11-11-2013
A here document is something that is just read (and expanded) and presented to stdin, and not executed per se. So the echo won't do anything. In the expansion phase, command substitution takes place, and expr will be executed. Are your positional parameters set? I guess not. Try set -- 3 5 and run again - does it succeed?
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expr(1) General Commands Manual expr(1)
Name
expr - evaluate expressions
Syntax
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 The relop is one of < <= = != >= > and 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
Yields addition or subtraction of the arguments.
expr * expr
expr / expr
expr % expr
Yields multiplication, division, or remainder of the arguments.
expr : expr The matching operator compares the string first argument with the regular expression second argument; regular expres-
sion syntax is the same as that of 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
The first example adds 1 to the Shell variable a:
a=`expr $a + 1`
The second example finds the file name part (least significant part) of the pathname stored in variable a,
expr $a : '.*/(.*)' '|' $a
Note the quoted Shell metacharacters.
Diagnostics
The command returns the following exit codes:
0 The expression is neither null nor '0'.
1 The expression is null or '0'.
2 The expression is invalid.
See Also
ed(1), sh(1), test(1)
expr(1)