Plain text

For the cryptography meaning, see Plaintext. For other uses, see Text (disambiguation).
Text file of The Human Side of Animals by Royal Dixon, displayed by the command cat in an xterm window

In computing, plain text is the data (e.g. file contents) that represent only characters of readable material but not its graphical representation nor other objects (images, etc.). It may also include a limited number of characters that control simple arrangement of text, such as line breaks or tabulation characters. Plain text is different from formatted text, where style information is included, and from "binary files" in which some portions must be interpreted as binary objects (encoded integers, real numbers, images, etc.).

The encoding has traditionally been either ASCII, sometimes EBCDIC. Unicode-based encodings such as UTF-8 and UTF-16 are gradually replacing the older ASCII derivatives limited to 7 or 8 bit codes.

Plain text and rich text

Files that contain markup or other meta-data are generally considered plain-text, as long as the entirety remains in directly human-readable form (as in HTML, XML, and so on (as Coombs, Renear, and DeRose argue,[1] punctuation is itself markup)). The use of plain text rather than bit-streams to express markup, enables files to survive much better "in the wild", in part by making them largely immune to computer architecture incompatibilities.

According to The Unicode Standard,

For instance, Rich text such as SGML, RTF, HTML, XML, and TEX relies on plain text. Wiki technology is another such example.

According to The Unicode Standard, plain text has two main properties in regard to rich text:

Plain text, the Unicode definition

Usage

The purpose of using plain text today is primarily independence from programs that require their very own special encoding or formatting. Plain text files can be opened, read, and edited with countless text editors and utilities.

A command-line interface allows people to give commands in plain text and get a response, also in plain text.

Many other computer programs are also capable of processing or creating plain text, such as countless programs in DOS, Windows, classic Mac OS, and Unix and its kin; as well as web browsers (a few browsers such as Lynx and the Line Mode Browser produce only plain text for display).

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 is also commonly used for configuration files, which are read for saved settings at the startup of a program, and for much e-mail.

A comment, a ".txt" file, or a TXT Record generally contains only plain text (without formatting) intended for humans to read.

Encoding

Character encodings

Main article: Character encoding

Before the early 1960s, computers were mainly used for number-crunching rather than for text, and memory was extremely expensive. Computers often allocated only 6 bits for each character, permitting only 64 characters—assigning codes for A-Z, a-z, and 0-9 would leave only 2 codes: nowhere near enough. Most computers opted not to support lower-case letters. Thus, early text projects such as Roberto Busa's Index Thomisticus, the Brown Corpus, and others had to resort to conventions such as keying an asterisk preceding letters actually intended to be upper-case.

Fred Brooks of IBM argued strongly for going to 8-bit bytes, because someday people might want to process text; and won. Although IBM used EBCDIC, most text from then on came to be encoded in ASCII, using values from 0 to 31 for (non-printing) control characters, and values from 32 to 127 for graphic characters such as letters, digits, and punctuation. Most machines stored characters in 8 bits rather than 7, ignoring the remaining bit or using it as a checksum.

The near-ubiquity of ASCII was a great help, but failed to address international and linguistic concerns. The dollar-sign ("$") was not so useful in England, and the accented characters used in Spanish, French, German, and many other languages were entirely unavailable in ASCII (not to mention characters used in Greek, Russian, and most Eastern languages). Many individuals, companies, and countries defined extra characters as needed—often reassigning control characters, or using value in the range from 128 to 255. Using values above 128 conflicts with using the 8th bit as a checksum, but the checksum usage gradually died out.

These additional characters were encoded differently in different countries, making texts impossible to decode without figuring out the originator's rules. For instance, a browser might display ¬A rather than ` if it tried to interpret one character set as another. The International Organisation for Standardisation (ISO) eventually developed several code pages under ISO 8859, to accommodate various languages. The first of these (ISO 8859-1) is also known as"Latin-1", and covers the needs of most (not all) European languages that use Latin-based characters (there was not quite enough room to cover them all). ISO 2022 then provided conventions for"switching" between different character sets in mid-file. Many other organisations developed variations on these, and for many years Windows and Macintosh computers used incompatible variations.

The text-encoding situation became more and more complex, leading to efforts by ISO and by the Unicode Consortium to develop a single, unified character encoding that could cover all known (or at least all currently known) languages. After some conflict, these efforts were unified. Unicode currently allows for 1,114,112 code values, and assigns codes covering nearly all modern text writing systems, as well as many historical ones and for many non-linguistic characters such as printer's dingbats, mathematical symbols, etc.

Text is considered plain-text regardless of its encoding. To properly understand or process it the recipient must know (or be able to figure out) what encoding was used; however, they need not know anything about the computer architecture that was used, or about the binary structures defined by whatever program (if any) created the data.

Control codes

Main article: Newline

The ASCII codes before SPACE (= 32 = 20H) are not intended as displayable characters, but instead as control characters. They are used for diverse 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 use just the LF, and Classic Mac OS (but not Mac OS X) uses just the code CR. This was once a slight problem when transferring files between Windows and Unix systems, but today most computer programs treat this seamlessly.

In 8-bit character sets such as Latin-1 and the other ISO 8859 sets, the first 32 characters of the"upper half" (128 to 159) are also control codes, known as the"C1 set" as opposed to the"C0" set just described. However, the commonplace Windows character set called code page 1252 assigns printing characters to these code points (other than this, cp1252 is the same as Latin-1). It is not uncommon that Web servers identify a document as being in Latin-1, when in fact it is in code page 1252, and uses characters in the C1 set as graphics. This may or may not lead to unexpected results.

See also

References

  1. ↑ Coombs, James H.; Renear, Allen H.; DeRose, Steven J. (November 1987). "Markup systems and the future of scholarly text processing". Communications of the ACM. ACM. 30 (11): 933–947. doi:10.1145/32206.32209.
  2. 1 2 The Unicode Standard, version 6.1, General Structure, page 14
  3. ↑ The Unicode Standard, version 6.1, General Structure, page 15
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