How do I clear DNS cache using BASH shell prompt under UNIX like operating systems?

DNS queries are cached to speed up DNS data access.

Linux – NSCD

Nscd caches libc-issued requests to the Name Service. If retrieving NSS data is fairly expensive, nscd is able to speed up consecutive access to the same data dramatically and increase overall system performance. Type the following command under Linux operating systems. Open a terminal and enter:
/etc/init.d/nscd restart
OR
sudo /etc/init.d/nscd restart

MAC OS X

Open a terminal and type the following command under OS X Leopard (v10.5):
dscacheutil -flushcache
Note if you are Mac OS X Tiger (v10.4) user, enter:
lookupd -flushcache