Users and groups

Related articles

• DeveloperWiki:UID / GID Database
• polkit
• File permissions and attributes
• Change username

Users and groups are used on GNU/Linux for access control — that is, to control access to the system's files, directories, and peripherals. Linux offers relatively simple/coarse access control mechanisms by default. For more advanced options, see ACL and LDAP Authentication.

Overview

A user is anyone who uses a computer. In this case, we are describing the names which represent those users. It may be Mary or Bill, and they may use the names Dragonlady or Pirate in place of their real name. All that matters is that the computer has a name for each account it creates, and it is this name by which a person gains access to use the computer. Some system services also run using restricted or privileged user accounts.

Managing users is done for the purpose of security by limiting access in certain specific ways. The superuser (root) has complete access to the operating system and its configuration; it is intended for administrative use only. Unprivileged users can use the su and sudo programs for controlled privilege escalation.

Any individual may have more than one account, as long as they use a different name for each account they create. Further, there are some reserved names which may not be used such as "root".

Users may be grouped together into a "group", and users may be added to an existing group to utilize the privileged access it grants.

Note: The beginner should use these tools carefully and stay away from having anything to do with any other existing user account, other than their own.

Permissions and ownership

From In UNIX Everything is a File:

The UNIX operating system crystallizes a couple of unifying ideas and concepts that shaped its design, user interface, culture and evolution. One of the most important of these is probably the mantra: "everything is a file," widely regarded as one of the defining points of UNIX.

This key design principle consists of providing a unified paradigm for accessing a wide range of input/output resources: documents, directories, hard-drives, CD-ROMs, modems, keyboards, printers, monitors, terminals and even some inter-process and network communications. The trick is to provide a common abstraction for all of these resources, each of which the UNIX fathers called a "file." Since every "file" is exposed through the same API, you can use the same set of basic commands to read/write to a disk, keyboard, document or network device.

From Extending UNIX File Abstraction for General-Purpose Networking:

A fundamental and very powerful, consistent abstraction provided in UNIX and compatible operating systems is the file abstraction. Many OS services and device interfaces are implemented to provide a file or file system metaphor to applications. This enables new uses for, and greatly increases the power of, existing applications — simple tools designed with specific uses in mind can, with UNIX file abstractions, be used in novel ways. A simple tool, such as cat, designed to read one or more files and output the contents to standard output, can be used to read from I/O devices through special device files, typically found under the /dev directory. On many systems, audio recording and playback can be done simply with the commands, "cat /dev/audio > myfile" and "cat myfile > /dev/audio," respectively.

Every file on a GNU/Linux system is owned by a user and a group. In addition, there are three types of access permissions: read, write, and execute. Different access permissions can be applied to a file's owning user, owning group, and others (those without ownership). One can determine a file's owners and permissions by viewing the long listing format of the ls command:

$ ls -l /boot/

total 13740
drwxr-xr-x 2 root root    4096 Jan 12 00:33 grub
-rw-r--r-- 1 root root 8570335 Jan 12 00:33 initramfs-linux-fallback.img
-rw-r--r-- 1 root root 1821573 Jan 12 00:31 initramfs-linux.img
-rw-r--r-- 1 root root 1457315 Jan  8 08:19 System.map26
-rw-r--r-- 1 root root 2209920 Jan  8 08:19 vmlinuz-linux

The first column displays the file's permissions (for example, the file initramfs-linux.img has permissions -rw-r--r--). The third and fourth columns display the file's owning user and group, respectively. In this example, all files are owned by the root user and the root group.

$ ls -l /media/

total 16
drwxrwx--- 1 root vboxsf 16384 Jan 29 11:02 sf_Shared

In this example, the sf_Shared directory is owned by the root user and the vboxsf group. It is also possible to determine a file's owners and permissions using the stat command:

Owning user:

$ stat -c %U /media/sf_Shared/

root

Owning group:

$ stat -c %G /media/sf_Shared/

vboxsf

Access rights:

$ stat -c %A /media/sf_Shared/

drwxrwx---

Access permissions are displayed in three groups of characters, representing the permissions of the owning user, owning group, and others, respectively. For example, the characters -rw-r--r-- indicate that the file's owner has read and write permission, but not execute (rw-), whilst users belonging to the owning group and other users have only read permission (r-- and r--). Meanwhile, the characters drwxrwx--- indicate that the file's owner and users belonging to the owning group all have read, write, and execute permissions (rwx and rwx), whilst other users are denied access (---). The first character represents the file's type.

List files owned by a user or group with the find utility:

# find / -group [group]

# find / -user [user]

A file's owning user and group can be changed with the chown (change owner) command. A file's access permissions can be changed with the chmod (change mode) command.

See man chown, man chmod, and Linux file permissions for additional detail.

File list

Warning: Do not edit these files by hand. There are utilities that properly handle locking and avoid invalidating the format of the database. See #User management and #Group management for an overview.

File	Purpose
/etc/shadow	Secure user account information
/etc/passwd	User account information
/etc/gshadow	Contains the shadowed information for group accounts
/etc/group	Defines the groups to which users belong
/etc/sudoers	List of who can run what by sudo
/home/*	Home directories

User management

To list users currently logged on the system, the who command can be used.

To add a new user, use the useradd command:

# useradd -m -g [initial_group] -G [additional_groups] -s [login_shell] [username]

• -m creates the user home directory as /home/username. Within their home directory, a non-root user can write files, delete them, install programs, and so on.
• -g defines the group name or number of the user's initial login group. If specified, the group name must exist; if a group number is provided, it must refer to an already existing group. If not specified, the behaviour of useradd will depend on the USERGROUPS_ENAB variable contained in /etc/login.defs. The default behaviour (USERGROUPS_ENAB yes) is to create a group with the same name as the username, with GID equal to UID.
• -G introduces 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 spaces. The default is for the user to belong only to the initial group.
• -s defines the path and file name of the user's default login shell. After the boot process is complete, the default login shell is the one specified here. Ensure the chosen shell package is installed if choosing something other than Bash.

Warning: The login shell should be one of those listed in /etc/shells, unless the shell is intended to be non-functional (e.g. /usr/bin/nologin), even if the actual path does not exist (e.g. bash actually resides in /usr/bin/bash but you need to use the /bin/bash path as described in /etc/shells otherwise your user will not be able to login). For programs that use PAM, this is checked by the pam_shells module.

Note: The password for the newly created user must then be defined, using passwd as explained below.

Tip: In case you need to create an user for a specific service, use /usr/bin/nologin as shell.

Example adding a user

On a typical desktop system, use the following command to add a new user named archie, specify Bash as their login shell and add them to the wheel group (see the entry in #User groups for details):

# useradd -m -G wheel -s /bin/bash archie

This command will also automatically create a group called archie with the same GID as the UID of the user archie and makes this the default group for archie on login. Making each user have their own group (with group name same as user name and GID same as UID) is the preferred way to add users.

You could also make the default group something else, e.g. users:

# useradd -m -g users -G wheel -s /bin/bash archie

However, this is not recommended for multi-user systems. Typically, the method for facilitating shared write access for specific groups of users while keeping home directories private is setting user umask value to 002, meaning the default group (users in the example above) will by default always have write access to any file you create. The user's home folder, which is owned by a group with group name same as user name, will be read-only for other system users, while shared files/folders can be made writeable by default for everyone in the operative group. The owning group can be automatically fixed to the group which owns the parent directory by setting the group sticky bit on this directory:

# chmod g+s our_shared_directory

Otherwise the file creator's default group (usually the same as the user name) is used.

Other examples of user management

To add a user to other groups use (additional_groups is a comma-separated list):

# usermod -aG additional_groups username

Alternatively, gpasswd may be used. Though the username can only be added (or removed) from one group at a time.

# gpasswd --add username group

Warning: If the -a option is omitted in the usermod command above, the user is removed from all groups not listed in additional_groups (i.e. the user will be member only of those groups listed in additional_groups).

To enter user information for the GECOS field (e.g. the full user name), type:

# chfn username

(this way chfn runs in interactive mode).

To specify the user's password, type:

# passwd username

To mark a user's password as expired, requiring them to create a new password the first time they log in, type:

# chage -d 0 username

User accounts may be deleted with the userdel command.

# userdel -r username

The -r option specifies that the user's home directory and mail spool should also be deleted.

Tip: The AUR packages adduser^AUR, adduser-defaults^AUR or adduser-deb^AUR provide an adduser script that allows carrying out the jobs of useradd, chfn and passwd interactively. See also FS#32893.

User database

Local user information is stored in the /etc/passwd file. To list all user accounts on the system:

$ cat /etc/passwd

There is one line per account, and each is of the format:

account:password:UID:GID:GECOS:directory:shell

where:

• account is the user name
• password is the user password
• UID is the numerical user ID
• GID is the numerical primary group ID for the user
• GECOS is an optional field used for informational purposes; usually it contains the full user name
• directory is the user's $HOME directory
• shell is the user command interpreter (defaults to /bin/sh)

Note: Arch Linux uses shadowed passwords. The passwd file is world-readable, so storing passwords (hashed or otherwise) in this file would be insecure. Instead, the password field will contain a placeholder character (x) indicating that the hashed password is saved in the access-restricted file /etc/shadow.

Group management

/etc/group is the file that defines the groups on the system (man group for details).

Display group membership with the groups command:

$ groups [user]

If user is omitted, the current user's group names are displayed.

The id command provides additional detail, such as the user's UID and associated GIDs:

$ id [user]

To list all groups on the system:

$ cat /etc/group

Create new groups with the groupadd command:

# groupadd [group]

Add users to a group with the gpasswd command:

# gpasswd -a [user] [group]

Modify an existing group with groupmod; e.g. to rename old_group group to new_group whilst preserving gid (all files previously owned by old_group will be owned by new_group):

# groupmod -n [new_group] [old_group]

To delete existing groups:

# groupdel [group]

To remove users from a group:

# gpasswd -d [user] [group]

If the user is currently logged in, he/she must log out and in again for the change to take effect.

Group list

User groups

Workstation/desktop users often add their non-root user to some of following groups to allow access to peripherals and other hardware and facilitate system administration:

Group	Affected files	Purpose
games	/var/games	Access to some game software.
rfkill	/dev/rfkill	Right to control wireless devices power state (used by rfkill).
users		Standard users group.
uucp	/dev/ttyS[0-9], /dev/tts/[0-9], /dev/ttyACM[0-9]	Serial and USB devices such as modems, handhelds, RS-232/serial ports.
wheel		Administration group, commonly used to give access to the sudo and su utilities (neither uses it by default, configurable in /etc/pam.d/su and /etc/pam.d/su-l). It can also be used to gain full read access to journal files.

System groups

The following groups are used for system purposes and are not likely to be used by novice Arch users:

Group	Affected files	Purpose
bin	none	Historical
daemon
dbus		used internally by dbus
ftp	/srv/ftp	used by FTP servers like Proftpd
fuse		Used by fuse to allow user mounts.
http
kmem	/dev/port, /dev/mem, /dev/kmem
mail	/usr/bin/mail
mem
nobody		Unprivileged group.
polkitd		polkit group.
root	/*	Complete system administration and control (root, admin).
smmsp		sendmail group.
systemd-journal	/var/log/journal/*	Provides access to the complete systemd logs. Otherwise, only user generated messages are displayed.
tty	/dev/tty, /dev/vcc, /dev/vc, /dev/ptmx	Eg. to acces /dev/ACMx

Software groups

These groups are used by certain non-essential software. Sometimes they are used just internally, in these cases you should not add your user into these groups. See the main page for the software for details.

Group	Affected files	Purpose
adbusers	devices nodes under /dev/	Right to access Android Debugging Bridge.
avahi
bumblebee	/run/bumblebee.socket	Right to launch applications with Bumblebee to utilize NVIDIA Optimus GPUs.
cdemu	/dev/vhba_ctl	Right to use cdemu drive emulation.
clamav	/var/lib/clamav/*, /var/log/clamav/*	Used by Clam AntiVirus.
gdm	X server authorization directory (ServAuthDir)	GDM group.
locate	/usr/bin/locate, /var/lib/locate, /var/lib/mlocate, /var/lib/slocate	Right to use updatedb command.
mpd	/var/lib/mpd/*, /var/log/mpd/*, /var/run/mpd/*, optionally music directories	MPD group.
networkmanager		Requirement for your user to connect wirelessly with NetworkManager. This group is not included with Arch by default so it must be added manually.
ntp	/var/lib/ntp/*	NTPd group.
thinkpad	/dev/misc/nvram	Used by ThinkPad users for access to tools such as tpb.
vboxsf	virtual machines' shared folders	Used by VirtualBox.
vboxusers	/dev/vboxdrv	Right to use VirtualBox software.
vmware		Right to use VMware software.
wireshark		Right to capture packets with Wireshark.

Deprecated or unused groups

Following groups are currently of no use for anyone:

Group	Purpose
log	Access to log files in /var/log/ created by syslog-ng.
ssh	Sshd can be configured to only allow members of this group to login. This is true for any arbitrary group; the ssh group is not created by default, hence non-standard.
stb-admin	Unused! Right to access system-tools-backends
kvm	Adding a user to the kvm group used to be required to allow non-root users to access virtual machines using KVM. This has been deprecated in favor of using udev rules, and this is done automatically.

Pre-systemd groups

These groups used to be needed before arch migrated to systemd. That is no longer the case, as long as the logind session is not broken (see General troubleshooting#Session permissions to check it). The groups can even cause some functionality to break. See Sysvinit#Migration_to_systemd for details.

Group	Affected files	Purpose
audio	/dev/audio, /dev/snd/*, /dev/rtc0	Direct access to sound hardware, for all sessions (requirement is imposed by both ALSA and OSS). Local sessions already have the ability to play sound and access mixer controls.
camera		Access to Digital Cameras.
disk	/dev/sda[1-9], /dev/sdb[1-9]	Access to block devices not affected by other groups such as optical, floppy, and storage.
floppy	/dev/fd[0-9]	Access to floppy drives.
lp	/etc/cups, /var/log/cups, /var/cache/cups, /var/spool/cups, /dev/parport[0-9]	Access to printer hardware; enables the user to manage print jobs.
network		Right to change network settings such as when using NetworkManager.
optical	/dev/sr[0-9], /dev/sg[0-9]	Access to optical devices such as CD and DVD drives.
power		Right to use Pm-utils (suspend, hibernate...) and power management controls.
scanner	/var/lock/sane	Access to scanner hardware.
storage		Access to removable drives such as USB hard drives, flash/jump drives, MP3 players; enables the user to mount storage devices.
sys		Right to administer printers in CUPS.
video	/dev/fb/0, /dev/misc/agpgart	Access to video capture devices, 2D/3D hardware acceleration, framebuffer (X can be used without belonging to this group). Local sessions already have the ability to use hardware acceleration and video capture.