Drush installation - setting symlink to /usr/local/bin/drush is a must?
This may not answer the question directly, but a preferred and much simpler way to install Drush, is to install it through the Composer.

Installing Composer

See Composer's website for greater detail.

curl -sS https://getcomposer.org/installer | php
# If the above code fails, use this one.
# php -r "readfile('https://getcomposer.org/installer');" | php
sed -i '1i export PATH="$HOME/.composer/vendor/bin:$PATH"' $HOME/.bashrc source $HOME/.bashrc


Installing Drush

See Drush instructions on GitHub for greater detail.

To install Drush 6:

composer global require drush/drush:6.*


Or to install Drush 7:

composer global require drush/drush:dev-master


Update

Per @leekit, he was able to solve his original issue by substituting the symbolic link with export PATH="$PATH:/path/to/drush:/usr/local/bin". For that and other alternatives, refer to step 3 of Install - Manual, on Drush instructions on GitHub.