Tobias Klein
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Learning Linux - Chapter 5: Section 6

How to switch users and sudo access

Learning Linux - Section 5.6

The sudo command is explored in this section along with how to switch users, using the command line. Only basic commands are used, that are available in any Linux distribution and Unix based system.

How to switch users and sudo access

Commands covered

  • su - username
  • sudo <command>
  • visudo

Relevant file

  • /etc/sudoers

Routine commands to run after login into a VM

Since a virtual machine is used for all the sections and exercises, one should make sure that the following values are what they should be.

whoami: Which user am I?
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~ $ whoami
tklein
~ $
pwd: What is my current absolute path?
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~ $ pwd
/home/tklein
~ $
hostname: On which host am I currently?
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~ $ hostname
localhost.localdomain
~ $

su and sudo

Command: su - username

One enters the ‘username’ of the account one wants to become. If left blank, it defaults to ‘username’ == root

Becoming user root and staying root until one decides to exit the root account, is done like demonstrated below. One must have sufficient permissions to become root and know the password for the root user account.

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~ $ su -
Password: 
Last login: Sun Dec 26 23:05:13 CET 2021 on pts/0
root *

While root, one can change into any other username, without being prompted to enter that username’s password.

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~ $ su -
Password: 
Last login: Mon Dec 27 09:56:48 CET 2021 on pts/0
root * su - kate
[kate@localhost ~]$

Exiting the root account again is done by entering exit after one’s prompt.

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root * exit
logout
~ $ whoami
tklein
~ $

Command: sudo <command>

The sudo command lets a user, who can not become root himself. It allows such a user to run commands, that only user root can run usually, if they are in group wheel for example or in the sudoers file.

Two commands that can only be run by root are dmidecode and fdisk -l, to run these one has to

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~ $ dmidecode
# dmidecode 3.2
/sys/firmware/dmi/tables/smbios_entry_point: Permission denied
Scanning /dev/mem for entry point.
/dev/mem: Permission denied
~ $ fdisk -l
fdisk: cannot open /dev/sda: Permission denied
fdisk: cannot open /dev/mapper/centos-root: Permission denied
fdisk: cannot open /dev/mapper/centos-swap: Permission denied
~ $

One can add or change the permissions that a user has, to let them become root for example or to let them run any command in the shell, if they are a member of group wheel by default. This group is by default listed in the file visudo.

Example

The example comes from a file that can be accessed by running visudo with root privileges.

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## Allow root to run any commands anywhere
root    ALL=(ALL)       ALL
will    ALL=(ALL)       ALL # Only an example
## Allows people in group wheel to run all commands
%wheel  ALL=(ALL)       ALL
## Same thing without a password
# %wheel        ALL=(ALL)       NOPASSWD: ALL
usermod -aG

Adding account ‘tklein’ to group wheel

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root * usermod -aG wheel tklein
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Verification
  • Using id <username> one gets all the groups the user is a member of.
  • grep "wheel" /etc/group will print a line from the /etc/group file, to verify the changes.
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root * id tklein
uid=1000(tklein) gid=1000(tklein) groups=1000(tklein),10(wheel)
root * grep "wheel" /etc/group
wheel:x:10:tklein
root *
Retrying dmidecode and fdisk commands

After it was not able to run either dmidecode or fdisk -l using my user account tklein, now it is possible to run dmidecode being part of group wheel:

dmidecode
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[will@localhost ~]$ su - tklein
Password:
Last login: Mon Dec 27 15:20:49 CET 2021 on pts/1
~ $ whoami
tklein
~ $ sudo dmidecode
[sudo] password for tklein:
# dmidecode 3.2
Getting SMBIOS data from sysfs.
SMBIOS 2.5 present.
10 structures occupying 450 bytes.
Table at 0x3EFFD020.

Handle 0x0000, DMI type 0, 20 bytes
BIOS Information
	Vendor: innotek GmbH
	Version: VirtualBox
	Release Date: 12/01/2006
	Address: 0xE0000
	Runtime Size: 128 kB
	ROM Size: 128 kB
	Characteristics:
		ISA is supported
		PCI is supported
		Boot from CD is supported
		Selectable boot is supported
		8042 keyboard services are supported (int 9h)
		CGA/mono video services are supported (int 10h)
		ACPI is supported
		UEFI is supported
Handle 0x0001, DMI type 1, 27 bytes
Handle 0x0001, DMI type 1, 27 bytes
System Information
	Manufacturer: innotek GmbH
	Product Name: VirtualBox
	Version: 1.2
	Serial Number: 0
	UUID: 008f59e3-ffb4-5947-aaba-763e1257b612
	Wake-up Type: Power Switch
	SKU Number: Not Specified
	Family: Virtual Machine
Handle 0x0008, DMI type 2, 15 bytes

...
The fdisk -l command:
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~ $ sudo fdisk -l
WARNING: fdisk GPT support is currently new, and therefore in an experimental phase. Use at your own discretion.

Disk /dev/sda: 10.7 GB, 10737418240 bytes, 20971520 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: gpt
Disk identifier: 8C3022AF-D83D-4714-A904-26629F38B595


#         Start          End    Size  Type            Name
 1         2048       411647    200M  EFI System      EFI System Partition
 2       411648      2508799      1G  Microsoft basic
 3      2508800     20969471    8.8G  Linux LVM

Disk /dev/mapper/centos-root: 8376 MB, 8376025088 bytes, 16359424 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/mapper/centos-swap: 1073 MB, 1073741824 bytes, 2097152 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

~ $

You are welcome to take a look and browse through some of my posts.

I adhere to two principals, in that order.

I follow two principles, in this order. A methodically clean and conscientious approach followed by the clear and aesthetically pleasing communication of information. In addition, I do my best to use the available tools efficiently and flexibly. Be it the commands provided by the CentOS (~Red Hat Linux) distribution for system administration, the workflow in Python for reading raw data from .csv files, custom web scraping algorithms or from a database directly, to a production-ready predictive machine learning model that can be deployed via Docker or AWS and serve the client's purposes.

A balance between low deployment costs through virtualization and the use of scalable, on-demand cloud services that keep costs in check, and a competitive advantage for the customer through the use of the finished product.

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