How can I prevent cron from filling up my syslog?

You can send the output of cron to a separate log facility add the following to your /etc/syslog.conf file:

# Log cron stuff
cron.*                                                  /var/log/cron

Remember to add /var/log/cron to your /etc/logrotate.d/syslog to ensure it gets rotated, eg

# /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
}

Change /etc/default/cron

  # Or, to log standard messages, plus jobs with exit status != 0:
  # EXTRA_OPTS='-L 5' 
  #
  # For quick reference, the currently available log levels are:
  #   0   no logging (errors are logged regardless)
  #   1   log start of jobs
  #   2   log end of jobs
  #   4   log jobs with exit status != 0
  #   8   log the process identifier of child process (in all logs)
  #
  EXTRA_OPTS="-L 0"

By default the EXTRA_OPTS line is ""

Ok,

The solution to my question was:

change

*.*;auth,authpriv.none     -/var/log/syslog

to

*.*;cron,auth,authpriv.none     -/var/log/syslog

within /etc/syslog.conf and then restart syslog

I also have cron being sent to /var/log/cron.log as suggested by Dave Cheney and stuck a logrotate on it. My fix with Daves suggestion is optimal for my situation because:

1. it keeps /var/log/syslog from being cluttered with cron messages
2. I still get cron messages (which is nice for troubleshooting)
3. logrotate keeps /var/log/cron.log from going too large.

In Ubuntu 14.04.5 (and likely elsewhere) there is rsyslogd instead of syslogd. TranslucentCloud hinted at this with Debian Jessie, but the same solution (but changing rsyslog.conf instead of syslogd.conf) doesn't appear to work in Ubuntu.

It appears that values set in /etc/rsyslog.d/50-default.conf will actually take precedence over stuff set in /etc/rsyslog.conf, so it's better to make the changes there and then restart rsyslog and cron. Otherwise you'll still end up with the default behavior of cron logging to syslog, plus cron logging to cronlog (but not exclusively).

First couple of lines in my /etc/rsyslog.d/50-default.conf:

*.*;cron,auth,authpriv.none     -/var/log/syslog
cron.*                          /var/log/cron.log

And voila!

Actually, the 'best' (one could claim) solution is a combination of what @DaveCheney suggested and what user7321 did eventually, plus a third action which I would recommend:

1. Preventing syslogd from appending cron-related log messages to /var/log/syslog
2. Ensuring cron log messages do get logged somewhere (specifically, in /var/log/cron) + ensuring log rotation for the cron log.
3. Preventing syslogd from appending cron-related log messages to /var/log/messages as well

In your /etc/syslog.conf, the combination of these suggestions changes something like the following:

*.*;cron,auth,authpriv.none                         -/var/log/syslog
auth,authpriv.none;daemon.none;mail,news.none       -/var/log/messages

into:

cron.*                                              /var/log/cron.log
*.*;cron,auth,authpriv.none                         -/var/log/syslog
auth,authpriv.none;cron,daemon.none;mail,news.none  -/var/log/messages

And don't forget to force-reload (or restart) both the cron and syslogd services, e.g. using:

/etc/init.d/syslogd force-reload
/etc/init.d/cron force-reload

Note: This works with rsyslogd as well.

I just solved this in a different way, but my aim was slightly different. I wanted to discard the cron log entries that were generated when atrun fired so that my hard drives could sleep and not be woken every 5 minutes.

You can have the target of a log event in syslog.conf be a shell command by prefixing it with the pipe, and so I used grep to throw away the ones I didn't want. So:

cron.*              | grep -v "(/usr/libexec/atrun)" >> /var/log/cron.log

I haven't investigated but I believe it should be possible to send those log entries to another target if they're still wanted.