Linux Users

On a Linux installation, user generally refers to a local user that a real person logs in as. It can also refer to system users that are created to establish permission restrictions on system processes. There is no real difference between these.

Adding Users

The low-level useradd command should be avoided.

To add a new user, call adduser. Most systems default to creating a home directory simultaneously, but it can be forced by including the --create-home option.

adduser --create-home USERNAME

To add a new user with a custom home directory, try:

adduser --create-home --home /var/USERNAME USERNAME

This can be useful for creating special logins for services, such as git.

Privileged Users

Standard practice is to use the wheel group to manage users with superuser privileges. This is managed by the sudoers file.

## Uncomment to allow members of group wheel to execute any command
%wheel ALL=(ALL) ALL

Either use visudo or edit /etc/sudoers directly to ensure that this line is uncommented.

Therefore, to add a privileged user, try:

adduser --create-home --groups wheel USERNAME

Adding Users to Groups

The low-level usermod command should be avoided. But for reference, the command would be usermod --append --groups GROUPNAME USERNAME.

To escalate the permissions of a user, following from the explanation above, run:

adduser USERNAME wheel

Adding System Users

There are a few considerations to make when creating a system user, but all of them are optional.

• To have a system user and group ID assigned, use the --system option.

• Generally a home directory isn't desireable, so suppress the creation of one using the -M option.

• To prevent login, set the login shell to /sbin/nologin (or something similar) using the --shell option.

adduser --system -M --shell /sbin/nologin USERNAME

To secure an existing user against login, either use chsh or edit /etc/passwd directly.

BusyBox

Note that the BusyBox implementation of adduser does not support the normal flags and options.

Configuration

adduser and useradd both look at /etc/default/useradd for a number of variables. The default login shell (SHELL), the base directory for home directory creation (HOME), and so on.

adduser additionally looks to /etc/logins.def. Much of this file relates to interactive login configuration, but a short list of variables are used in user creation. UID_MIN and UID_MAX define the range for local user IDs, while SYS_UID_MIN and SYS_UID_MAX define the range for system user IDs. (The parallel GID_MIN, GID_MAX, SYS_GID_MIN, and SYS_GID_MAX variables do the same for group IDs.)

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