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When digging into logging from shell scripts and applications I found that the logger utility gives the option to log directly to systemd's journals. The man page advocates in these cases to fill in at least the MESSAGE_ID field, which consists of a unique UUID which can be generated with systemd-id128, according to the man page of systemd.journal-fields.

Looking into ways it is used by applications (journalctl -o verbose is your friend) across multiple systems, I see a lot of identical MESSAGE_IDS, although only about 10% of the messages have a MESSAGE_ID field, and suspect a relation with certain executables, although systemd entries use several.

Does anyone know what this field signifies?

Related question: How is MESSAGE_ID set in journalctl?

Thanks in advance for any help.

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MESSAGE_ID points to entries in the journal message catalogs, which provide expanded information which can’t easily fit in a typical syslog entry (including links to external documentation).

To use such identifiers, you need to identify the messages you which to enhance, generate identifiers for them, add corresponding catalog entries (as files in .../systemd/catalog), and register them (technically, update the journal catalog index).

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