I have a script that periodically checks /var/log/system.log and I've noticed across our network that some machines keep logfiles for a month through reboots and other activities, while some purge the file each night or each reboot.

Does anyone know the exact rules here, or if I can specify intervals? Thanks in advance.


On OS X, /var/log/system.log is rotated by the newsyslog command, which is run at half-past each hour (see /System/Library/LaunchDaemons/com.apple.newsyslog.plist) whenever the Mac is running and awake. The rotation rules are in /etc/newsyslog.conf, from which the relevant lines are:

# logfilename          [owner:group]    mode count size when  flags [/pid_file] [sig_num]
/var/log/system.log                     640  7     *    @T00  J

If I understand this right (see the newsyslog.conf man page), the "@T00" in the "when" field means that the log will be rotated if newsyslog runs between midnight and 1am -- i.e. at the 12:30am run. But if the Mac is off or asleep at 12:30, this run won't happen and the log won't get rotated that day, which is probably why you see such variable results.

If you want to change the rotation criteria, feel free to edit /etc/newsyslog.conf; most of the other logs get rotated based on size, and I'm not sure why system.log is different.

UPDATE: Starting in 10.9, the rotation control for system.log moved from /etc/newsyslog.conf to /etc/asl.conf (the config for the Apple System Log facility, which does the writing to system.log). It'll have a section like this:

# Rules for /var/log/system.log
> system.log mode=0640 format=bsd rotate=seq compress file_max=5M all_max=50M
? [= Sender kernel] file system.log
? [<= Level notice] file system.log
? [= Facility auth] [<= Level info] file system.log
? [= Facility authpriv] [<= Level info] file system.log

The "rotate=seq compress file_max=5M all_max=50M" section controls archiving and retention. See man asl.conf for more info and options.

The logging system got a pretty thorough rewrite in 10.12, but this doesn't appear to have changed.

  • Great answer, wish I could upvote twice. Thanks a ton! – David Houde Apr 29 '11 at 12:15
  • Has this changed in El Capitan? I don't see system.log in newsyslog.conf or in newsyslog.d. – Barmar Dec 31 '16 at 18:59
  • @Barmar It moved to /etc/asl.conf; see update. – Gordon Davisson Jan 1 '17 at 6:28
  • Where do the old (rotated) log files go to? – Jason Harrison Jan 3 '18 at 18:30
  • @JasonHarrison That's controlled by the rotate=NAME_STYLE, compress, and dest=PATH options in the asl.conf file. See the "FILE ROTATION" section of the man page. In the case of system.log, rotate=seq compress means they'll be stored as /var/log/system.log.0.gz, .../system.log.1.gz, etc. When they hit a total of 50MB (the all_max parameter), the oldest gets deleted. – Gordon Davisson Jan 3 '18 at 18:47

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