Vertical bar

Vertical bar, or pipe is the name of the ASCII character at position 124 (decimal). The character is depicted as either a solid vertical bar ("|") or a vertical bar with a break in the middle (broken bar "¦"). The character is usually depicted as a broken bar on IBM PC keyboards to distinguish it from other characters. However, in Unicode the "broken bar" is a separate character, U+00A6 (¦).

Contents

1 Usage: 1.1 Mathematics 1.2 Cursors 1.3 Wiki markup 1.4 Backus-Naur form 1.5 Unix 1.6 Regular Expression 1.7 Disjunction 1.8 Delimiter 2 Etymology

Usage:

Mathematics

The vertical bar is used as a mathematical symbol in

• absolute value: | x | , read "the absolute value (or modulus) of x".
• norms: | | (x₁,x₂) | | , 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: {x | x < 2}, 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: P(X | Y), read "the probability of X given Y".
• divisibility: a | b, read "a divides b".

Cursors

In text that cannot be edited but can be selected, for example, on a web page, the cursor can also be a vertical bar, possibly with serifs at the top and bottom.

Wiki markup

The vertical bar is used in internal links to separate a link from the displayed text. E.g., [[c programming language|C]] displays as C.

Backus-Naur form

In BNF 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>

Unix

A pipe is an operating system mechanism originating in Unix which allows the output of one process to be used as input to another. Used in a shell command, this is represented by the vertical bar character. See Pipe (Unix).

Regular Expression

In regular expressions, the pipe indicates alternation. E.g., grep 'foo|bar' matches lines containing 'foo' or 'bar'.

Disjunction

In the C, Perl, and Java programming languages, among many others, it is used to designate bitwise or (e.g. a|b) or doubled up logical or (e.g. a||b).

Delimiter

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

Etymology

The name pipe for the character "|" came into common usage due to the fact that it is used in the Unix shell to pipe the output of one program into another program.