Linux also has a file system user ID (fsuid) which is used explicitly for access control to the file system. It matches the euid unless explicitly set otherwise. It may be root's user ID only if ruid, suid, or euid is root. Whenever the euid is changed, the change is propagated to the fsuid.
The intent of fsuid is to permit programs (e.g. the NFS server) to limit themselves to the file system rights of some given uid without giving that uid permission to send them signals.
Read more about this topic: User Identifier
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