3 Analyzing and Managing System Log Files #

System log files are always located under the /var/log directory. The following list presents an overview of all system log files from SUSE Linux Enterprise Server present after a default installation. Depending on your installation scope, /var/log also contains log files from other services and applications not listed here. Some files and directories described below are “placeholders” and are only used, when the corresponding application is installed. Most log files are only visible for the user root.

To view log files, you can use any text editor. There is also a simple YaST module for viewing the system log available in the YaST control center under Miscellaneous › System Log.

For viewing log files in a text console, use the commands less or more. Use head and tail to view the beginning or end of a log file. To view entries appended to a log file in real-time use tail -f. For information about how to use these tools, see their man pages.

To search for strings or regular expressions in log files use grep. awk is useful for parsing and rewriting log files.

Log files under /var/log grow on a daily basis and quickly become very large. logrotate is a tool that helps you manage log files and their growth. It allows automatic rotation, removal, compression, and mailing of log files. Log files can be handled periodically (daily, weekly, or monthly) or when exceeding a particular size.

logrotate is usually run daily by systemd, and thus usually modifies log files only once a day. However, exceptions occur when a log file is modified because of its size, if logrotate is run multiple times a day, or if --force is enabled. Use /var/lib/misc/logrotate.status to find out when a particular file was last rotated.

The main configuration file of logrotate is /etc/logrotate.conf. System packages and programs that produce log files (for example, apache2) put their own configuration files in the /etc/logrotate.d/ directory. The content of /etc/logrotate.d/ is included via /etc/logrotate.conf.

# see "man logrotate" for details
# rotate log files weekly
weekly

# keep 4 weeks worth of backlogs
rotate 4

# create new (empty) log files after rotating old ones
create

# use date as a suffix of the rotated file
dateext

# uncomment this if you want your log files compressed
#compress

# comment these to switch compression to use gzip or another
# compression scheme
compresscmd /usr/bin/bzip2
uncompresscmd /usr/bin/bunzip2

# RPM packages drop log rotation information into this directory
include /etc/logrotate.d

Important: Avoid Permission Conflicts

The create option pays heed to the modes and ownerships of files specified in /etc/permissions*. If you modify these settings, make sure no conflicts arise.

logwatch is a customizable, pluggable log-monitoring script. It parses system logs, extracts the important information and presents them in a human readable manner. To use logwatch, install the logwatch package.

logwatch can either be used at the command line to generate on-the-fly reports, or via cron to regularly create custom reports. Reports can either be printed on the screen, saved to a file, or be mailed to a specified address. The latter is especially useful when automatically generating reports via cron.

On the command line, you can tell logwatch for which service and time span to generate a report and how much detail should be included:

# Detailed report on all kernel messages from yesterday
logwatch --service kernel --detail High --range Yesterday --print

# Low detail report on all sshd events recorded (incl. archived logs)
logwatch --service sshd --detail Low --range All --archives --print

# Mail a report on all smartd messages from May 5th to May 7th to root@localhost
logwatch --service smartd --range 'between 5/5/2005 and 5/7/2005' \
--mailto root@localhost --print

The --range option has got a complex syntax—see logwatch --range help for details. A list of all services that can be queried is available with the following command:

ls /usr/share/logwatch/default.conf/services/ | sed 's/\.conf//g'

logwatch can be customized to great detail. However, the default configuration should usually be sufficient. The default configuration files are located under /usr/share/logwatch/default.conf/. Never change them because they would get overwritten again with the next update. Rather place custom configuration in /etc/logwatch/conf/ (you may use the default configuration file as a template, though). A detailed HOWTO on customizing logwatch is available at /usr/share/doc/packages/logwatch/HOWTO-Customize-LogWatch. The following configuration files exist:

logger is a tool for making entries in the system log. It provides a shell command interface to the rsyslogd system log module. For example, the following line outputs its message in /var/log/messages or directly in the journal (if no logging facility is running):

logger -t Test "This message comes from $USER"

Depending on the current user and host name, the log contains a line similar to this:

Sep 28 13:09:31 venus Test: This message comes from tux