Just wanted to say that in linux you have extended attributes on files, check the man page on chattr. I believe the -i option makes files immutable.

I picked this little trick up watching a red team discuss how they set themselves persistence on the target system by making /etc/shadow immutable this way.. Fun bit is, root can't even remove the file until the flag is removed, and you can't see the immutable flag on the file unless you know what you are looking for via lsattr.