expr

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: expr expression

The operators available

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

The following is an 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 length and substr commands is not recommended.

• List of Unix commands

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