Hi, I have a log file from Munin like this:

2012/12/04 13:45:31 [INFO]: Munin-update finished (29.68 sec)
2012/12/04 13:50:01 Opened log file
2012/12/04 13:50:01 [INFO]: Starting munin-update
2012/12/04 13:50:01 [ERROR] Error occured in under [] in the configuration.
2012/12/04 13:50:01 [Warning] Could not parse datafile /var/lib/munin/datafile: [ERROR] Error occured in under [] in the configuration.  Please refer to the log if details are missing here.
2012/12/04 13:50:11 [WARNING] Call to accept timed out.  Remaining workers: localdomain;localhost.localdomain
2012/12/04 13:50:21 [WARNING] Call to accept timed out.  Remaining workers: localdomain;localhost.localdomain
2012/12/04 13:50:31 [WARNING] Call to accept timed out.  Remaining workers: localdomain;localhost.localdomain
2012/12/04 13:50:33 [INFO] Reaping Munin::Master::UpdateWorker<localdomain;localhost.localdomain>.  Exit value/signal: 0/0
2012/12/04 13:50:33 [INFO]: Munin-update finished (31.71 sec)
2012/12/04 13:55:02 Opened log file
2012/12/04 13:55:02 [INFO]: Starting munin-update
2012/12/04 13:55:02 [ERROR] Error occured in under [] in the configuration.
2012/12/04 13:55:02 [Warning] Could not parse datafile /var/lib/munin/datafile: [ERROR] Error occured in under [] in the configuration.  Please refer to the log if details are missing here.
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label pending_stable
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label hold_unstable
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label pending_testing
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label pending_unstable
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label hold_stable
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label hold_testing
2012/12/04 13:55:12 [WARNING] Call to accept timed out.  Remaining workers: localdomain;localhost.localdomain
2012/12/04 13:55:22 [WARNING] Call to accept timed out.  Remaining workers: localdomain;localhost.localdomain
2012/12/04 13:55:32 [WARNING] Call to accept timed out.  Remaining workers: localdomain;localhost.localdomain
2012/12/04 13:55:37 [INFO] Reaping Munin::Master::UpdateWorker<localdomain;localhost.localdomain>.  Exit value/signal: 0/0
2012/12/04 13:55:37 [INFO]: Munin-update finished (35.48 sec)

2012/12/04 13:55:02 Opened log file
2012/12/04 13:55:02 [INFO]: Starting munin-update
2012/12/04 13:55:02 [ERROR] Error occured in under [] in the configuration.
2012/12/04 13:55:02 [Warning] Could not parse datafile /var/lib/munin/datafile: [ERROR] Error occured in under [] in the configuration.  Please refer to the log if details are missing here.
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label pending_stable
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label hold_unstable
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label pending_testing
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label pending_unstable
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label hold_stable
2012/12/04 13:55:02 [WARNING] Service apt_all on localhost.localdomain/127.0.0.1:4949 returned no data for label hold_testing
2012/12/04 13:55:12 [WARNING] Call to accept timed out.  Remaining workers: localdomain;localhost.localdomain
2012/12/04 13:55:22 [WARNING] Call to accept timed out.  Remaining workers: localdomain;localhost.localdomain
2012/12/04 13:55:32 [WARNING] Call to accept timed out.  Remaining workers: localdomain;localhost.localdomain
2012/12/04 13:55:37 [INFO] Reaping Munin::Master::UpdateWorker<localdomain;localhost.localdomain>.  Exit value/signal: 0/0
2012/12/04 13:55:37 [INFO]: Munin-update finished (35.48 sec)

Try:

perl -lp0e 's/.*\n([^\n]*Opened log file)/$1/s' file.log

You can try this.

sed -n '/Opened log file/,$p' logfilename

I would like to have all data from last found Opened log file , not from first found
Prefer sed/awk soultion. My knowledge about perl is missing.

Did you try my code?

Yes, your solution works
But prefer not to use perl

awk '/Opened log file/ {i=NR} END {for (x=i;x>i;i++) {print $NR}}' munin-update.log

Then you can open the log file and type in the end

set nu

sed -n '4,50'p logfilename