f943ab6efb
--lines=1 means that we show the last message about the unit, whatever it is:
$ SYSTEMD_LOG_LEVEL=debug systemctl --verbose start demo2
...
Invoking 'journalctl -q --follow --no-pager --lines=1 --synchronize-on-exit=yes --unit=demo2.service' as child.
...
Directory /var/log/journal/7d16833bfa924410851e2a193bcfd4ba added.
Journal effective settings seal=no keyed_hash=yes compress=ZSTD compress_threshold_bytes=8B
...
Reiterating files to get inotify watches established.
Considering root directory '/run/log/journal'.
Considering root directory '/var/log/journal'.
Considering directory '/var/log/journal/7d16833bfa924410851e2a193bcfd4ba'.
Nov 26 17:24:35 rawhide systemd[1]: Finished demo2.service.
...
Executing dbus call org.freedesktop.systemd1.Manager StartUnit(demo2.service, replace)
...
Job for demo2.service finished.
Got result done/Success for job demo2.service.
journal: Sending message: {"method":"io.systemd.Journal.Synchronize","parameters":{"offline":false}}
Nov 27 13:05:30 rawhide systemd[1]: Starting demo2.service...
Nov 27 13:05:30 rawhide systemd[1]: demo2.service: Deactivated successfully.
Nov 27 13:05:30 rawhide systemd[1]: Finished demo2.service.
...
We obviously should only show _new_ messages, hence change to --lines=0.
This works properly after the fix in the previous commit.
Fixes #39048.
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