su, you become another user — root by default, but potentially another user. If you say
su -, your environment gets replaced with that user's login environment as well, so that what you see is indistinguishable from logging in as that user. There is no way the system can tell what you do while
su'd to another user from actions by that user when they log in.
Things are very different with
Commands you run through
sudo execute as the target user — root by default, but changeable with
-u — but it logs the commands you run through it, tagging them with your username so blame can be assigned afterward. :)
sudo is very flexible. You can limit the commands a given user or group of users are allowed to run, for example. With
su, it's all or nothing.
This feature is typically used to define roles. For instance, you could define a "backups" group allowed to run
tar, each of which needs root access to properly back up the system disk.
I mention this here because it means you can give someone
sudo privileges without giving them
sudo -s or
sudo bash abilities. They have only the permissions they need to do their job, whereas with
su they have run of the entire system. You have to be careful with this, though: if you give someone the ability to say
sudo vi, for example, they can shell out of
vi and have effectively the same power as with
Because it takes the sudoer's password instead of the root password,
sudo isolates permission between multiple sudoers.
This solves an administrative problem with
su, which is that when the root password changes, all those who had to know it to use
su had to be told.
sudo allows the sudoers' passwords to change independently. In fact, it is common to password-lock the root user's account, so that all sysadmin tasks have to be done via
sudo. In a large organization with many trusted sudoers, this means when one of the sysadmins leaves, you don't have to change the root password and distribute it to those admins who remain.
The main difference between
sudo bash and
sudo -s is spoofability.
sudo -s looks in trusted locations to determine which shell to execute, whereas
sudo bash causes
sudo to run the first
bash program in the PATH, which may not be the shell you intended it to run. There could be multiple
bash executables on the system, in which case you might be tricked into running the wrong one; if someone knew you had
$HOME/bin in your
PATH ahead of
/bin and could somehow get a
bash program into your
$HOME/bin directory, they could cause that program to do nasty things.
sudo -s effectively can't be tricked that way, since it would require that the attacker gain root access to start with, and at that point dirty tricks are no longer necessary.
sudo -s gives precedence to the
SHELL variable over
/etc/passwd when determining the shell to use, it may still be susceptible to spoofing by another path. If someone can modify a sudoer's environment prior to a
sudo -s, they could get it to run any command they desired.
sudo -s could also cause a security breach less directly through other environment variables like
The solution to that is
sudo -i, which is a relatively recent addition to
sudo. This always gives you a root login shell and resets all but a few key environment variables. Roughly speaking,
sudo -i is to
sudo -s as
su - is to
sudo -i only looks at
/etc/passwd and replaces the
PATH, its security is as good as the security of the root account.
You might still want to use
sudo -s for those situations where you know you want to remain in the same directory you were
cd'd into when you ran
sudo. It's still safer to
sudo -i and
cd back to where you were, though.