How to fix ORA-12547 TNS lost contact when try to connect to Oracle

Problem:
While login to oracle, the following error may happen:

sqlplus scott/tiger
ERROR:
ORA-12547: TNS:lost contact

Solution:
Step: #1
cd $ORACLE_HOME/bin
ls -ltr oracle
-rwxr-xr-x    1 oracle   oinstall       136803483 Mar 16 20:32 oracle

Step: #2
Change permissions as below:

chmod 6751 oracle
ls -ltr oracle
-rwsr-s--x    1 oracle   oinstall       136803483 Mar 16 20:32 oracle

Source:
http://www.dadbm.com/how-to-fix-ora-12547-tns-lost-contact-when-try-to-connect-to-oracle/