Users and groups: Difference between revisions
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== Overview == | == Overview == | ||
:''"root" redirects here. For the root directory, see [[Partitioning#/]].'' | |||
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. | 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. |
Revision as of 09:08, 30 July 2018
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
- "root" redirects here. For the root directory, see Partitioning#/.
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.
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 chown(1), chmod(1), and Linux file permissions for additional detail.
File list
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
/--create-home
- 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
/--gid
- 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, withGID
equal toUID
. -G
/--groups
- 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
/--shell
- 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.
/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
.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
To add a new user named archie
, creating its home directory and otherwise using all the defaults in terms of groups, folder names, shell used and various other parameters:
# useradd --create-home archie
useradd --default
. The default is Bash, a different shell can be specified with the -s
/--shell
option.Although it is not required to protect the newly created user archie
with a password, it is highly recommended to do so:
# passwd archie
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 using the -g
option, but 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. If a user must be a member of a specific group specify that group as a supplementary group when creating the user.
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
Change a user's login name or home directory
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.
# ln -s /my/new/home/ /my/old/home
Make sure there is no trailing /
on /my/old/home
.
To change a user's login name:
# usermod -l newname oldname
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 openingcrontab -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} *
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
-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.
To change the user's login shell:
# usermod -s /bin/bash username
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: Thepasswd
file is world-readable, so storing passwords (hashed or otherwise) in this file is insecure. Instead, Arch Linux uses shadowed passwords: thepassword
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
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
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.
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 purposes:
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]* |
Access to parallel port devices (printers and others). |
/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. Also used in JACK to give users realtime processing permissions. |
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 | Used for lockfile access. Required by e.g. gnokii. | |
mem | ||
network | Unused by default. Can be used e.g. for granting access to NetworkManager (see NetworkManager#Set up PolicyKit permissions). | |
power | ||
uuidd |