Vertical bar

Punctuation

Note: "broken bar" and the glyph "¦" redirect here.

Vertical bar, verti-bar, vertical line, divider line, or pipe is the name of the character (|). Broken bar (¦) is a separate character. The typical keyboard used in the United Kingdom features separate keys for vertical bar and broken bar, however most UK keyboard drivers today map both keys onto the vertical bar, as broken bar has hardly any practical application. Keyboards in other countries do not make this distinction, and there, the vertical bar character is often depicted as a broken bar, to distinguish it from the "I" (uppercase "i") character.

Etymology

The name pipe for the character "|" was a natural consequence of its usage in Unix shells, where it is used to notate the usage of the unix pipeline construct.

Usage

Mathematics

The vertical bar is used as a mathematical symbol in

• absolute value: <math>|x|</math>, read "the absolute value of x".
• norms: <math>\|(x_1,x_2)\|</math>, read "the norm of x sub one, x sub two"; though Unicode also provides a special double vertical line symbol U+2016: ‖x‖
• set definition: <math>\{x|x<2\}</math>, read "the set of x such that x is less than two". Often sets are written with a colon ':' rather than a vertical bar.
• conditional probability: <math>P(X|Y)</math>, read "the probability of X given Y".
• divisibility: <math>a | b</math>, read "a divides b".
• the Sheffer stroke.
• evaluate: <math>a+3|a=4</math>, read "a plus 3 evaluated for when a equals 4", or <math>4+3=7</math>.

Backus-Naur form

In Backus-Naur form the expression consists of sequences of symbols and/or sequences separated by '|', indicating a choice, the whole being a possible substitution for the symbol on the left.

<personal-name> ::= <name> | <initial>

Computing

Unix

A pipe is an inter-process communication mechanism originating in Unix which allows the output of one process to be used as input to another. In most Unix shells (command interpreters), this is represented by the vertical bar character. This feature is found in most versions of DOS (and hence Windows) as well.

Regular expression

In regular expression syntax, the pipe indicates alternation. For example: the Unix command grep -E 'foo|bar' matches lines containing 'foo' or 'bar'.

Disjunction

In many programming languages, it is used to designate the logic operation or, either bitwise or or logical or.

Specifically, in C and other languages following C syntax, such as Perl, Java and C#, (a | b) denotes a bitwise or; whilst a double vertical bar (a || b) denotes a (short-circuited) logical or.

Concatenation

In PL/I and SQL, the operator "||" denotes string concatenation.

Delimiter

Although not as common as commas or tabs, the vertical bar can be used as a delimiter in a flat file. An example of a pipe-delimited standard data format is LEDES 1998B.

Concurrency operator

In calculi of communicating processes (like Pi-calculus), the vertical bar is used to indicate that processes execute in parallel.

Khoisan orthography

In the Khoisan languages, the pipe is used to write the dental click.

MediaWiki

The vertical bar is made use of in Wiki markup:

1. to separate parameters in MediaWiki templates;
2. to pipe links, displaying a token different from the article title;
3. to separate cells in table layout.

Encoding

Despite the vertical bar being an accessible keystroke on typical North American and UK keyboards, there are cases where Web designers, Wiki editors, computer programmers, and others need to encode the vertical bar — usually because the vertical bar is a special symbol in their source code and yet a literal vertical bar needs to be displayed to the end user.

Set forth in 1963, American Standard Code for Information Interchange (ASCII-1963), one of the first widely used character maps only had 69 display ("printable") characters; and A–Z, and 0–9 accounts for 36 of those characters. Vertical bar ("|") is the ASCII-1963 character at position 124 (decimal). Broken bar ("¦") is not part of the ASCII character set (any version), but a separate character that appeared (along with vertical bar) first in the EBCDIC family of character sets, and was copied from there into ISO 8859-1 and Unicode. The typical computer keyboard used in the United Kingdom features separate keys for "vertical bar" and "broken bar", however most UK keyboard drivers today map both keys onto the ASCII character "vertical bar", as "broken bar" has hardly any practical application.

In common character maps

Unicode

hexidecimal (base-16): 007C

UTF-8

hexidecimal (base-16): 7C

ASCII

decimal (base-10): 124, or hexidecimal (base-16): 7C

ISO/IEC 8859-1

hexidecimal (base-16): 7C

Shift-JIS Men-Ku-Ten

1-01-35

EBCDIC (CCSID 500 variant)

hexidecimal (base-16): BB

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