General: Unable to start the postgresql after copying postgresql.log

Dear sir/mam,
I'm using postgresql 9.4.6 version.
I've copied the postgresql.log as postgresql.log_bk.
While copying the postgresql.log, I've restarted the postgresql
process.
Then the process got failed and I've got the following error,

psql: could not connect to server: Connection refused
Is the server running locally and accepting
connections on Unix domain socket "/tmp/.s.PGSQL.5432"?

And then I've check with log, it gives the result as follow,
FATAL: pre-existing shared memory block is still in use
HINT: If you're sure there are no old server processes still running,
remove the shared memory block or just delete the file
"postmaster.pid".

After that unable to restart the postgresql, it always fails.
After rebooting it gets succeed and truncating postmaster.pid gets
succeed.
Kindly help me how to fix this.

--
with regards,
Neelaveni