Unix-like operating systems identify users within the kernel by an unsigned integer value called a user identifier, often abbreviated to UID or User ID. The range of UID values varies amongst different systems; at the very least, a UID represents a 15-bit integer, ranging between 0 and 32767, with the following restrictions:
The UID value references users in the /etc/passwd file. Shadow password files and Network Information Service also refer to numeric UIDs. The user identifier has become a necessary component of Unix file systems and processes. Some operating systems support 16-bit UIDs, making 65536 unique IDs possible; a modern system with 32-bit UIDs will potentially make 4,294,967,296 (232) distinct values available.
In POSIX-compliant environments the command-line command id gives the user's user identifier number as well as more information on the user account like the user name, primary user group and group identifier (GID).
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The effective UID (euid) and effective GID (egid) affect file creation and access. During file creation, the kernel sets the owner attributes of the file to the effective UID and effective GID of the creating process. During file access, the kernel uses the effective UID and effective GID of the process to determine if it can access the file.
The saved user ID (suid) is used when a program running with elevated privileges needs to temporarily do some unprivileged work: it changes its effective user ID from a privileged value (typically root) to some unprivileged one, and this triggers a copy of the privileged user ID to the saved user ID slot. Later, it can set its effective user ID back to the saved user ID (an unprivileged process can only set its effective user ID to three values: its real user ID, its saved user ID, and its effective user ID—i.e., unchanged) to resume its privileges.
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.
The real UID (ruid) and real GID rgid identify the real owner of the process and affect the permissions for sending signals. A process without superuser privilege can signal another process only if the sender’s real UID matches with the real UID of the receiver. Since child processes inherit the credentials from the parent, they can signal each other.