Run os.system as if python was not executed with sudo

Question

When I run the following command, everyhting works as expected. No error and I get a system notification saying "Hello":

$ python3
>>> import os
>>> os.system("notify-send Hello")
0

However, when I do this:

$ sudo python3
>>> import os
>>> os.system("notify-send Hello")

The script gets stuck and nothing happens.

I then tried to do this:

$ sudo python3
>>> import os
>>> os.seteuid(1000)
>>> os.system("notify-send Hello")

(1000 being my normal non-root user account)
But still, the script gets stuck and nothing happens.

I also tried this:

$ sudo python3
>>> import os
>>> os.system("su my-user-name -c 'notify-send Hello'")

and this:

$ sudo python3
>>> import os
>>> os.seteuid(1000)
>>> os.system("su my-user-name -c 'notify-send Hello'")

They all have the same issue...

I'm not looking for an alternative way of creating notifications. I'm not interested in subprocess or things like notify2 which cause a whole new category of problems on my system. Oh and please don't tell me not to use sudo. I have my reasons.

Answer 1

The implementation detail I've discovered through trial-and-error is notify-send requires the XDG_RUNTIME_DIR environment variable to function -- at least with these versions:

$ dpkg -l | grep libnotify
ii  libnotify-bin                              0.7.7-3                                      amd64        sends desktop notifications to a notification daemon (Utilities)
ii  libnotify4:amd64                           0.7.7-3                                      amd64        sends desktop notifications to a notification daemon

I first determined it needed some sort of environment variable by using env -i notify-send hello, which produced no notification.

I then bisected the environment with a modified version of this script

How you get that environment variable is up to you, but you need to be running notify-send as the proper user and with that variable set.

Here's a sample python script, I refuse to use os.system due to its security issues:

import os
import pwd
import subprocess
import sys


def main():
    if len(sys.argv) != 2:
        raise SystemExit(f'usage `{sys.argv[0]} USER`')
    if os.getuid() != 0:
        raise SystemExit('expected to run as root')

    # find the `gnome-session` executable, we'll use that to grab
    # XDG_RUNTIME_DIR
    cmd = ('pgrep', '-u', sys.argv[1], 'gnome-session')
    pid = int(subprocess.check_output(cmd))

    # read that process's environment
    with open(f'/proc/{pid}/environ') as f:
        for line in f.read().split('\0'):
            if line.startswith('XDG_RUNTIME_DIR='):
                _, _, xdg_runtime_dir = line.partition('=')
                break
        else:
            raise SystemExit('Could not find XDG_RUNTIME_DIR')

    # run the notify send as the right user
    uid = pwd.getpwnam(sys.argv[1]).pw_uid
    os.seteuid(uid)
    os.environ['XDG_RUNTIME_DIR'] = xdg_runtime_dir
    os.execvp('notify-send', ('notify-send', 'ohai'))


if __name__ == '__main__':
    exit(main())

DISCLAIMER: this script is doing some very hacky things that I wouldn't necessarily suggest in production code. Notably:

• shelling out to pgrep to find a process
• reading another process's environment variables
• sudo

Sample usage:

$ python3 t.py
usage `t.py USER`
$ python3 t.py asottile
expected to run as root
$ sudo python3 t.py asottile
# (I get a notification for `ohai`)