[systemd-devel] Question about changing systemd target during boot

[Michael Chapman]

On Mon, 1 Aug 2016, Andrei Borzenkov wrote:
[...]
>> So here goes what I've done:
>>
>> 1. Create a service and put it in the network-online.target:
>>
>> /etc/systemd/system/change-target.service:
>> [Unit]
>> Description=Change Target
>> Wants=network-online.target
>> After=network-online.target
>>
>> [Service]
>> Type=oneshot
>> ExecStart=/tmp/script.sh
>> TimeoutSec=60s
>>
>> [Install]
>> WantedBy=network-online.target
>>
>
> This unit have conflicting requirements - on one hand it is
> After=network-online.target, OTOH WantedBy=network-online.target
> implies Before=network-online.target.

I've seen this asserted on this list a few times, but as far as I can tell 
it isn't actually correct. After/Before are meant to be completely 
orthogonal to Wants/Requires/etc., according to the documentation.

If A.service has:

   Wants=B.service
   Before=B.service

then any time you ask A.service to be started, systemd will add a job to 
start B.service... but it still ensures that A.service is started *before* 
B.service.

Is my understanding on this incorrect? I have been making use of this 
behaviour for quite some time, and as far as I can tell it has never 
failed.

- Michael