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. Whenever the euid is changed, the change is propagated to the fsuid. It may be root's user ID only if ruid, suid, or euid is root. It matches the euid unless explicitly set otherwise. Linux also has a file system user ID ( fsuid) which is used explicitly for access control to the file system. Most filesystems implement a method to select whether BSD or AT&T semantics should be used regarding group ownership of a newly created file BSD semantics are selected for specific directories when the S_ISGID (s-gid) permission is set. According to AT&T UNIX System V semantics (also adopted by Linux variants), a newly created file is normally given the group ownership specified by the egid of the process that creates the file. According to BSD Unix semantics, the group ownership given to a newly created file is unconditionally inherited from the group ownership of the directory in which it is created. The effective GID ( egid) of a process also affects access control and may also affect file creation, depending on the semantics of the specific kernel implementation in use and possibly the mount options used. It is also used as the owner for files created by that process. The effective UID ( euid) of a process is used for most access checks. The POSIX standard introduced three different UID fields into the process descriptor table, to allow privileged processes to take on different roles dynamically: In POSIX-compliant environments, the command-line command id gives the current user's UID, as well as more information such as the user name, primary user group and group identifier (GID). UIDs are stored in the inodes of the Unix file system, running processes, tar archives, and the now-obsolete Network Information Service. The password file maps textual user names to UIDs. The UID, along with the group identifier (GID) and other access control criteria, is used to determine which system resources a user can access. Unix-like operating systems identify a user by a value called a user identifier, often abbreviated to user ID or UID. Value identifying a user account in Unix and Unix-like operating systems
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