Can't filter using "_SYSTEMD_UNIT"

[cryzed]

I want to monitor the journal of a running vpn-client unit. To do so I first used sudo journalctl -f --output=json to get an idea of the general structure of each entry's catalog (unimportant information truncated):

{
  "SYSLOG_IDENTIFIER" : "openvpn",
  "_COMM" : "openvpn",
  "_EXE" : "/usr/bin/openvpn",
  "_CMDLINE" : "/usr/sbin/openvpn --suppress-timestamps --nobind --config VPN.conf",
  "_SYSTEMD_CGROUP" : "/system.slice/system-openvpn\\x2dclient.slice/openvpn-client@VPN.service",
  "_SYSTEMD_UNIT" : "openvpn-client@VPN.service",
  "_SYSTEMD_SLICE" : "system-openvpn\\x2dclient.slice",
  "MESSAGE" : "...",
  "_PID" : "4586"
}

Based on this, I figured I could use .add_match(_SYSTEMD_UNIT='openvpn-client@VPN.service') as documented to get access to the corresponding journal.

However I can't seem to fetch and journal entries when doing so:

reader = systemd.journal.Reader()
reader.this_boot()
reader.add_match(_SYSTEMD_UNIT='openvpn-client@VPN.service')
for entry in reader:
    print(entry)

Will simply print nothing, while confusingly (to me) specifying the SYSLOG_IDENTIFIER instead will work:

reader = systemd.journal.Reader()
reader.this_boot()
reader.add_match(SYSLOG_IDENTIFIER='openvpn')
for entry in reader:
    print(entry)

However this fetches the entries of all openvpn journals, which I don't want. I also made sure using journalctl --boot --unit openvpn-client@VPN.service that journal entries matching the specified filter exist. (Removing the this_boot()-filter also doesn't lead to any entries being fetched).

Any advice?

[keszybz]

I run your script with

  reader.add_match('_SYSTEMD_UNIT=user@1000.service')

and

  reader.add_match(_SYSTEMD_UNIT='user@1000.service')

and I get a bunch of entries with both. I don't use openvpn, but your script seems to work OK.

You should check if you get the expected entries with journalctl. Your script corresponds to journalctl -b _SYSTEMD_UNIT=openvpn-client@VPN.service -o json, and if that gives you entries, but the script doesn't, something is very wrong.

[keszybz]

Closing because of lack of feedback. Please reopen if needed.