Users and Groups¶
In this chapter we’ll learn about user and group management on your system, and also about basic access control.
In Linux everything is associated to an user and a group. Based on these values, the system figures out, who can access what part of the system. That includes files, directories, network ports etc.
Finding the owner of file¶
We use the ls -l command to find the owner, and group of a file or directory.
In the above example, fedora is the name of the owner and group both. The first value talks about who can access this file (we will learn about this in a while.)
/etc/passwd file¶
/etc/passwd contains all the users available in the system. This is a plain text file (this means you can view the information by using cat command.)
$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
systemd-timesync:x:999:998:systemd Time Synchronization:/:/sbin/nologin
systemd-network:x:192:192:systemd Network Management:/:/sbin/nologin
systemd-resolve:x:193:193:systemd Resolver:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
chrony:x:998:995::/var/lib/chrony:/sbin/nologin
systemd-coredump:x:994:994:systemd Core Dumper:/:/sbin/nologin
fedora:x:1000:1000:Fedora:/home/fedora:/bin/bash
polkitd:x:993:993:User for polkitd:/:/sbin/nologin
tss:x:59:59:Account used by the trousers package to sandbox the tcsd daemon:/dev/null:/sbin/nologin
Each line has seven entries separated by :.
username:password:uid:gid:gecos:/home/dirname:shell
| FIELD | MEANING | |
|---|---|---|
| username | the username | |
| password | the password of the user | |
| uid | Numeric user id | |
| gid | Numeric group id of user | |
| gecos | arbitary field | |
| /home/dirname | Home directory of the user | |
| shell | Which shell to use for the user | ||
You’ll see accounts with /sbin/nologin as their shell. These are generally accounts for various services, which are not supposed to be used by a normal human user; (which is why, no shell is needed.)
The actual user passwords are stored in an encrypted form in /etc/shadow file, with only the root user having access to this file.
$ ls -l /etc/shadow
----------. 1 root root 2213 Jun 22 15:20 /etc/shadow
If you want to know more about the current user, use the id command.
$ id
uid=1000(vagrant) gid=1000(vagrant) groups=1000(vagrant) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
Details about groups¶
Group details are stored inside the /etc/group file. Each user has one primary group, and zero or more supplementary groups.
wheel group¶
If your user is part of the wheel group, then it has sudo access. If you remember the Fedora Installer, it actually gives you the option to mark a new user to be part of the wheel group during installation.
Becoming superuser¶
Have you noticed the silent command sudo in front of many commands in the lab before? We use that sudo command to become root user temporarily. The root user is also known as the superuser of the system, it has all the access power to change anything on the system. It is the administrator account of any Linux system.
Try the following command.
$ sudo id
Now, you will find the id* command worked as root instead of your regular user.
If you want to become root user for more than one command, then use the following command, and provide the root password to the input.
$ su -
Important
To be able to use sudo command, you must have your user mentioned in the /etc/sudoers file. The best way to edit the file is to use visudo command as root user.
Important
Read the man pages of su and sudo command.
Adding a new user¶
The useradd command adds a new user to the system. As you can well guess, this command has to execute as root, otherwise anyone can add random user accounts in the system. The following command adds a new user babai to the system.
$ sudo useradd babai
In Fedora, the initial user you create gets the uid 1000.
Changing user passwords¶
The passwd command helps to change any user password.
$ sudo passwd babai
Changing password for user babai.
New password:
Retype new password:
passwd: all authentication tokens updated successfully.
Modifying existing user details¶
The usermod command can help to modify an existing user. You can use the same command to lock user account in the system.
$ sudo usermod -L babai
$ su - babai
Password:
su: Authentication failure
$ sudo usermod -U babai
The last command in the above example unlocks the user account.
Deleting a user¶
We use the userdel command to delete a user from the system.
Adding a new group¶
The groupadd command adds a new group. You can also pass the group id as an option. In the following example we are adding a new group called firejumpers.
$ sudo groupadd -g 4001 firejumpers
Adding new group to an user¶
We can use usermod command to add any extra group to any of our system user. In the following example, we are adding firejumpers group to our vagrant user.
$ sudo usermod -aG firejumpers vagrant
Important
It is important to use -a flag to the usermod command. Without the -a flag usermod command will delete all the existing groups of the user. With usermod -a we append the user to the supplemental groups. And -G flag specifies the new list of supplementary GROUPS. Therefore with usermond -aG we append the new list of supplementary groups to the user’s existing group/groups.
