Editing User identifier (section)

====File system user ID====
Linux also has a file system user ID (<code>fsuid</code>) which is used explicitly for access control to the file system. It matches the <code>euid</code> unless explicitly set otherwise. It may be [[Superuser|root]]'s user ID only if <code>ruid</code>, <code>suid</code>, or <code>euid</code> is root. Whenever the <code>euid</code> is changed, the change is propagated to the <code>fsuid</code>.

The intent of <code>fsuid</code> is to permit programs (e.g., the [[Network File System|NFS]] server) to limit themselves to the file system rights of some given <code>uid</code> without giving that <code>uid</code> permission to send them signals. Since kernel 2.0, the existence of <code>fsuid</code> is no longer necessary because Linux adheres to [[Single UNIX Specification#2001: Single UNIX Specification, version 3, POSIX.1-2001|SUSv3]] rules for sending signals, but <code>fsuid</code> remains for compatibility reasons.<ref name='Kerrisk'>Kerrisk, Michael. ''The Linux Programming Interface''. No Starch Press, 2010, p.  171.</ref>