sample cron to capture Dmesg

[D Block 2]

can someone please supply a sample cron entry
for capturing output from "dmesg".

thanks in advance.
Tom

[Peter Godron]

Tom,
could you please clarify?
dmesg is a cyclic buffer, so do you want to have the output of dmesg redirected into a log file (dmesg >> /tmp/log.log) ?
Regards

[Stephen Keane]

From man dmesg ...


DESCRIPTION
dmesg looks in a system buffer for recently printed diagnostic messages and prints them on the standard output. The messages are those printed by the system when unusual events occur (such as when system tables overflow or the system crashes). If the - argument is specified, dmesg computes incrementally) the new messages since the last time it was run and places these on the standard output. This is typically used with cron (see cron(1)) to produce the error log /var/adm/messages by running the command:

/usr/sbin/dmesg - >> /var/adm/messages

every 10 minutes.

so something like


10 * * * * /usr/sbin/dmesg - >> /var/adm/messages

Or whatever log file you want to use.

[A. Clay Stephenson]

There is really a flaw in your logic. It's already been explained to you that dmesg is a circular buffer and therefore of limited capacity. Even if the "-" options is used to only output the delta's there is a non-zero proability of missing data --- even with a very short sampling period. If messages are rapidly hitting the dmesg buffer then even a cron every minute might lose data. Dmesg is really only intended to record data when no other facilities (like syslog) are available such as boot time.

[twang]

you may find output of dmesg from /var/sam/log/samlog
ypu don't have to capture its output.

[D Block 2]

I wanted to clarify where my request came from... after a system audit (analysis of both h/w and s/w and SPOFs).. one item reads:

"there is no entry in cron to capture the output of dmesg at a regular interval."


so, maybe what I have to do to resolve this item from the audit is:

run "dmesg" only once a week or once a day.

[A. Clay Stephenson · Accepted solution]

Okay, given that constraint then I would create 2 cron entries:

1) One per day to print the actual dmesg buffer contents (no "-")
2) One every 10 minutes or so to print the delta's


1 0 * * * /usr/sbin/dmesg >> /var/adm/dmesg.log
2,12,22,32,42,52 * * * * /usr/bin/dmesg - >> /var/adm/dmesg.log

You might want to actually write a script that outputs the "date" output followed by dmesg.

[D Block 2]

ok, thanks for all your inputs here!

[Bill Hassell]

The security audit:

"there is no entry in cron to capture the output of dmesg at a regular interval."

is not aware that dmesg (for HP-UX) has an easy workaround. The "-" option does two things: it reports only what has been added since the last dmesg - command, and two: it timestamps the new messages, something is missing from other Unix-flavor cron solutions. The man page and the default cron job example in /usr/newconfig/var/spool/cron/crontab.root shows you exactly how to log every dmesg entry into a log file. The 1-liner:

05,15,25,35,45,55 * * * * /usr/sbin/dmesg - >>/var/adm/messages

takes care of the audit requirement. Although a lot of Unix flavors suggest the filename "messages" , I think dmesg.log is a lot more intuitive.

[D Block 2]

The dmesg in cron - bill's suggestion is a good one.