I have a bash script (x11docker) that needs to run some commands as root (docker), and some commands as unprivileged user (X servers like Xephyr). The script prompts for password at some point. It should run on arbitrary linux systems without configuring the system first.

Some systems use su, some use sudo to get root privileges. How can I recognize which one will work? I tried sudo -l docker. That should tell me if sudo dockeris allowed. Unfortunately, it needs a password even for this information.

The point is, root may or may not have a password (that is needed to use su -c), and sudo may or may not be allowed to run docker. How to decide which one will do the job (=executing a command with root privileges)?

Checking for group sudo may be a good guess, but is not reliable, as it does not tell me if /etc/sudoers is configured to allow group sudo arbitrary root access. Also, docker can be allowed in /etc/sudoers without the user being member of group sudo.

pkexec should be a solution, but is not reliable. The password prompt fails on console, fails on X servers different from DISPLAY=:0, and fails on OpenSuse at all.

Currently, the script defaults to use su, and a switch --sudo allows to use sudo. Possible, but not nifty.

I am working on an update allowing to run the script as root at all and checking for the "real" unprivileged user with logname, SUDO_USER and PKEXEC_UID. Not nifty, too.

Is there a way to know if I should use su or sudo?

  • su switches to a user with an interactive shell (switch user). sudo just executes a command as that user (switch user & do). If no user is specified root will be used.
    – jesse_b
    Commented Aug 4, 2017 at 12:23
  • 1
    Basically you probably shouldn't use either in a script, but it would probably be best to execute the script as root (or sudo script) and then within the script su - docker before executing the commands needed for it.
    – jesse_b
    Commented Aug 4, 2017 at 12:30
  • The point is, root may or may not have a password (that is needed to use su -c), and sudo may or may not be allowed to run docker. How to decide which one will do the job (=executing a command with root privileges)?
    – mviereck
    Commented Aug 4, 2017 at 14:19
  • @Jesse_b: Not sure what you mean with su - docker, I have no user called docker. Do you mean su -c docker? Starting the script with root privileges and than switching back to unprivileged user is possible, if I can rely on logname, SUDO_USER and PKEXEC_USER, as I said. But that is not a good way, I think.
    – mviereck
    Commented Aug 4, 2017 at 14:26
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    @Jesse_b You can use su to run a single command with the -c option. The big difference between sudo and su is that sudo has a configuration file listing what's allowed, and sudo asks for the user's own password rather than the target user's password, and can be configured to not ask for a password.
    – Barmar
    Commented Aug 5, 2017 at 0:33

2 Answers 2


You can at least test whether root has a password. In that case you probably want to try sudo:

if [ foo = "$(echo | su -c "echo foo")" ]; then
  su -c whatever
  sudo whatever
  • Thanks, but this fails at least on debian 9 and fedora 25, although root has a password. I always get error code 1, and "else ..sudo" comes up.
    – mviereck
    Commented Aug 6, 2017 at 10:53
  • @mviereck The idea is that sudo does get executed if there is a password for root... Commented Aug 6, 2017 at 12:15
  • Your example always defaults to sudo, regardless of root having a password or not. The check if root has a password does not work. (now checked on Ubuntu 16.04, too)
    – mviereck
    Commented Aug 6, 2017 at 13:30
  • @mviereck OK, that's bad. I tested it with a non-root account on openSUSE Tumbleweed. Would it be possible to make sure that certain software (like expect) is installed on all affected systems? Commented Aug 6, 2017 at 13:43
  • As I provide just a bash script, I want to avoid dependencies that are not commonly already installed. Looking at debian 9, expect is not installed by default. Software that is common for all main distributions may be expectable. I try to avoid hard dependencies. The script uses for instance xrandr and xdpyinfo, but does not fail if they are missing.
    – mviereck
    Commented Aug 6, 2017 at 14:42

The su command switches to the super user – or root user – when you execute it with no additional options. You’ll have to enter the root account’s password. This isn’t all the su command does, though – you can use it to switch to any user account. If you execute the su bob command, you’ll be prompted to enter Bob’s password and the shell will switch to Bob’s user account.

Once you’re done running commands in the root shell, you should type exit to leave the root shell and go back to limited-privileges mode.

Sudo runs a single command with root privileges. When you execute sudo command, the system prompts you for your current user account’s password before running command as the root user. By default, Ubuntu remembers the password for fifteen minutes and won’t ask for a password again until the fifteen minutes are up.

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    Thanks, but that does not answer my question, whether a system provides either su or sudo to get root privileges. Apart from that, both su and sudo can be used for single commands or for interactive sessions. (su -c or sudo -s to change default behaviour).
    – mviereck
    Commented Aug 6, 2017 at 16:16

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