quck question about expr and (( ))

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LEARN ABOUT V7

expr

EXPR(1) 						      General Commands Manual							   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 precedence, 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 expression 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

       ed(1), 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.

																	   EXPR(1)

Shell Programming and Scripting