Topic: journald
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Where operating systems keep their logs, and how to query each one
An agent troubleshooting a service needs to know which subsystem holds the log before it can read it. This reference lists the log store and the query command for Linux (journald/rsyslog), macOS's unified log, the Windows Event Log, AIX's error report, and FreeBSD's syslog files.
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Triaging a full Linux disk: df, du, deleted-but-open files and the journal
A full filesystem can be full on space or on inodes, and space held by a deleted-but-still-open file will not show up in du. This methodology walks the checks in order — df -h, df -i, du -x, lsof +L1, and journald vacuuming — before anything is deleted.
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Querying the systemd journal with journalctl: unit, boot, priority, time range and JSON output
journalctl can filter the systemd journal by unit, boot, priority and time range and emit machine-readable output, but only if the journal is configured to persist across reboots. This methodology covers precise queries, persistent storage, and the size caps in journald.conf.
Machine-readable: JSON