Rotating Linux Log Files - Part 2: logrotate

June 6, 2006 4-minute read

logrotate is the default application used to rotate all other log files not handled by syslog itself (details on rotating system log files can be found in part 1 of the article). It allows automatic rotation, compression, removal, and mailing of log files. Each log file may be handled daily, weekly, monthly, or when it grows too large.

Normally, logrotate is run as a daily cron job. Let’s look into the script that was installed in /etc/cron.daily for this:

cat /etc/cron.daily/logrotate
#!/bin/sh

test -x /usr/sbin/logrotate || exit 0
/usr/sbin/logrotate /etc/logrotate.conf

Logrotate will look into /etc/logrotate.conf for its configuration directives.

cat /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

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

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

# no packages own wtmp, or btmp -- we'll rotate them here
/var/log/wtmp {
missingok
monthly
create 0664 root utmp
rotate 1
}

/var/log/btmp {
missingok
monthly
create 0664 root utmp
rotate 1
}

# system-specific logs may be configured here

So we can see it defines some default parameters (weekly, rotate 4, create, compress) and includes all the files from /etc/logrotate.d/. Also it defines the rotation for some files that are not handled by syslog itself, like wtmp. For example, I would want to keep more than one month of old wtmp logs, then I would have to change the parameter rotate 1.

Inside the /etc/logrotate.d/ various packages will install their own configuration file that will ensure their logs are properly rotate (on my fresh Debian install I have the following files: acpid apache2 aptitude base-config dpkg exim4-base). As long as you don’t change the paths to those logs the rotation will work out of the box. But in case you change them you might want to look inside this folder and make the proper adjustments to the log file definitions, to assure they will be rotated. For example, let’s look at the apache rotation file created here by the apache2 package:

cat /etc/logrotate.d/apache2
/var/log/apache2/*.log {
weekly
missingok
rotate 52
compress
delaycompress
notifempty
create 640 root adm
sharedscripts
postrotate
if [ -f /var/run/apache2.pid ]; then
/etc/init.d/apache2 restart > /dev/null
fi
endscript
}

We can see that by default it will rotate apache logs found in /var/log/apache2/ that have the extension .log, on a weekly basis and keep 52 archives (about 1 year) of the old data. Once the rotation is completed it will restart the apache daemon. You can check logrotate manual page for all the available parameters, as they are self-explanatory. Now, if I would like to keep my own apache log files in a different location (/var/weblogs for example) and rotate them monthly then I will need to make the following changes:

/var/weblogs/*.log {
monthly
...

Probably, I will also want to change the default hour when the daily cron is running to have it on midnight. Anyway this is just an example and you will most certainly configure this based on your needs.

Even though I didn’t intended with this article to describe what each configuration parameter of logrotate means (as you can easily find out yourself), but to show what is the logic and its functionality, I would like to add that while configuring and testing this you might find very useful the debug option:

logrotate -d file

This will show you what it will do, without actually rotating anything, and this is most valuable while testing complex setups that you don’t want to ‘play’ with the logs to see if your configuration will work as you want it.

Also logrotate -f file will force the rotation even if that would have normally not occurred (logrotate will only assume it need to run and rotate logs once per day).

Note: as mentioned also in part 1, RedHat based systems (RHEL, Centos, Fedora, etc.) will also rotate by default the ‘system logs’ using **logrotate **and not syslog’s internal method as Debian systems. This is handled by default with the logrotate configuration file:

cat /etc/logrotate.d/syslog
/var/log/messages /var/log/secure /var/log/maillog /var/log/spooler /var/log/boot.log /var/log/cron {
sharedscripts
postrotate
/bin/kill -HUP

cat /var/run/syslogd.pid 2> /dev/null
 2> /dev/null || true
endscript
}

The sharedscripts parameter means that the postrotate script will only be run once (after the old logs have been compressed), not once for each log which is rotated. So nothing special defined here, besides the log files that will be rotated, and it will use the defaults from /etc/logrotate.conf.

Go to:
Rotating Linux Log Files - Introduction
Rotating Linux Log Files - Part 1: syslog

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