expr

expr

Example of expr command that does basic summation
Original author(s)	Dick Haight (AT&T Bell Laboratories)
Developer(s)	Various open-source and commercial developers
Initial release	1979; 43 years ago (1979)
Operating system	Unix, Unix-like, IBM i
Platform	Cross-platform
Type	Command

expr is a command line utility on Unix and Unix-like operating systems which evaluates an expression and outputs the corresponding value. It first appeared in Unix v7. The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities.^[1] The expr command has also been ported to the IBM i operating system.^[2]

Overview[]

expr evaluates integer or string expressions, including pattern matching regular expressions. Each symbol (operator, value, etc.) in the expression must be given as a separate parameter. Most of the challenge posed in writing expressions is preventing the invoking command line shell from acting on characters intended for expr to process.

Syntax[]

Syntax: expr expression

The operators available

• for integers: addition, subtraction, multiplication, division and modulus
• for strings: match a regular expression; in some versions: find a set of characters in a string ("index"), find substring ("substr"), length of string ("length")
• for either: comparison (equal, not equal, less than, etc.)

Example[]

The following is a (non-POSIX-compliant) example involving boolean expressions:

expr length  "abcdef"  "<"  5  "|"  15  -  4  ">"  8

This example 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 is often more convenient to use bc. For example:

echo "3 * 4 + 14 / 2" | bc

since it accepts the expression as a single argument.

For portable shell programming, use of the "index", "length", "match" and "substr" commands must be avoided; string matching remains possible but it must use the "string : regexp" syntax.

See also[]

• List of Unix commands

References[]

External links[]

The Wikibook Guide to Unix has a page on the topic of: Commands

• expr: evaluate arguments as an expression – Commands & Utilities Reference, The Single UNIX Specification, Issue 7 from The Open Group
• expr invocation in GNU coreutils manual

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