Sometimes it is still easiest (especially when there is only a command line terminal available) to directly edit files like /etc/passwd, /etc/hosts etc. with a plain text editor. When I today just wanted to change the login shell of a user on an Oracle Enterprise Linux installation, I simply changed /bin/csh to /bin/bash for the corresponding entry in /etc/passwd. However, the surprise was that the new entry was not considered when re-logging in. This is where nscd comes into play. From the man page:
NSCD(8) Linux Programmer's Manual NSCD(8)
NAME
/usr/sbin/nscd - name service cache daemon
DESCRIPTION
Nscd is a daemon that provides a cache for the most common name service requests. ...
Among those most common name service requests are lookups into the /etc/hosts, /etc/group and /etc/hosts files. Actually the nscd daemon also watches the respective files for changes, but it might take several minutes until it picks up the modified files.To clear the nscd cache immediately, simply call nscd with the -i switch and the name of the lookup database which needs to be reloaded:
$ /usr/sbin/nscd -i passwd
By the way, stopping and re-starting the nscd daemon through /etc/init.d/nscd is not enough – the daemon still remembers its cached entries afterwards.