R

radio button --
An onscreen button used for switching an option off or on. When the radio button is highlighted or colored, that option is selected. Radio buttons are used when one (and only one) of a group of options must be selected. See also checkbox.

Random Access Memory (RAM) --
The computer's working storage area. All processing of data and running programs are stored in RAM.

RARP --
Reverse Address Resolution Protocol, a protocol that is the reverse of ARP. RARP maps Ethernet addresses to its corresponding IP addresses.

raw device --
A block device for which read and write operations are synchronized to natural records of the physical device (not buffered).

raw I/O --
(programming) Movement of data directly between user address spaces and the device. Raw I/O is used primarily for administrative functions where the speed of a specific operation is more important than overall system performance.

raw mode --
(programming) The method of transmitting data from a terminal to a user without processing. This mode is defined in the line discipline modules.

rcp --
A file transfer program for copying files to and from a remote computer in a network.

read access --
Authorization for a subject to read the information in an object.

read queue --
(programming) In a stream, the message queue in a module or driver containing messages moving upstream.

realized --
(programming) In the context of the X Toolkit Intrinsics, the point at which all the data structures of a widget have been allocated. Windows and other information are not created when the widget is created with the XtCreateWidget routine, but are created in a later call to XtRealizeWidget on the widget itself or on an ancestor widget.

real user ID --
(programming) Each user allowed on the system is identified by a positive integer (0 to UID_MAX) called a real user ID. Each user is also a member of a group. The group is identified by a positive integer called the real group ID. An active process has a real user ID and real group ID that are set to the real user ID and real group ID, respectively, of the user responsible for the creation of the process.

reboot --
See boot.

reference page --
See manual page.

region --
A group of machine addresses that refer to a base address.

register --
(programming) 1. Also known as ``registration''. To make a routine name known to the API. When the application programmer develops a callback routine, that routine must be registered when the widget is created so that it can be properly invoked. 2. For SNMP, the process whereby an SMUX peer informs an SNMP agent that the peer is assuming responsibility for servicing management station requests involving objects contained in a specific MIB module (or subtree). See also priority.

regular expression --
A notation for matching any sequence of characters. The notation is used to describe the form of a sequence of characters, rather than the characters themselves. Regular expressions consist of literal characters, which match only themselves, and metacharacters.

relative pathname --
A pathname that does not start with a slash (/); for example; Tutorial, Reports/September, or ../tmp. A relative pathname is searched for, starting from the current working directory and can use the notation ``..'' to indicate ``one directory up from the current working directory.'' See also absolute pathname and pathname.

release --
One of multiple, sequentially produced versions of a software product, each of which contains improvements on the last. A distribution of fixes or new functions for an existing software product.

remote computer --
A computer other than the computer that you originally logged in to. It might be on a LAN or a WAN or accessed using a dial-up connection.

remote printer --
Any printer connected to a remote system.

Remote Procedure Call (RPC) --
Procedures that provide the means by which one process (the caller) can have another process (the server) execute a procedure call as if the caller had done so itself locally.

remote program --
(programming) Software that implements one or more remote procedures.

request-ID --
A number assigned to each print job when you send it to the printer. The number is displayed on the screen when you print a file.

reserved word --
A word that is reserved because it has special meaning to the system, or as part of the grammar of a programming language.

resize --
To change the height or width of a window.

resize corners --
Hollow, L-shaped symbols located at the corners of a window which, when ``grabbed'', are used to change the size of the window. See also grab.

resource --
1. For the Network File System (NFS), a resource represents a remote file or directory that can be connected to your filesystem. 2. When used with the X windowing system, a resource represents a particular attribute that defines how a part of your X session looks or acts. 3. (programming) An attribute of a widget or a widget class. A resource is a named data value in the defining structure of a widget.

resource translation --
(programming) The mechanism by which resource values are made accessible to widgets. The list of resources is contained in the app-defaults files. Each entry in these files consists of a resource name/value pair of the form: app_name.resource_name: value. Using an asterisk in place of the app_name makes the entry available to any application that recognizes the resource_name. Any hardcoded value takes precedence over what is set in the resource file.

response time --
The time taken between issuing a command and receiving some feedback from the system. This is not to be confused with turnaround time, which is a measure of how long a particular task takes from invocation to completion.

restore --
1. To return an iconified or maximized window to its normal size. See also iconify. 2. To copy files from a backup onto the system. See also backup.

retension --
The process of rewinding a tape in a cartridge tape device to get the amount of tautness necessary for accurate recording of data.

return value --
A number from 0 to 255 which is returned by each command executed from a shell. By convention, a value of zero (TRUE) represents a successful exit; other values indicate that the command might have failed.

RFC --
Internet technical bulletins known as Requests for Comments.

rlogin --
A program for logging in to a remote computer.

root --
1. The top directory of a UNIX filesystem, represented as a slash (/). 2. The login name of the superuser, a user who has the widest form of computer privileges.

root directory --
The top directory of a UNIX filesystem, represented as a slash (/).

rotational gap --
The gap between the actual disk locations of blocks of data belonging to the same file. The rotational gap compensates for the continuous, high-speed rotation of the disk so that when the controller is ready to reference the next physical block, the read-write head is positioned correctly at the beginning of that block.

roundtrip delay --
The time it takes for a host to send an NTP packet to another host and get an NTP packet back from that host in reply.

router --
A device used to connect two or more similar or dissimilar networks and provide routing services based on end-to-end connections.

routine --
(programming) See function.

RPC language --
(programming) A C-like programming language recognized by the rpcgen(1tcp) compiler.

RPC package --
(programming) The collection of software and documentation used to implement and support remote procedure calls in UNIX System V. The RPC Package implements and is a superset of the functionality of the RPC protocol.

RPC protocol --
(programming) The message-passing protocol that is the basis of the RPC package.

RPC/XDR --
(programming) See RPC language.

RreqPDU --
A registration request Protocol Data Unit. This is issued by an SMUX peer to request registration for a specific MIB module at either a specific priority or at priority -1. See also register and priority.

RrspPDU --
A registration response Protocol Data Unit. This is issued by an SNMP agent in response to a peer's registration request. This PDU contains the integer priority value that the agent has assigned to the peer's MIB module registration. See also RreqPDU, register, and priority.

run level --
The UNIX system executes in a number of different states known as ``run levels''. Depending on the run level occupied by the system, user logins might or might not be possible, and printing or networking facilities might or might not be activated.