to append the login name to
default_home and use that as the login directory name.

-e expire_date
The date on which the user account will be disabled. The date is specified in the format YYYY-MM-DD.

-f inactive_days
The number of days after a password expires until the account is permanently disabled. A value of 0 disables the account as soon as the
password has expired, and a value of -1 disables the feature. The default value is -1.

-g initial_group
The group name or number of the user's initial login group. The group name must exist. A group number must refer to an already existing
group. The default group number is 1 or whatever is specified in /etc/default/useradd.

-G group,[...]
A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening
whitespace. The groups are subject to the same restrictions as the group given with the -g option. The default is for the user to belong
only to the initial group.

-m The user's home directory will be created if it does not exist. The files contained in skeleton_dir will be copied to the home directory
if the -k option is used, otherwise the files contained in /etc/skel will be used instead. Any directories contained in skeleton_dir or
/etc/skel will be created in the user's home directory as well. The -k option is only valid in conjunction with the -m option. The
default is to not create the directory and to not copy any files. This option may not function correctly if the username has a / in it.

-o Allow create user with duplicate (non-unique) UID.

-p passwd
The encrypted password, as returned by crypt(3). The default is to disable the account.

-s shell
The name of the user's login shell. The default is to leave this field blank, which causes the system to select the default login shell.

-u uid The numerical value of the user's ID. This value must be unique, unless the -o option is used. The value must be non-negative. The
default is to use the smallest ID value greater than 99 and greater than every other user. Values between 0 and 99 are typically reserved
for system accounts.
Changing the default values
When invoked with the -D option, useradd will either display the current default values, or update the default values from the command line. The
valid options are

-b default_home
The initial path prefix for a new user's home directory. The user's name will be affixed to the end of default_home to create the new
directory name if the -d option is not used when creating a new account.

-e default_expire_date
The date on which the user account is disabled.

-f default_inactive
The number of days after a password has expired before the account will be disabled.

-g default_group
The group name or ID for a new user's initial group. The named group must exist, and a numerical group ID must have an existing entry .

-s default_shell
The name of the new user's login shell. The named program will be used for all future new user accounts.

If no options are specified, useradd displays the current default values.

The system administrator is responsible for placing the default user files in the /etc/skel directory.

You may not add a user to an NIS group. This must be performed on the NIS server.

/etc/passwd - user account information
/etc/shadow - secure user account information
/etc/group - group information
/etc/default/useradd - default information
/etc/skel - directory containing default files