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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?

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  • 1
    a "login shell" often means interactive shell. This is vs. a shell that just executes a script and exits.
    – Shahbaz
    Jul 26, 2012 at 16:41
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    It's right there in the same manpage you link to: "-, -l, --login: make the shell a login shell"
    – ArjunShankar
    Jul 26, 2012 at 16:43
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    you don't even have to google it, just type su --help.
    – Art Shayderov
    Jul 26, 2012 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, 2012 at 16:55

2 Answers 2

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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).

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  • this site -> 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, 2012 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, 2012 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, 2012 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 4, 2015 at 3:56
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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.

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  • 1
    What are the kinds of problems one could experience by using su - instead of su --login? Unfortunately neither the wiki nor man page provides examples.
    – xorinzor
    Sep 7, 2023 at 9:57

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