Plain text
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In computing, plain text is a term used for an ordinary "unformatted" sequential file readable as textual material without much processing.
The encoding has traditionally been either ASCII, one of its many derivatives such as ISO/IEC 646 etc, or sometimes EBCDIC. No other encodings are used in plain text files which neither contain any (character-based) structural tags such as heading marks, nor any typographic markers like bold face, italics, etc.
Unicode is today gradually replacing the older ASCII derivatives limited to 7 or 8 bit codes. It will probably serve much the same purposes, but this time permitting almost any human language as well as important punctuation and symbols such as mathematical relations (≠ ≤ ≥ ≈), multiplication (× •), etc, which are not included in the very rudimentary and incomplete ASCII set.
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[edit] Usage
The purpose of using plain text today is primarily a "lowest common denominator" independence from programs that require their very own special encoding or formatting (with due sacrifices and limitations). Plain text files can be opened, read, and edited with text editors. Examples include Notepad, EditPlus (Windows), edlin (DOS), MS-DOS Editor, ed/vi/Emacs (Unix, Linux, and elsewhere), pico, nano, SimpleText (Mac OS), or TextEdit (Mac OS X). In practice many other computer programs are also capable of importing text without formatting. It can also be used by simple computer tools such as line printing text commands, in DOS and Windows type, and in Unix cat.
Plain text files are almost universal in programming, a source code file containing instructions in a programming language is almost always a plain text file. Plain text was also commonly used for configuration files, who were read for saved settings at the startup of a program. Nowadays XML is becoming a widespread replacement for plain text.
[edit] Related terms
The related term, plaintext, is most commonly used in a cryptographic context, while cleartext usually refers to lack of protection from eavesdropping. Usage of these terms is such that there is some confusion amongst them, especially among those new to computers, cryptography, or data communications.
[edit] Philosophy
This reveals that plain text is in fact the technical user's way to regard a file or a sequence of bytes. In this sense, there is no plain text, since bits in the computer are stored as states of latches, charges on transistor gates, magnetic or optic dots on a disk, intact or broken electrical connections, etc, and humans don't have an electromagnetic sense. The information must thus appear as text (on screen or on paper) in order to be text in this absolute sense of the word.
Plain text is a way to represent generic text without attributes such as fonts, subscripts, and boldface; due to this simplicity, it is readable and processable by almost any computer program. In a way a HTML, SGML and an XML file is regarded as plain text, since no control codes (see below) are used, but real structural tags are actually included in these formats. As regards to the SGML and XML author, these tags are "human readable" since that format author understands the structure by reading the format. This may illuminate the complications of the usage of terms within computer science: it's all about your relative view point.
[edit] Encoding
[edit] Character encodings
Text was once commonly encoded in ASCII, using 8 bits for one letter or other character, encoding 7 bits, allowing 128 values, and using the 8th as a checksum bit when transferring a file. This just allowed the ordinary Latin alphabet, transfer control codes, parentheses and interpunction, which annoyed especially Portuguese and Swedish computer users. Therefore, when data transfer became more stable, the remaining 128 values were encoded, everywhere differently, and in a way that made multilingual texts impossible to encode. At last Unicode was defined, which currently allows for 1,114,112 code values used for any modern text writing system, and a lot of extinct ones. For example Unicode codes Chinese, Hebrew, Cyrillic as well as Latin. Some of these text formats may be pretty complicated to process correctly, but they still contain no structural data, such as bold start and end markers, and are therefore plain text.
[edit] Control codes
The ASCII codes before Space, ' ', are not intended as displayable characters, but instead as control characters. They are used for a diversity of interpreted meanings, for example the code NULL (= 0, sometimes denoted Ctrl-@) is used as string end markers in the programming language C and successors. Most troublesome of these are the codes LF (= LINE FEED = 10 = 0AH) and CR (= CARRIAGE RETURN = 13 = 0DH). Windows and OS/2 require the sequence CR,LF to represent a newline, while Unix and relatives uses just the LF. This was once a slight problem when transferring files between Windows and Unices, but today most computer programs treat this seamlessly.
