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I get the impression that there might be a way to write to the systemd journal, json data directly without first converting it to the format the sd_journal* functions expect. Is this possible or not?

My suspicion is because of some comments about an inbuilt json parser. However the man pages suggest otherwise.

Also, I note that if your write to stdout in the format 

<priority> message

The priority will end up in the PRIORITY="priority" field and message will end up in MESSAGE="message" field. But can other structured field data be input?

Note: The man pages do not talk about the last feature I mention. So I wouldn't be surprised if they are slightly out of date which is why I am asking.

hookenz
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1 Answers1

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journald doesn't accept arbitary JSON. Just Key/Value pairs. So it's not possible to send nested data structures. You can send data directly via the Unix Domain socket:

echo -e  "MESSAGE=Hello\nFOO=BAR\nMY_ID=12345\n" |socat  - UNIX-SENDTO:/run/systemd/journal/socket

results in:

{
    "__CURSOR" : "s=46dc1bd66d0e4a48a6809e45228511e2;i=84cc;b=fd9144999d6846c8827d58f56c2635db;m=850161136;t=55669a307fdd6;x=887a021a37840789",
    "__REALTIME_TIMESTAMP" : "1502386590318038",
    "__MONOTONIC_TIMESTAMP" : "35703361846",
    "_BOOT_ID" : "fd9144999d6846c8827d58f56c2635db",
    "_TRANSPORT" : "journal",
    "_UID" : "1001",
    "_GID" : "1001",
    "_CAP_EFFECTIVE" : "0",
    "_SYSTEMD_OWNER_UID" : "1001",
    "_SYSTEMD_SLICE" : "user-1001.slice",
    "_SYSTEMD_USER_SLICE" : "-.slice",
    "_MACHINE_ID" : "6e7b40640bf6473189165f19f8be2536",
    "_HOSTNAME" : "samson",
    "_SYSTEMD_UNIT" : "user@1001.service",
    "_SYSTEMD_INVOCATION_ID" : "e5ed32fbb1004545b1ddf73a0d928d87",
    "_SYSTEMD_CGROUP" : "/user.slice/user-1001.slice/user@1001.service/gnome-terminal-server.service",
    "_SYSTEMD_USER_UNIT" : "gnome-terminal-server.service",
    "_COMM" : "socat",
    "_EXE" : "/usr/bin/socat",
    "_CMDLINE" : "socat - UNIX-SENDTO:/run/systemd/journal/socket",
    "FOO" : "BAR",
    "MESSAGE" : "Hello",
    "MY_ID" : "12345",
    "_PID" : "19868",
    "_SOURCE_REALTIME_TIMESTAMP" : "1502386590317991"
}
Jürgen Hötzel
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  • Thanks. That's interesting. My data is coming from Lua. So I'm wanting to take a flat table of key/value pairs and inject them into the journal. I found some C ode that basically took JSON data as input on stdin and wrote it to the journal like that. So I adapted it to my needs and it works. – hookenz Aug 10 '17 at 21:05