In one of the machines I work with I'm seeing a strange behaviour. I've manually changed the shell of one user in /etc/passwd but when I try to login with that user, it still uses the old one. getent also shows the old one. But a grep on the /etc/passwd file confirms I saved the file correctly.
Is there (on Linux) any kind of /etc/passwd caching?
How to avoid it?
Do you have a process named nscd running? That's the name service caching daemon.
If it is running, you may need to run sudo nscd -i passwd to tell it that it should forget any cached /etc/passwd file data. It has other caches too: the respective keywords for them are group, hosts, services and netgroup.
It is more commonly used with LDAP/AD/NIS-based authentication schemes, but may improve system performance if you're running some process that makes a very large number of user/group/service/hosts-file look-ups. Or if you have installed some services you're not actually using (perhaps to use just a small part of them), some service may have pulled nscd in as a dependency.
grep <that_user> /etc/passwd?chsh <username>...