BSD 2.11  man page for expr (bsd section 1) 
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EXPR(1) EXPR(1)
NAME
expr  evaluate arguments as an expression
SYNOPSIS
expr arg ...
DESCRIPTION
The arguments are taken as an expression. After evaluation, the result is written on the
standard output. Each token of the expression is a separate argument.
The operators and keywords are listed below. The list is in order of increasing prece
dence, with equal precedence operators grouped.
expr  expr
yields the first expr if it is neither null nor `0', otherwise yields the second
expr.
expr & expr
yields the first expr if neither expr is null or `0', otherwise yields `0'.
expr relop expr
where relop is one of < <= = != >= >, yields `1' if the indicated comparison is
true, `0' if false. The comparison is numeric if both expr are integers, otherwise
lexicographic.
expr + expr
expr  expr
addition or subtraction of the arguments.
expr * expr
expr / expr
expr % expr
multiplication, division, or remainder of the arguments.
expr : expr
The matching operator compares the string first argument with the regular expres
sion second argument; regular expression syntax is the same as that of ed(1). The
\(...\) pattern symbols can be used to select a portion of the first argument.
Otherwise, the matching operator yields the number of characters matched (`0' on
failure).
( expr )
parentheses for grouping.
Examples:
To add 1 to the Shell variable a:
a=`expr $a + 1`
To find the filename part (least significant part) of the pathname stored in variable a,
which may or may not contain `/':
expr $a : '.*/\(.*\)' '' $a
Note the quoted Shell metacharacters.
SEE ALSO
sh(1), test(1)
DIAGNOSTICS
Expr returns the following exit codes:
0 if the expression is neither null nor `0',
1 if the expression is null or `0',
2 for invalid expressions.
7th Edition April 29, 1985 EXPR(1) 



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