Expr
Encyclopedia
expr is a command line Unix
utility which evaluates an expression
and outputs the corresponding value. It first appeared in Unix v7
as a standalone program, and was later incorporated into the shell as a built-in command.
Syntax: expr (expression)
expr evaluates integer
or string
expressions, including pattern matching regular expression
s. Most of the challenge posed in writing expressions is preventing the command line shell from acting on characters intended for expr to process.
The operators available
Also, boolean
expressions involving and and or, such as
expr length "abcdef" "<" 5 "|" 15 - 4 ">" 8
outputs "1". This is because length "abcdef" is 6, which is not less than 5 (so the left side of the | returns zero). But 15 minus 4 is 11 and is greater than 8, so the right side is true, which makes the or true, so 1 is the result. The program exit status is zero for this example.
For pure arithmetic, it it often more convenient to use bc
. For example:
echo "3*4+14/2" | bc
since it accepts the expression as a single argument.
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
utility which evaluates an expression
Expression (mathematics)
In mathematics, an expression is a finite combination of symbols that is well-formed according to rules that depend on the context. Symbols can designate numbers , variables, operations, functions, and other mathematical symbols, as well as punctuation, symbols of grouping, and other syntactic...
and outputs the corresponding value. It first appeared in Unix v7
Version 7 Unix
Seventh Edition Unix, also called Version 7 Unix, Version 7 or just V7, was an important early release of the Unix operating system. V7, released in 1979, was the last Bell Laboratories release to see widespread distribution before the commercialization of Unix by AT&T in the early 1980s...
as a standalone program, and was later incorporated into the shell as a built-in command.
Syntax: expr (expression)
expr evaluates integer
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...
or string
String (computer science)
In formal languages, which are used in mathematical logic and theoretical computer science, a string is a finite sequence of symbols that are chosen from a set or alphabet....
expressions, including pattern matching regular expression
Regular expression
In computing, a regular expression provides a concise and flexible means for "matching" strings of text, such as particular characters, words, or patterns of characters. Abbreviations for "regular expression" include "regex" and "regexp"...
s. Most of the challenge posed in writing expressions is preventing the command line shell from acting on characters intended for expr to process.
The operators available
- for integers: addition, subtraction, multiplication, division and modulus
- for strings: find substring, find regular expression, find a set of characters in a string, length of string
- for either: comparison (equal, not equal, less than, etc.)
Also, boolean
Boolean logic
Boolean algebra is a logical calculus of truth values, developed by George Boole in the 1840s. It resembles the algebra of real numbers, but with the numeric operations of multiplication xy, addition x + y, and negation −x replaced by the respective logical operations of...
expressions involving and and or, such as
expr length "abcdef" "<" 5 "|" 15 - 4 ">" 8
outputs "1". This is because length "abcdef" is 6, which is not less than 5 (so the left side of the | returns zero). But 15 minus 4 is 11 and is greater than 8, so the right side is true, which makes the or true, so 1 is the result. The program exit status is zero for this example.
For pure arithmetic, it it often more convenient to use bc
Bc programming language
bc, for bench calculator, is "an arbitrary precision calculator language" with syntax similar to the C programming language. bc is typically used as either a mathematical scripting language or as an interactive mathematical shell....
. For example:
echo "3*4+14/2" | bc
since it accepts the expression as a single argument.