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I'm quite new to Linux terminal and I'm not quite sure what the difference between su with a hyphen and su without a hyphen is, for example: su - username vs. su username.

I looked into the documentation but in there, this was not mentioned. Could someone please help me out?

Community
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user574183
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    a "login shell" often means interactive shell. This is vs. a shell that just executes a script and exits. – Shahbaz Jul 26 '12 at 16:41
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    It's right there in the [same manpage you link to](http://unixhelp.ed.ac.uk/CGI/man-cgi?su): "*-, -l, --login: make the shell a login shell*" – ArjunShankar Jul 26 '12 at 16:43
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    you don't even have to google it, just type `su --help`. – Art Shayderov Jul 26 '12 at 16:51
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    Of noteworthyness: This is particularly useful when su-ing to root as without using the hypen to start a new login shell, your `$PATH` won't get updated and thus you won't be able to directly call root-only binaries in `/sbin` and `/usr/sbin` – Garrett Jul 26 '12 at 16:55

2 Answers2

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The difference between "-" and "no hyphen" is that the latter keeps your existing environment (variables, etc); the former creates a new environment (with the settings of the actual user, not your own).

https://wiki.archlinux.org/index.php/Su

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).

paulsm4
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  • this site -> http://www.admon.org/difference-between-login-shell-and-non-login-shell/ says that when used without the hyphen the files /etc/profile and ~/.bash_profile are not executed. What significance do these two files have on the user environment ? – user574183 Jul 26 '12 at 17:02
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    Those two things are the files that *set* the user environment :) *Not* executing them means *keeping* your original environment. Executing them (by using "-") means making *your* environment the same as the user you're "su'ing" to. Using the hyphen is usually what you *want* - I use "-" habitually whenever I "su". – paulsm4 Jul 26 '12 at 18:40
  • does the order of execution really matter ? Iam asking this because on that site they have mentioned that the two files /etc/bashrc ~/.bashrc are executed in reverse order when in a non login shell. Somebody please help me – user574183 Jul 28 '12 at 18:03
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    @user574183 of course, e.g. if one set something but the other unset something or set to something else – fikr4n Jul 04 '15 at 03:56
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Year 2022 update:

It is discouraged shorthand for --login.

See: https://man.archlinux.org/man/su.1#DESCRIPTION

It is recommended to always use the --login option (instead of its shortcut -) to avoid side effects caused by mixing environments.

And https://wiki.archlinux.org/title/Su :

You will sometimes encounter su being used to get a login shell with su -, rather than su -l/su --login. This shorthand is discouraged because the - option can encounter parsing limitations.

user124
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