/usr/bin/expr

Print the value of EXPRESSION to standard output

Usage: expr [EXPRESSION]
       expr [OPTIONS]

Arguments:
  [expression]...

Options:
      --version  output version information and exit
      --help     display this help and exit

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.

Exit status is 0 if EXPRESSION is neither null nor 0, 1 if EXPRESSION
is null or 0, 2 if EXPRESSION is syntactically invalid, and 3 if an
error occurred.

Environment variables:

  - EXPR_DEBUG_TOKENS=1: dump expression's tokens
  - EXPR_DEBUG_RPN=1: dump expression represented in reverse polish notation
  - EXPR_DEBUG_SYA_STEP=1: dump each parser step
  - EXPR_DEBUG_AST=1: dump expression represented abstract syntax tree

Errors are reported with the expression echoed back and a caret under the
offending argument whenever stderr is a terminal; anything else reading
stderr gets the usual single-line message.