# Expr

**expr** is a command line utility which evaluates an expression and outputs the corresponding value.

*expr* evaluates integer or string expressions, including pattern matching regular expressions. 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 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 (the backslash escapes a less than from the shell) 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 return value—the program exit status—is zero for this example.)

## man page

Usage: expr EXPRESSION or: expr OPTION

Print the value of EXPRESSION to standard output. A blank line below separates increasing precedence groups. EXPRESSION may be:

ARG1 | ARG2 ARG1 if it is neither null nor 0, otherwise ARG2 ARG1 & ARG2 ARG1 if neither argument is null or 0, otherwise 0 ARG1 < ARG2 ARG1 is less than ARG2 ARG1 <= ARG2 ARG1 is less than or equal to ARG2 ARG1 = ARG2 ARG1 is equal to ARG2 ARG1 != ARG2 ARG1 is unequal to ARG2 ARG1 >= ARG2 ARG1 is greater than or equal to ARG2 ARG1 > ARG2 ARG1 is greater than ARG2 ARG1 + ARG2 arithmetic sum of ARG1 and ARG2 ARG1 - ARG2 arithmetic difference of ARG1 and ARG2 ARG1 * ARG2 arithmetic product of ARG1 and ARG2 ARG1 / ARG2 arithmetic quotient of ARG1 divided by ARG2 ARG1 % ARG2 arithmetic remainder of ARG1 divided by ARG2 STRING : REGEXP anchored pattern match of REGEXP in STRING match STRING REGEXP same as STRING : REGEXP substr STRING POS LENGTH substring of STRING, POS counted from 1 index STRING CHARS index in STRING where any CHARS is found, or 0 length STRING length of STRING + TOKEN interpret TOKEN as a string, even if it is a keyword like `match' or an operator like `/' ( EXPRESSION ) value of EXPRESSION

Beware that many operators need to be escaped or quoted for shells. Comparisons are arithmetic if both ARGs are numbers, else lexicographical. Pattern matches return the string matched between \( and \) or null; if \( and \) are not used, they return the number of characters matched or 0.