On Wed, Aug 17, 2022 at 5:05 AM Peter Hoeg <peter@xxxxxxxx> wrote:
Hi all,
I have a small program that queries an external web API, massages the data and passes it off to something else. As I only need it to run during certain windows and I didn't want to have to bother with making sure it was alive, I am starting it using a systemd timer which is sufficiently flexible. All good and it's super nice to be able to "outsource" all that to systemd.
My question is - what is the canonical way to detect that I'm running under systemd so that I can adjust accordingly? Currently I'm checking for the existence of the "_SYSTEMD_INVOCATION_ID" environment variable in order to change the logging function as the default logger includes a timestamp which is redundant when journald is picking it up.
I'm also using LoadCredential for passing tokens but that just comes down to looking for the right file name where the CREDENTIALS_DIRECTORY variable points, so that really isn't systemd specific and there is a fallback in case that isn't set or the directory/file doesn't exist anyway.
I could of course also add a --systemd flag that toggles this but if I can do "TheRightThing(tm)" out of the box, why not.
What *is* specific to systemd that you need to check for and how would that change the program's behavior? It sounds to me that being started by pid1 is irrelevant here – if you want to alter your logging, then you want to check specifically whether stderr is tied to the journal, for which you can use JOURNAL_STREAM (like in https://github.com/systemd/systemd/blob/main/src/basic/log.c#L217).
(In addition to disabling the timestamps you should *at least* add message priority indicators as well, either by using SyslogLevelPrefix=, or by avoiding stderr entirely and using the syslog() API for logging – if not going all the way with sd_journal_send*().)
--
Mantas Mikulėnas
