Is it possible to set Ubuntu a way that it does not shutdown before a script is finished?

For Ubuntu 16.04+ using systemd (the default).

systemd-inhibit --why="Wait for this script to finish" bash script.sh

===

Test:

$ systemctl poweroff
Operation inhibited by "bash script.sh" (PID 23912 "systemd-inhibit", user rinzwind),
reason is "Wait for this script to finish".
Please retry operation after closing inhibitors and logging out other users.

===

There are 7 locks:

• sleep inhibits system suspend and hibernation requested by (unprivileged) users
• shutdown inhibits high-level system power-off and reboot requested by (unprivileged) users
• idle inhibits that the system goes into idle mode, possibly resulting in automatic system suspend or shutdown depending on configuration.
• handle-power-key inhibits the low-level (i.e. logind-internal) handling of the system power hardware key, allowing (possibly unprivileged) external code to handle the event instead.
• handle-suspend-key inhibits the low-level handling of the system hardware suspend key.
• handle-hibernate-key inhibits the low-level handling of the system hardware hibernate key.
• handle-lid-switch inhibits the low-level handling of the systemd hardware lid switch.

You probably also want to prevent suspend, idle and hibernate.

Example using "package manager":

fd = Inhibit("shutdown:idle", "Package Manager", "Upgrade in progress...", "block");
/* ...
      do your work
                 ... */
close(fd);

Similar to this you can code your version and add a "shutdown" at the end of this script (or add a way to determine a shutdown needs to be the next action).

In BackInTime I'm using couple different DBus methods to work on all major DEs. Only downside is this won't work for root because root has no dbus.SessionBus.

#!/usr/bin/env python3
import sys
import dbus
from time import sleep

INHIBIT_LOGGING_OUT = 1
INHIBIT_USER_SWITCHING = 2
INHIBIT_SUSPENDING = 4
INHIBIT_IDLE = 8

INHIBIT_DBUS = (
               {'service':      'org.gnome.SessionManager',
                'objectPath':   '/org/gnome/SessionManager',
                'methodSet':    'Inhibit',
                'methodUnSet':  'Uninhibit',
                'interface':    'org.gnome.SessionManager',
                'arguments':    (0, 1, 2, 3)
               },
               {'service':      'org.mate.SessionManager',
                'objectPath':   '/org/mate/SessionManager',
                'methodSet':    'Inhibit',
                'methodUnSet':  'Uninhibit',
                'interface':    'org.mate.SessionManager',
                'arguments':    (0, 1, 2, 3)
               },
               {'service':      'org.freedesktop.PowerManagement',
                'objectPath':   '/org/freedesktop/PowerManagement/Inhibit',
                'methodSet':    'Inhibit',
                'methodUnSet':  'UnInhibit',
                'interface':    'org.freedesktop.PowerManagement.Inhibit',
                'arguments':    (0, 2)
               })

def inhibitSuspend(app_id = sys.argv[0],
                    toplevel_xid = None,
                    reason = 'take snapshot',
                    flags = INHIBIT_SUSPENDING | INHIBIT_IDLE):
    """
    Prevent machine to go to suspend or hibernate.
    Returns the inhibit cookie which is used to end the inhibitor.
    """
    if not app_id:
        app_id = 'backintime'
    if not toplevel_xid:
        toplevel_xid = 0

    for dbus_props in INHIBIT_DBUS:
        try:
            bus = dbus.SessionBus()
            interface = bus.get_object(dbus_props['service'], dbus_props['objectPath'])
            proxy = interface.get_dbus_method(dbus_props['methodSet'], dbus_props['interface'])
            cookie = proxy(*[(app_id, dbus.UInt32(toplevel_xid), reason, dbus.UInt32(flags))[i] for i in dbus_props['arguments']])
            print('Inhibit Suspend started. Reason: %s' % reason)
            return (cookie, bus, dbus_props)
        except dbus.exceptions.DBusException:
            pass
    print('Inhibit Suspend failed.')

def unInhibitSuspend(cookie, bus, dbus_props):
    """
    Release inhibit.
    """
    assert isinstance(cookie, int), 'cookie is not int type: %s' % cookie
    assert isinstance(bus, dbus.bus.BusConnection), 'bus is not dbus.bus.BusConnection type: %s' % bus
    assert isinstance(dbus_props, dict), 'dbus_props is not dict type: %s' % dbus_props
    try:
        interface = bus.get_object(dbus_props['service'], dbus_props['objectPath'])
        proxy = interface.get_dbus_method(dbus_props['methodUnSet'], dbus_props['interface'])
        proxy(cookie)
        print('Release inhibit Suspend')
        return None
    except dbus.exceptions.DBusException:
        print('Release inhibit Suspend failed.')
        return (cookie, bus, dbus_props)

if __name__ == '__main__':
    cookie, bus, dbus_props = inhibitSuspend()
    print('do something here')
    sleep(10)
    unInhibitSuspend(cookie, bus, dbus_props)