Unlock a logfile locked by another process linux

I have written a script to empty/nullify a logfile once a day on my ubuntu server.

LogFile Path: /var/log/turn.log

I have tried below commands for this purpose:

Commands : sudo cp /dev/null /var/log/turn.log sudo truncate -s 0 /var/log/turn.log

The size of the file does not reduce by using any of the either commands. I suspect that the file is locked by the process currently writing logs into it.

$ sudo fuser /var/log/turn.log
/var/log/turn.log :    964 
$ ps 964
PID    TTY    STAT    TIME    COMMAND

964    ?      Ssl    0:54     /usr/bin/turnserver -c  /etc/turnserver.conf -o -v
$

How can I unlock this file so that my script can nullify the logfile everyday at a given time.

How can I unlock this file so that my script can nullify the logfile everyday at a given time.

Stop turnserver before you do

sudo cp /dev/null /var/log/turn.log 
sudo truncate -s 0 /var/log

and start it after this is done. If supported only a restart after emptying the file is also possible. I would have used > /var/log/turn.log as root or from crontab as root; but this also keeps the file as is until released. Truncate is not needed.

Besides altering turnsever to not lock the file all the time (using a service I would lock the file when needed and somewhere in the loop close/reopen the log) the more generic method would be to add the log to logrotate and have logrotate deal with this: compress the file and remove compressed files when older than x days. Those older logs are never touched by turnserver.

Try this command line, it should exchange your old log file with a new empty one. The old log file will have the *.old file extension.

sudo cp -a --attributes-only /var/log/turn.log /var/log/turn.log.new &&\
sudo ln -f /var/log/turn.log /var/log/turn.log.old &&\
sudo ln -f /var/log/turn.log.new /var/log/turn.log &&\
sudo unlink /var/log/turn.log.new && sync ||\
echo "Error: Something went wrong."