Users and groups

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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 PAM#Configuration How-Tos.

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 list all existing user accounts including their properties stored in the user database, run passwd -Sa as root. See passwd(1) for the description of the output format.

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: In order to be able to log in, the login shell must be one of those listed in /etc/shells, otherwise the PAM module pam_shell will deny the login request. In particular, do not use the /usr/bin/bash path instead of /bin/bash, unless it is properly configured in /etc/shells.
Note: The password for the newly created user must then be defined, using passwd as shown in #Example adding a user.

When the login shell is intended to be non-functional, for example when the user account is created for a specific service, /usr/bin/nologin may be specified in place of a regular shell to politely refuse a login (see nologin(8)).

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:

# useradd -m -G wheel -s /bin/bash archie
Tip: Do not worry about adding the user to other groups now. If later on you will install a particular application, or perform a specific configuration, that requires explicitly adding the user to a group, the wiki will remind you to do it in the specific article that you will be following.

Although it is not required to protect the newly created user archie with a password, it is highly recommend to do so:

# passwd archie
Tip: Bash is the default value for the shell (as indicated by useradd -D), so you can omit the -s option except if you want to use something else.

The above useradd 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. study:

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

Note that, in multi-user systems, using a single default group (e.g. users) for every user is not recommended. The reason is that typically, the method for facilitating shared write access for specific groups of users is setting user umask value to 002, which means that the default group will by default always have write access to any file you create. See also User Private Groups.

In the recommended scenario, where the default group has the same name as the user name, all files are by default writeable only for the user who created them. To allow write access to a specific group, shared files/folders can be made writeable by default for everyone in this group and the owning group can be automatically fixed to the group which owns the parent directory by setting the setgid 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.

If a GID change is required temporarily you can also use the newgrp command to change the user's default GID to another GID at runtime. For example, after executing newgrp groupname files created by the user will be associated with the groupname GID, without requiring a re-login. To change back to the default GID, execute newgrp without a groupname.

Example adding a system user

System users can be used to run processes/daemons under a different user, protecting (e.g. with chown) files and/or directories and more examples of computer hardening.

With the following command a system user without shell access and without a home directory is created (optionally append the -U parameter to create a group with the same name as the user, and add the user to this group):

# useradd -r -s /usr/bin/nologin username

Other examples of user management

To change a user's login name:

# usermod -l newname oldname

To change a user's home directory:

# usermod -d /my/new/home -m username

The -m option also automatically creates the new directory and moves the content there.

Tip: You can create a link from the user's former home directory to the new one. Doing this will allow programs to find files that have hardcoded paths.
# ln -s /my/new/home/ /my/old/home

Make sure there is no trailing / on /my/old/home.

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

# usermod -aG additional_groups username
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).

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

# gpasswd --add username group

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

# chfn username

(this way chfn runs in interactive mode).

Alternatively the GECOS comment can be set more liberally with:

# usermod -c "Comment" 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 adduserAUR script allows carrying out the jobs of useradd, chfn and passwd interactively. See also FS#32893.

Username change tips

Warning: Make certain that you are not logged in as the user whose name you are about to change. Open a new tty (Ctrl+Alt+F1) and log in as root or as another user and su to root. usermod should prevent you from doing this by mistake.

Changing a username is safe and easy when done properly, just use the usermod command. If the user is associated to a group with the same name, you can rename this with the groupmod command.

Alternatively, the /etc/passwd file can be edited directly, see #User database for an introduction to its format.

Also keep in mind the following notes:

  • If you are using sudo make sure you update your /etc/sudoers to reflect the new username(s) (via the visudo command as root).
  • Personal crontabs need to be adjusted by renaming the user's file in /var/spool/cron from the old to the new name, and then opening crontab -e to change any relevant paths and have it adjust the file permissions accordingly.
  • Wine's personal folders/files' contents in ~/.wine/drive_c/users, ~/.local/share/applications/wine/Programs and possibly more need to be manually renamed/edited.
  • Certain Thunderbird addons, like Enigmail, may need to be reinstalled.
  • Anything on your system (desktop shortcuts, shell scripts, etc.) that uses an absolute path to your home dir (i.e. /home/oldname) will need to be changed to reflect your new name. To avoid these problems in shell scripts, simply use the ~ or $HOME variables for home directories.
  • Also do not forget to edit accordingly the configuration files in /etc that relies on your absolute path (i.e. Samba, CUPS, so on). A nice way to learn what files you need to update involves using the grep command this way: grep -r {old_user} *

User database

Local user information is stored in the plain-text /etc/passwd file: each of its lines represents a user account, and has seven fields delimited by colons.

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

Where:

  • account is the user name. This field can not be blank. Standard *NIX naming rules apply.
  • password is the user password.
    Warning: The passwd file is world-readable, so storing passwords (hashed or otherwise) in this file is insecure. Instead, Arch Linux uses shadowed passwords: the password field will contain a placeholder character (x) indicating that the hashed password is saved in the access-restricted file /etc/shadow. For this reason it is recommended to always change passwords using the passwd command.
  • UID is the numerical user ID. In Arch, the first login name (after root) is UID 1000 by default; subsequent UID entries for users should be greater than 1000.
  • GID is the numerical primary group ID for the user. Numeric values for GIDs are listed in /etc/group.
  • GECOS is an optional field used for informational purposes; usually it contains the full user name, but it can also be used by services such as finger and managed with the chfn command. This field is optional and may be left blank.
  • directory is used by the login command to set the $HOME environment variable. Several services with their own users use /, but normal users usually set a folder under /home.
  • shell is the path to the user's default command shell. This field is optional and defaults to /bin/bash.

Example:

jack:x:1001:100:Jack Smith,some comment here,,:/home/jack:/bin/bash

Broken down, this means: user jack, whose password is in /etc/shadow, whose UID is 1001 and whose primary group is 100. Jack Smith is his full name and there is a comment associated to his account; his home directory is /home/jack and he is using Bash.

The pwck command can be used to verify the integrity of the user database. It can sort the user list by GID at the same time, which can be helpful for comparison:

# pwck -s

Note that the Arch Linux defaults of the files are created as .pacnew files by new releases of the filesystem package. Unless Pacman outputs related messages for action, these .pacnew files can, and should, be disregarded/removed. New required default users and groups are added automatically by the packages' install script.

Group management

/etc/group is the file that defines the groups on the system (see group(5) 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
Note: This will change a group name but not the numerical GID of the 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 must log out and in again for the change to take effect.

The grpck command can be used to verify the integrity of the system's group files.

Updates to the filesystem package create .pacnew files. Alike the .pacnew files for the #User database, these can be disregarded/removed, because the install script adds any new required groups.

Group list

This section explains the purpose of the essential groups from the core/filesystem package. There are many other groups, which will be created with correct GID when the relevant package is installed. See the main page for the software for details.

Note: A later removal of a package does not remove the automatically created user/group (UID/GID) again. This is intentional because any files created during its usage would otherwise be left orphaned as a potential security risk.

User groups

Non-root workstation/desktop users often need to be added to some of following groups to allow access to hardware peripherals and facilitate system administration:

Group Affected files Purpose
adm Administration group, similar to wheel.
ftp /srv/ftp/ Access to files served by FTP servers.
games /var/games Access to some game software.
http /srv/http/ Access to files served by HTTP servers.
log Access to log files in /var/log/ created by syslog-ng.
rfkill /dev/rfkill Right to control wireless devices power state (used by rfkill).
sys Right to administer printers in CUPS.
systemd-journal /var/log/journal/* Can be used to provide read-only access to the systemd logs, as an alternative to adm and wheel [1]. Otherwise, only user generated messages are displayed.
users Standard users group.
uucp /dev/ttyS[0-9]+, /dev/tts/[0-9]+, /dev/ttyUSB[0-9]+, /dev/ttyACM[0-9]+, /dev/rfcomm[0-9]+ RS-232 serial ports and devices connected to them.
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, an assignment to users is only required for dedicated purposos:

Group Affected files Purpose
dbus used internally by dbus
kmem /dev/port, /dev/mem, /dev/kmem
locate /usr/bin/locate, /var/lib/locate, /var/lib/mlocate, /var/lib/slocate See Core utilities#locate.
lp /dev/lp[0-9]*, /dev/parport[0-9]*, /etc/cups, /var/log/cups, /var/cache/cups, /var/spool/cups Access to parallel port devices (printers and others) and read-only access to CUPS files. If you run a non-printer parallel port device, see FS#50009 for implied problems.
mail /usr/bin/mail
nobody Unprivileged group.
proc /proc/pid/ A group authorized to learn processes information otherwise prohibited by hidepid= mount option of the proc filesystem. The group must be explicitly set with the gid= mount option.
root /* Complete system administration and control (root, admin).
smmsp sendmail group.
tty /dev/tty, /dev/vcc, /dev/vc, /dev/ptmx
utmp /run/utmp, /var/log/btmp, /var/log/wtmp

Pre-systemd groups

Before arch migrated to systemd, users had to be manually added to these groups in order to be able to access the corresponding devices. This way has been deprecated in favour of udev marking the devices with a uaccess tag and logind assigning the permissions to users dynamically via ACLs according to which session is currently active. Note that the session must not be broken for this to work (see General troubleshooting#Session permissions to check it).

There are some notable exceptions which require adding a user to some of these groups: for example if you want to allow users to access the device even when they are not logged in. However, note that adding users to the groups can even cause some functionality to break (for example, the audio group will break fast user switching and allows applications to block software mixing).

Group Affected files Purpose
audio /dev/audio, /dev/snd/*, /dev/rtc0 Direct access to sound hardware, for all sessions. It is still required to make ALSA and OSS work in remote sessions, see ALSA#User privileges.
disk /dev/sd[a-z][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.
input /dev/input/event[0-9]*, /dev/input/mouse[0-9]* Access to input devices. Introduced in systemd 215 [2].
kvm /dev/kvm Access to virtual machines using KVM.
optical /dev/sr[0-9], /dev/sg[0-9] Access to optical devices such as CD and DVD drives.
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.
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).

Unused groups

The following groups are currently not used for any purpose:

Group Affected files Purpose
bin none Historical
daemon
lock
mem
network Unused by default. Can be used e.g. for granting access to NetworkManager (see NetworkManager#Set up PolicyKit permissions).
power
uuidd