Working with Users and Groups

• Based on a user's ID and the groups it is a part of, Linux manages access to resources.

Commands

• Print information about users who are currently logged in.

users, who, w, whoami

users
# Output:
	user

who
# Output:
	user     :0           2022-09-02 19:26 (:0)

who -a
	system boot  2022-11-03 21:44
	run-level 5  2022-11-03 21:44
user     + tty2         2022-11-03 21:45  old         1574 (tty2)
	pts/1        2022-11-05 10:44              6168 id=ts/1  term=0 exit=130
	pts/2        2022-11-05 10:30              6064 id=ts/2  term=0 exit=0
	pts/3        2022-11-05 10:30              6065 id=ts/3  term=0 exit=1

w
# Output info and processes about the logged on users:
	19:56:27 up  9:19,  1 user,  load average: 0,03, 0,01, 0,00
	USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
	user     :0       :0               02set22 ?xdm?   3:15   0.00s /usr/lib/gdm3/gdm-x-session --run-sc

whoami
# Output:
	user

Users

• An account is based on a user, a password, a home directory and other information, that can be seen in some of the Linux O.S. configuration text files.

/etc/passwd

cat /etc/passwd
    root:x:0:0:root:/root:/bin/bash
    daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
    bin:x:2:2:bin:/bin:/usr/sbin/nologin
    [...]
    gdm:x:125:130:Gnome Display Manager:/var/lib/gdm3:/bin/false
    user:x:1000:1000:user,,,:/home/user:/bin/bash

user:x:1000:1000:user,,,:/home/user:/bin/bash

• 1st column - user name

• 2nd column used to be the hash of the user's password, replaced by an x

  • * in the second column disable logins to the account

  • Password hashes were moved to the /etc/shadow file

• 3rd column - user's UID (User's Unique Identifier)

• 4th column - user's primary group ID - GID

  • both UIDs are used for file permissions and process managing

• 5th column - additional text information, each info separated by a comma , - GECOS

• 6th column - user's home directory

• 7th (last) column - user's default login shell

  • /usr/sbin/nologin and /bin/false are used to prevent interactive shell logon with that user

• Normal user accounts start with UID 1000

📌 /etc/passwd explained.

📌 /etc/shadow explained.

ls -la /home
# List users home directories, owned by specific users and groups

Groups

/etc/group

# List all groups with:
cat /etc/group
# or
getent group
# includes LDAP users if present
	root:x:0:
    daemon:x:1:
    bin:x:2:
    sys:x:3:
    adm:x:4:syslog,user
    tty:x:5:sysl
    [...]
    sudo:x:27:user
    [...]
    gdm:x:130:
    lxd:x:131:user
    user:x:1000:
    sambashare:x:132:user
    systemd-coredump:x:999:
    mlocate:x:133:

groups
# List current user's groups
	user adm cdrom sudo dip plugdev lpadmin lxd sambashare
	
id user
# List UID, GID, secondary groups
	uid=1000(user) gid=1000(user) groups=1000(user),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),120(lpadmin),131(lxd),132(sambashare)

user:x:1000:

• 1st column - group name. Primary group of the user is the unique group associated only with that user.

• 2nd column - group password, not used, replaced by an x

• 3rd column - group's unique identifier - GID

• 4th last column - comma separated list of group members

• adm (administrator level privileges) and sudo (run commands as any user/group) groups are very important groups with high level privileges.

📌 List groups explained.

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