Apr 16

Today i just got a question on how actually a useradd command works and is it possible for us to create user without useradd command. And here is my answer after some googling around ­čść

what is useradd ?

useradd COMMAND:
useradd – Adds new user to the linux system, with specified user-name. When a new user is added then a corresponding entry is made in files /etc/passwd, /etc/group and /etc/shadow

SYNTAX:
The Syntax is
useradd [options] [username]

OPTIONS:

# useradd –help

Usage: useradd [options] LOGIN

Options:

-b, –base-dir BASE_DIR base directory for the new user account home directory
-c, –comment COMMENT set the GECOS field for the new user account
-d, –home-dir HOME_DIR home directory for the new user account
-D, –defaults print or save modified default useradd configuration
-e, –expiredate EXPIRE_DATE set account expiration date to EXPIRE_DATE
-f, –inactive INACTIVE set password inactive after expiration to INACTIVE
-g, –gid GROUP force use GROUP for the new user account
-G, –groups GROUPS list of supplementary groups for the new user account
-h, –help display this help message and exit
-k, –skel SKEL_DIR specify an alternative skel directory
-K, –key KEY=VALUE overrides /etc/login.defs defaults
-m, –create-home create home directory for the new user account
-o, –non-unique allow create user with duplicate (non-unique) UID
-p, –password PASSWORD use encrypted password for the new user account
-r, –system create a system account
-s, –shell SHELL the login shell for the new user account
-u, –uid UID force use the UID for the new user account

EXAMPLE:

  1. To add new user:
    # useradd test

    This command will add a new user with name test.

  2. To add user but not allow to login in the system:
    useradd -s /bin/nologin test

    This command will add user hiox but not allow to login. Or you can go to the /etc/passwd and set the test username  Shell access tobe /usr/sbin/nologin or /usr/sbinfalse

    Understanding fields in /etc/passwd

    The /etc/passwd contains one entry per line for each user (or user account) of the system. All fields are separated by a colon (:) symbol. Total seven fields as follows.

    Generally, passwd file entry looks as follows (click to enlarge image):

    1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
    2. Password: An x character indicates that encrypted password is stored in /etc/shadow file.
    3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
    4. Group ID (GID): The primary group ID (stored in /etc/group file)
    5. User ID Info: The comment field. It allow you to add extra information about the users such as user’s full name, phone number etc. This field use by finger command.
    6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
    7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell. Please note that it does not have to be a shell.
    In above command:
    test -Is the user-name
    /bin/nologin -Is Shell assigned to the user
  3. To set expire date of the user:
    useradd -e 2008-06-30 test

    This command will add user test and set the expire date to 2009-06-30.

    In above command:
    test -Is the user-name
    2009-06-30 -Is date on which the user-account will be expired
  4. To create user without creating home directory:
    useradd -M test

    The above command will create user test but home directory will not be created.

How to add user without useradd command ?

as you probably know useradd command actualy create a user on /etc/passwd to determine the user id home directory comman shell etc, /etc/group to determine te group id and /etc/shadow for password. I will create a newtest username but since the password was encrpty MD5 i will use the previous username password on the newtest username that we will create withiut useradd command

Code:
# useradd testing
# passwd testing
New password: password
Re-enter new password: password
Password changed.
#
# cat /etc/shadow | grep testing
testing:$1$AdHBr9EJ$VwEqlF.GprlX28oWWYprZ0:14313:0:99999:7:::
#
# cat /etc/passwd | grep testing
testing:x:1001:1001::/home/tesing:/bin/sh
# cat /etc/group | grep testing
testing:x:1001:

Create newtest username without adduser command
# vi /etc/shadow
add newtest password and save
newtest:$1$AdHBr9EJ$VwEqlF.GprlX28oWWYprZ0:14313:0:99999:7:::
# vi /etc/group
add newtest username group and id (makesure u have use the new id number
newtest:x:1002:
# vi /etc/passwd
add newtest username,id,home directory ,shell command
newtest:x:1002:1002::/home:/bin/sh

Of course, this is my version of creating a user without useradd command. Please correct me if i am wrong

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