Linux: Users and Groups

This page is a basic tutorial on managing Linux users and groups.

On a linux machine, there are:

1. A set of users.
2. A set a groups.
3. Each user has a name and a user ID called uid. (positive integer)
4. Each group has a name and a group ID called gid. (positive integer)
5. Each user belongs to at least one group.
6. One of the group the user belongs to is called his primary group.
7. The default admin user is named “root”, and its uid is 0.

List user/group, Find uid/gid

Show a user's uid

Show the id number of a user, and all groups he belongs to.

# show a user's uid
id myLoginName

List all users

cat /ect/passwd

List all users

# list all users on a machine
cat /etc/passwd | awk -F\: '{print $1}'

Here's a sample passwd file:

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
…

Each line is a colon separated field. The values mean these:

1. login name
2. optional encrypted password
3. numerical user ID
4. numerical group ID
5. user name or comment field
6. user home directory
7. optional user command interpreter (shell path)

For detail, type man 5 passwd.

Show a group's gid

It's stored in the file /etc/group.

# list all group names
cat /etc/group

Here's a sample group file content:

root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:joe
tty:x:5:
disk:x:6:
lp:x:7:
mail:x:8:
news:x:9:
uucp:x:10:
man:x:12:
proxy:x:13:
kmem:x:15:

…

Each line is a colon separated field. The values mean these:

1. group name
2. password (not really used in practice)
3. group id (gid)
4. list of users that belongs to this group, separated by comma.

See man 5 group.

List all groups

getent group

List all groups. See getent --help

cat /etc/group

List all groups.

Find the primary group of a user

id userName

Sample output:

uid=1000(joe) gid=1000(joe) groups=1000(joe),4(adm),20(dialout),24(cdrom),46(plugdev),116(lpadmin),118(admin),124(sambashare)

The “gid=1000(joe)” is the primary group the user belongs to. In this example, my user name is “joe”, uid is 1000, and there's a group also named “joe” with gid 1000, and it's primary group joe belongs to.

The “groups=…” are all the groups the user belongs to.

Creating/Modifying {user, group}

Create a new user

useradd userName

Create a new user account. (On Debian based Linuxes, there's higher-level adduser command, written in Perl .)

When you create a user, several other things must happen too. For example, creating a home dir for the user, specify the user's login shell path, specify the user's primary group.

Use useradd -D to see the defaults.

joe@joe-VirtualBox ◆  2012-10-13 ◆ 04:00  ◆ ~
useradd -D

GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/sh
SKEL=/etc/skel
CREATE_MAIL_SPOOL=no

Create a new group

groupadd newName

create a new group.
(on some linux distro, there is also addgroup)

Change the primary group for a user

sudo usermod -g new_group_name userName

The group for all files of user's home dir will be changed too. But for other files, you'll need to change yourself.

Add a user to a group

sudo usermod -a -G new_group_name userName

The user needs to re-login for his new group to have effect.

Remove a user from a group

First, find out all the groups the user belongs, by id userName, then use sudo usermod -G comma_separated_group_names userName. The “-G” option take a list of all the groups the user should belong to.

Create multiple users in batch

newusers users_data_file. The users_data_file is a text file containing user info. Each line should have the same format as the /etc/passwd file. See: man newusers.

Change User's Password

passwd userName

Change a user's password.

Run a command as root

sudo commandString

Run a command as “root” (“root” is the name of default admin account.)

su

Switch to “root”

sudo su root

Switch to “root”. (useful when root isn't setup as a login account. For example, default Ubuntu)

Show who is logged in

w

Show who is logged in.

who -a

List all users that have logged in recently.