What are the differences between "su", "sudo -s", "sudo -i", "sudo su"?

The main difference between these commands is in the way they restrict access to their functions.

su (which means "substitute user" or "switch user") - does exactly that, it starts another shell instance with privileges of the target user. To ensure you have the rights to do that, it asks you for the password of the target user. So, to become root, you need to know root password. If there are several users on your machine who need to run commands as root, they all need to know root password - note that it'll be the same password. If you need to revoke admin permissions from one of the users, you need to change root password and tell it only to those people who need to keep access - messy.

sudo (hmm... what's the mnemonic? Super-User-DO?) is completely different. It uses a config file (/etc/sudoers) which lists which users have rights to specific actions (run commands as root, etc.) When invoked, it asks for the password of the user who started it - to ensure the person at the terminal is really the same "joe" who's listed in /etc/sudoers. To revoke admin privileges from a person, you just need to edit the config file (or remove the user from a group which is listed in that config). This results in much cleaner management of privileges.

As a result of this, in many Debian-based systems root user has no password set - i.e. it's not possible to login as root directly.

Also, /etc/sudoers allows to specify some additional options - i.e. user X is only able to run program Y etc.

The often-used sudo su combination works as follows: first sudo asks you for your password, and, if you're allowed to do so, invokes the next command (su) as a super-user. Because su is invoked by root, it does not require you to enter the target user's password. So, sudo su allows you to open a shell as another user (including root), if you're allowed super-user access by the /etc/sudoers file.

sudo lets you run commands in your own user account with root privileges. su lets you switch user so that you're actually logged in as root.

sudo -s runs a shell with root privileges. sudo -i also acquires the root user's environment.

To see the difference between su and sudo -s, do cd ~ and then pwd after each of them. In the first case, you'll be in root's home directory, because you're root. In the second case, you'll be in your own home directory, because you're yourself with root privileges.

There's more discussion of this exact question here.

This answer is a dupe of my answer on a dupe of this question, put here on the canonical answer so that people can find it!

The major difference between sudo -i and sudo -s is:

• sudo -i gives you the root environment, i.e. your ~/.bashrc is ignored.
• sudo -s gives you the user's environment, so your ~/.bashrc is respected.

Here is an example, you can see that I have an application lsl in my ~/.bin/ directory which is accessible via sudo -s but not accessible with sudo -i. Note also that the Bash prompt changes as will with sudo -i but not with sudo -s:

dotancohen@melancholy:~$ ls .bin
lsl

dotancohen@melancholy:~$ which lsl
/home/dotancohen/.bin/lsl

dotancohen@melancholy:~$ sudo -i

root@melancholy:~# which lsl

root@melancholy:~# exit
logout

dotancohen@melancholy:~$ sudo -s
Sourced .bashrc

dotancohen@melancholy:~$ which lsl
/home/dotancohen/.bin/lsl

dotancohen@melancholy:~$ exit
exit

Though sudo -s is convenient for giving you the environment that you are familiar with, I recommend the use of sudo -i for two reasons:

1. The visual reminder that you are in a 'root' session.
2. The root environment is far less likely to be poisoned with malware, such as a rogue line in .bashrc.

• su Asks root password, becomes root, opens an interactive non-login shell.
• su - Asks root password, becomes root, opens an interactive login shell.

• sudo -s Asks your passwords, becomes root, opens a interactive non login shell.
• sudo -i Asks your passwords, becomes root, opens a interactive login shell.

Best practice is to use the above two commands.

• sudo su Asks your password, becomes root momentarily to run su as root.
• sudo su - Asks your password, becomes root momentarily to run su - as root.

So in this case you are running su using sudo and you don't have to know root's actual password. The results are same as su and su -.

su asks for the password of the user "root".

sudo asks for your own password (and also checks if you're allowed to run commands as root, which is configured through /etc/sudoers -- by default all user accounts that belong to the "admin" or "sudo" groups are allowed to use sudo).

sudo -s launches a shell as root, but doesn't change your working directory. sudo -i simulates a login into the root account: your working directory will be /root, and root's .profile etc. will be sourced as if on login.

In Ubuntu or a related system, I don't find much use for su in the traditional, super-user sense. sudo handles that case much better. However, su is great for becoming another user in one-off situations where configuring sudoers would be silly.

For example, if I'm repairing my system from a live CD/USB, I'll often mount my hard drive and other necessary stuff and chroot into the system. In such a case, my first command is generally:

su - myuser  # Note the '-'. It means to act as if that user had just logged in.

That way, I'm operating not as root, but as my normal user, and I then use sudo as appropriate.