I use nginx to as the front server, I have modified the CSS files, but nginx is still serving the old ones.
I have tried to restart nginx, to no success and I have Googled, but not found a valid way to clear it.
Some articles say we can just delete the cache directory: var/cache/nginx, but there is no such directory on my server.
What should I do now?
I had the exact same problem - I was running my nginx in Virtualbox. I did not have caching turned on. But looks like sendfile was set to on in nginx.conf and that was causing the problem. @kolbyjack mentioned it above in the comments.
When I turned off sendfile - it worked fine.
Sendfile is used to ‘copy data between one file descriptor and another‘ and apparently has some real trouble when run in a virtual machine environment, or at least when run through Virtualbox. Turning this config off in nginx causes the static file to be served via a different method and your changes will be reflected immediately and without question
It is related to this bug: https://www.virtualbox.org/ticket/12597