Linux: Users and Groups

How to show a user's uid?

# show a user's uid
id myLoginName

How to list all users?

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

# 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.

How to 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.

How to list all groups?

cat /etc/group

How to find the primary group of a user?

id user_name

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.

How to create a new user?

useradd new_name. The “useradd” is a low-level command. On Debian based Linuxes, there's a higher-level command adduser you can use.

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

How to create a new group?

groupadd new_name, or addgroup.

See their man page for detail.

How to change the primary group for a user?

sudo usermod -g new_group_name user_name

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

How to add a user to a group?

sudo usermod -a -G new_group_name user_name

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

How to remove a user from a group?

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

How 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.