Runlevel

Runlevel is a mode of operation in one of the computer operating systems that implement Unix System V-style initialization. Conventionally, seven runlevels exist, numbered from zero to six; though up to ten, from zero to nine, may be used. S is sometimes used as a synonym for one of the levels. Only one runlevel is executed on startup; run levels are not executed one after another, i.e. either the runlevel 2, 3 or 4 is executed, not more of them sequentially or in any other order.

A runlevel defines the state of the machine after boot. Different runlevels are typically assigned (not necessarily in any particular order) to the single-user mode, multi-user mode without network services started, multi-user mode with network services started, system shutdown, and system reboot system states. The exact setup of these configurations varies between operating systems and Linux distributions. For example, runlevel 4 might be a multi-user GUI no-server configuration on one distribution, and nothing on another. Runlevels commonly follow the general patterns described in this article; however, some distributions employ certain specific configurations.

Hash

Hash, Hashes, Hash mark or Hashing may refer to:

Hash (food)

Hash is a dish consisting of diced or chopped meat, potatoes, and spices that are mixed together and then cooked either alone or with other ingredients such as onions. The name is derived from the French verb hacher (to chop).

Corned beef hash became especially popular in some countries including in Britain and France during and after World War II as rationing limited the availability of fresh meat.

In many locations, hash is served primarily as a breakfast food on restaurant menus and as home cuisine, often served with eggs and toast (or biscuits), and occasionally fried potatoes (hash browns, home fries, etc.). The dish may also use corned beef or roast beef.

Hash has recently made a comeback as more than just a dish for leftovers or breakfasts of last resort. High-end restaurants now offer sophisticated hashes and the first cookbook dedicated exclusively to a wide variety of hashes was self-published in 2012.

United States

The meat packing company Hormel claims that it introduced corned beef hash and roast beef hash to the U.S. as early as 1950, but "hash" of many forms was part of the American diet since at least the 18th century, as is attested by the availability of numerous recipes and the existence of many "hash houses" named after the dish. In the United States, September 27 is "National Corned Beef Hash Day."

Hash function

A hash function is any function that can be used to map data of arbitrary size to data of fixed size. The values returned by a hash function are called hash values, hash codes, hash sums, or simply hashes. One use is a data structure called a hash table, widely used in computer software for rapid data lookup. Hash functions accelerate table or database lookup by detecting duplicated records in a large file. An example is finding similar stretches in DNA sequences. They are also useful in cryptography. A cryptographic hash function allows one to easily verify that some input data maps to a given hash value, but if the input data is unknown, it is deliberately difficult to reconstruct it (or equivalent alternatives) by knowing the stored hash value. This is used for assuring integrity of transmitted data, and is the building block for HMACs, which provide message authentication.

Hash functions are related to (and often confused with) checksums, check digits, fingerprints, randomization functions, error-correcting codes, and ciphers. Although these concepts overlap to some extent, each has its own uses and requirements and is designed and optimized differently. The Hash Keeper database maintained by the American National Drug Intelligence Center, for instance, is more aptly described as a catalogue of file fingerprints than of hash values.