Ubuntu – Difference between /var/log/messages, /var/log/syslog, and /var/log/kern.log

Syslog is a standard logging facility. It collects messages of various programs and services including the kernel, and stores them, depending on setup, in a bunch of log files typically under /var/log. In some datacenter setups there are hundreds of devices each with its own log; syslog comes in handy here too. One just sets up a dedicated syslog server which collects all the individual device logs over the network. Syslog can also save logs to databases, and other clients.

According to my /etc/syslog.conf, default /var/log/kern.log captures only the kernel's messages of any loglevel; i.e. the output of dmesg.

/var/log/messages instead aims at storing valuable, non-debug and non-critical messages. This log should be considered the "general system activity" log.

/var/log/syslog in turn logs everything, except auth related messages.

Other insteresting standard logs managed by syslog are /var/log/auth.log, /var/log/mail.log.

2020 update

You may still stumble upon syslog; but the defaults have changed.

journald has replaced syslog, in quite a big portion of systems, including Ubuntu.

This is relevant because you won't be finding /var/log/messages that often anymore. journald doesn't write plaintext logs — it uses its own, compressed and partially authenticated format.

Search online for e.g. journalctl cheatsheet, or just study man 8 systemd-journald, man 1 journalctl yourself.

Syslog and journald are, to a degree, cross-compatible; you can transport logs between them in either direction. However, you won't get plaintext logs a-la /var/log/messages with journald; and you won't get structured (journalctl -o json-pretty) and authenticated logging with syslog.