convert dmesg timestamp to date time

If you have messages in dmesg that log the event time as “seconds since last boot”, it can be difficult to tell when they happened. Here’s an example of one of these messages:

# tail -1 /var/log/dmesg
[8969653.483175] poorcoding.php[14798]: segfault at 7f2efca36ed0 ip 00007f2efca36ed0 sp 00007f2efaf0be98 error 14

You could use something like this to parse out the timestamp and convert it to a date:

# date --date=@$((`date +%s --date="\`who -b | awk '{print $3" "$4}'\`"` + `dmesg | tail -1 | awk '{print $1}' | sed -e 's/\[//g' -e 's/\..*//g'`))
Thu Nov  5 01:21:53 PDT 2015

If your application logs to kern.log, it uses timestamps as well as seconds since uptime, so the error will already have a timestamp on it

Aug 14 09:57:59 myhostname kernel: [8969653.483177] poorcoding.php[14800]: segfault at 7f2efca36ec8 ip 00007f2efca36ec8 sp 00007f2ef10c4e98 error 14

TODO: make a quick function; convert all the messages not just a tailed or greped one;

convert log time seconds to readable date

[1122633.028643] end_request: I/O error, dev sdc, sector 0

When looking in logs, like dmesg, error messages are preceded by a number that represents the uptime on the server in seconds at the time of the error. So this I/O error happened 1122633 seconds after the machine booted. This means nothing to us. In order to see when the error happened, you need to convert the seconds of uptime into a readable date.

First get the date/time at which the server booted using who -b and convert to seconds. Then add the seconds of uptime from the error message, and then convert back to a human readable date:

# date --date="@$(echo $(date --date="`who -b | awk '{print $3" "$4}'`" +%s)+1122633|bc)"
# Tue Oct 22 00:03:33 PDT 2013

So this error happened shortly after midnight. Very interesting…