Regarding the below point :
c) The service prints to stdout/stderr, but systemd attaches the service's stdout/stderr to a pipe which is read by journald (using sd_journal_stream_fd(3) from libsystemd). See [Service] StandardOutput= in systemd.service(5).
I did not see StandardOutput field in [Service] sections of a service file, for example sshd.service, but its logs are visible in journalctl.
Is it by default piped to journal and we need to explicitly mention it (StandardOutput=) only when we want to redirect it somewhere else?
Even in my self made service I did not mention StandardOutput=, but I can see the logs in journalctl.
Is there a place where I can see .service file defaults ?
Nishant
On Fri, 20 Aug 2021 at 15:20, Mantas Mikulėnas <grawity@xxxxxxxxx> wrote:
On Fri, Aug 20, 2021 at 12:31 PM Nishant Nayan <nayan.nishant2000@xxxxxxxxx> wrote:Hi,My query is how does systemd-journald talk to other services so that it stores their logs/output in journal files, which could be displayed using journalctl utlity.Journald doesn't talk to services, services talk to journald:a) The service uses the standard syslog(3) call to send basic messages through the /dev/log socket, where journald (or a traditional syslogd) is listening.b) The service uses sd_journal_print(3) from libsystemd to send structured messages through /run/systemd/journal/socket (systemd-journald.socket). Some frameworks, such as GLib, have their own implementations of this protocol without needing libsystemd.c) The service prints to stdout/stderr, but systemd attaches the service's stdout/stderr to a pipe which is read by journald (using sd_journal_stream_fd(3) from libsystemd). See [Service] StandardOutput= in systemd.service(5).d) Journald also reads kernel messages (dmesg) from the /dev/kmsg device. Programs can actually write to /dev/kmsg to generate dmesg messages and journald will capture them.--Mantas Mikulėnas
