Linux · System Logging (syslog & journald)

A service refuses to start and gives no clue why. Why is /var/log the first place to look, and what second job do those same files do once the service is running?

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More in System Logging (syslog & journald)

syslog groups messages by facility. Which facilities are reserved for admins to use freely, and why is authpriv treated more carefully than auth?

Apache, MySQL, CUPS, and Samba all write logs, yet none of them are affected when you edit the central syslog configuration. Why do they escape it, and where do their logs come from instead?

An rsyslogd rule looks like mail.err written before /var/log/mail.err. What are the two structural halves of every such rule, and what does each one decide?

Kernel messages land in a fixed 16 KiB RAM ring buffer. What happens to old kernel messages over time, and how does that structure make persistent kernel logs a separate concern?

Left alone, log files grow without bound. What sequence of actions does logrotate perform on a log, and how often is it triggered?

logwatch is meant to save you from reading raw logs, but on a freshly configured box its daily report never arrives. What does logwatch do, and what dependency silently blocks it?

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