See Linux Boot Process for kernel ring buffer & logs during boot.
Syslog
Syslog is a standard for message logging. It allows separation of the software that generates messages, the system that stores them, and the software that reports and analyzes them. Each message is labeled with a facility code, indicating the software type generating the message, and assigned a severity level.
# rsyslog configurations
cat /etc/rsyslog.conf# Logs
/var/log
# Apache
/var/log/apache2/Create log messages
logger [-p FACILITY.SEVERITY] [-t TAG] messagelogger -p mail.info -t mailtest "Testing logger"
sudo tail -1 /var/log/mail.logRotate log files
cat /etc/logrotate.confFacility codes
Facility codes are used to know where the message originated from.
Facility code Keyword Description
0 kern Kernel messages
1 user User-level messages
2 mail Mail system
3 daemon System daemons
4 auth Security/authentication messages
5 syslog Messages generated internally by syslogd
6 lpr Line printer subsystem
7 news Network news subsystem
8 uucp UUCP subsystem
9 cron Clock daemon
10 authpriv Security/authentication messages
11 ftp FTP daemon
12 ntp NTP subsystem
13 security Log audit
14 console Log alert
15 solaris-cron Scheduling daemon
16–23 local0 – local7 Locally used facilities Severity Levels
Value Severity Keyword
0 Emergency emerg
1 Alert alert
2 Critical crit
3 Error err
4 Warning warning
5 Notice notice
6 Informational info
7 Debug debug