Part of the mystery is solved and part not.
SOLVED:
The problem was that journal entries were not being stored on disk and
journalctl was not showing entries from memory.
The default Ubuntu config has settings
Storage=auto
SplitMode=uid
Also, the directory /var/log/journal does not exist by default and does
not get created in Storage=auto mode. Creating the /var/log/journal
directory and restarting journald so that entries get stored to disk
fixes the problem.
NOT SOLVED:
It is still unclear why journalctl would display output when run from
the shell but not when run from systemd. It would seem that in a systemd
--user unit it could not access the in-memory logs.
I would suggest that the default Ubuntu install should have the
/var/log/journal directory in place so that journalctl will work correctly
out-of-the-box.
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https://bugs.launchpad.net/bugs/1810351
Title:
journalctl fails to work inside systemd service
Status in systemd package in Ubuntu:
New
Bug description:
Using
Ubuntu 16.04.4 LTS
and
Package: systemd
Architecture: amd64
Version: 229-4ubuntu21.10
Multi-Arch: foreign
Priority: required
Section: admin
Origin: Ubuntu
The journalctl(1) command cannot be used in a systemd OnFailure
service to get error logs. To replicate the issue
1) Create a service unit
[Unit]
Description=%n
[Service]
#ExecStart=/bin/sh -xv -c 'systemctl --user status -o cat -n 20 |
mail -s "Unit failed" '
ExecStart=/bin/sh -c 'journalctl --user-unit= -o verbose -q -n 20
| mail -s "Unit failed" '
2) enable the unit
3) start the unit
You will get a failure message from journalctl and mail of
sh[28225]: No journal files were opened due to insufficient permissions.
sh[28225]: mail: Null message body; hope that's ok
4) switch the comment # to the journalctl ExecStart and uncomment the
systemctl ExecStart line
From systemctl you will get journal output in the email.
The man page for systemctl states "In addition, journalctl
--unit=NAME or journalctl --user-unit=NAME use a similar filter for
messages and might be more convenient." and indeed it would if it
worked properly.
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