In Ubuntu Server, user and group management is an important aspect of system administration. Users are individuals who are able to log in to the server and perform various tasks. Groups are collections of users that can be managed as a single entity. Ubuntu Server uses the concept of users and groups to control access to resources on the system. By default, Ubuntu Server comes with a number of predefined users and groups, but you can also create your own custom users and groups as needed.

• Creating and Modifying Users on Ubuntu Server
• Managing User Passwords and SSH Access on Ubuntu Server
• Creating and Modifying Groups on Ubuntu Server
• Assigning Users to Groups on Ubuntu Server
• Understanding File and Directory Permissions in Ubuntu Server
• Setting File and Directory Permissions on Ubuntu Server
• Managing Special Permissions: SUID, SGID, and Sticky Bits
• Understanding and Setting Default Permissions for New Files and Directories
• Troubleshooting User and Group Permission Issues on Ubuntu Server

In this tutorial, we will cover the basics of user and group management in Ubuntu Server. We will cover topics such as creating and modifying users, managing user passwords and SSH access, creating and modifying groups, assigning users to groups, and troubleshooting user and group permission issues. By the end of this tutorial, you should have a good understanding of how to manage users, groups, and permissions on Ubuntu Server.

Creating and Modifying Users on Ubuntu Server

To create a new user on Ubuntu Server, you can use the adduser command. This command will prompt you for various information about the user, such as their name, login name, and password.

For example, to create a new user named “john” with the login name “john”, you can run the following command:

sudo adduser john

You will then be prompted to enter a password for the user and provide additional information such as their full name and contact information.

To modify an existing user, you can use the usermod command. This command allows you to change various attributes of a user, such as their name, login name, and password.

For example, to change the full name of the user “john” to “John Doe”, you can run the following command:

sudo usermod -c "John Doe" john

To view a list of all users on the system, you can use the cut and awk commands to extract the username and real name columns from the /etc/passwd file:

cut -d: -f1,5 /etc/passwd | awk -F: '{print $1 " - " $2}'

This will output a list of all users on the system, along with their real names. You can use this information to manage users on your Ubuntu Server.

Managing User Passwords and SSH Access on Ubuntu Server

On Ubuntu Server, you can manage user passwords and SSH access using the passwd and ssh commands, respectively.

To change a user’s password, use the passwd command followed by the username. For example, to change the password for the user “john”, you can run the following command:

sudo passwd john

You will be prompted to enter the new password for the user.

To allow or restrict SSH access for a user, you can edit the /etc/ssh/sshd_config file and modify the AllowUsers and DenyUsers options. For example, to allow the user “john” to log in via SSH, you can add the following line to the AllowUsers option:

AllowUsers john

To restrict the user “john” from logging in via SSH, you can add the following line to the DenyUsers option:

DenyUsers john

Remember to restart the sshd service after making any changes to the /etc/ssh/sshd_config file:

sudo systemctl restart ssh

By managing user passwords and SSH access, you can ensure that only authorized users are able to log in to your Ubuntu Server.

Creating and Modifying Groups on Ubuntu Server

On Ubuntu Server, you can create and modify groups using the groupadd, groupmod, and groupdel commands.

To create a new group, use the groupadd command followed by the group name. For example, to create a new group named “developers”, you can run the following command:

sudo groupadd developers

To modify an existing group, use the groupmod command followed by the group name and the desired attribute. For example, to change the name of the group “developers” to “project_team”, you can run the following command:

sudo groupmod -n project_team developers

To delete a group, use the groupdel command followed by the group name. For example, to delete the group “project_team”, you can run the following command:

sudo groupdel project_team

By creating and modifying groups, you can easily manage and organize users on your Ubuntu Server. Groups can be used to control access to resources, such as files and directories, on the system.

Assigning Users to Groups on Ubuntu Server

To add a user to a group, use the usermod command followed by the -aG option and the group name. For example, to add the user “john” to the group “developers”, you can run the following command:

sudo usermod -aG developers john

To remove a user from a group, use the usermod command followed by the -G option and the group name. For example, to remove the user “john” from the group “developers”, you can run the following command:

sudo usermod -G "" john

You can also use the id command to view the groups that a user is a member of. For example, to view the groups that the user “john” is a member of, you can run the following command:

id -Gn john

By assigning users to groups, you can control access to resources on your Ubuntu Server. For example, you can grant or restrict access to certain files and directories based on group membership.

Keep in mind that group membership is only effective when the group is used to set permissions on resources. Simply adding a user to a group does not automatically grant them access to any resources. You must also set the appropriate permissions on the resources themselves.

Understanding File and Directory Permissions in Ubuntu Server

File and directory permissions control who is able to access and modify resources on the system. Each file and directory has an associated owner and group, and permissions can be set for the owner, the group, and other users (also known as “others”).

Permissions are represented by a series of digits and letters. The first digit represents the permissions for the owner of the resource, the second digit represents the permissions for the group, and the third digit represents the permissions for others.

There are three types of permissions that can be set for each category: read (r), write (w), and execute (x). These permissions are represented by the digits 4, 2, and 1, respectively.

For example, the permission string “rwxrw-r–” can be interpreted as follows:

• The owner has read (4), write (2), and execute (1) permissions, or a total of 7 (4+2+1).
• The group has read (4) and write (2) permissions, or a total of 6 (4+2).
• Others have read (4) permission, or a total of 4.

To view the permissions of a file or directory, you can use the ls -l command. For example, to view the permissions of the file “example.txt”, you can run the following command:

ls -l example.txt

This will output information about the file, including its permissions, owner, and group.

By understanding file and directory permissions, you can control access to resources on your Ubuntu Server and ensure that only authorized users are able to access and modify them.

Setting File and Directory Permissions on Ubuntu Server

The chmod command allows you to set permissions using either octal notation or symbolic notation. Octal notation represents permissions using a three-digit number, with each digit corresponding to the permissions for the owner, group, and others, respectively. Symbolic notation uses letters and symbols to represent the permissions for the owner, group, and others.

Here are some examples of using chmod in octal notation:

• To give the owner read and write permissions, the group read permission, and no permissions to others for the file “example.txt”, you can run the following command:

chmod 640 example.txt

• To give the owner and group read and execute permissions, and no permissions to others for the directory “example_dir”, you can run the following command:

chmod 750 example_dir

Here are some examples of using chmod in symbolic notation:

• To give the owner read and write permissions, the group read permission, and no permissions to others for the file “example.txt”, you can run the following command:

chmod u=rw,g=r,o= example.txt

• To give the owner and group read and execute permissions, and no permissions to others for the directory “example_dir”, you can run the following command:

chmod ug=rx,o= example_dir

By using chmod, you can set the appropriate permissions for files and directories on your Ubuntu Server to control access to resources.

You must have the appropriate permissions to modify the permissions of a file or directory. For example, to modify the permissions of a file owned by another user, you must have root privileges.

Managing Special Permissions: SUID, SGID, and Sticky Bits

you can use special permissions known as SUID, SGID, and sticky bits to control access to resources on the system.

SUID (Set User ID) is a special permission that can be set on a file. When SUID is set on a file, it allows any user who executes the file to have the permissions of the owner of the file, rather than their own permissions. This can be useful for allowing users to execute a file that requires higher privileges than they normally have.

SGID (Set Group ID) is a special permission that can be set on a file or directory. When SGID is set on a file, it allows any user who executes the file to have the permissions of the group owner of the file, rather than their own group. When SGID is set on a directory, it allows any file created within the directory to have the group owner of the directory, rather than the group owner of the user creating the file.

Sticky bit is a special permission that can be set on a directory. When sticky bit is set on a directory, it allows only the owner of a file to delete or rename the file, regardless of the permissions set on the file. This can be useful for preventing users from deleting or renaming files in a shared directory.

To set SUID, SGID, or sticky bit on a file or directory, you can use the chmod command with the appropriate symbolic notation.

For example, to set SUID on the file “example.sh”, you can run the following command:

chmod u+s example.sh

To set SGID on the directory “example_dir”, you can run the following command:

chmod g+s example_dir

To set sticky bit on the directory “example_dir”, you can run the following command:

chmod +t example_dir

By using SUID, SGID, and sticky bit, you can fine-tune the permissions of files and directories on your Ubuntu Server to control access to resources.

Understanding and Setting Default Permissions for New Files and Directories

The umask command specifies the default permissions that will be applied to newly created files and directories. It is a four-digit octal number, with the first three digits representing the default permissions for the owner, group, and others, respectively. The fourth digit represents the default permissions for special permissions such as SUID, SGID, and sticky bit.

For example, to set the default permissions for new files to 644 (rw-r–r–) and for new directories to 755 (rwxr-xr-x), you can run the following command:

umask 022

This will set the default permissions for new files to 644 and for new directories to 755.

To set the default permissions for new files to 600 (rw——-) and for new directories to 700 (rwx——), you can run the following command:

umask 077

This will set the default permissions for new files to 600 and for new directories to 700.

By setting default permissions using umask, you can ensure that newly created files and directories have the appropriate permissions on your Ubuntu Server.

The umask command only sets default permissions and does not modify the permissions of existing files and directories. To modify the permissions of existing files and directories, you must use the chmod command.

Troubleshooting User and Group Permission Issues on Ubuntu Server

If you encounter issues with user and group permissions on Ubuntu Server, there are a few steps you can take to troubleshoot and resolve the problem.

1. Verify that the user has the correct permissions: Make sure that the user has the appropriate permissions to access the resource in question. You can use the ls -l command to view the permissions of the resource and determine if the user has the necessary permissions.
2. Check for special permissions: Check if the resource has any special permissions such as SUID, SGID, or sticky bit set. These special permissions can override the normal permissions of a resource and cause issues. You can use the ls -l command to view the permissions of the resource and determine if any special permissions are set.
3. Check for incorrect ownership: Make sure that the resource has the correct owner and group. You can use the ls -l command to view the owner and group of the resource. If the resource has the wrong owner or group, you can use the chown command to change the ownership.
4. Check for incorrect permissions on parent directories: If you are having issues accessing a resource within a directory, make sure that you have the necessary permissions for the parent directories as well. In some cases, the permissions of the parent directories can prevent access to the resource.
5. Check for incorrect SELinux contexts: If you are using SELinux (Security-Enhanced Linux) on your Ubuntu Server, make sure that the SELinux context of the resource is correct. Incorrect SELinux contexts can cause permissions issues. You can use the ls -Z command to view the SELinux context of the resource.

By following these steps, you should be able to troubleshoot and resolve user and group permission issues on your Ubuntu Server.