Operator precedence determines how operators are grouped, when
different operators appear close by in one expression. For example,
`*' has higher precedence than `+'; thus, `a + b * c'
means to multiply b
and c
, and then add a
to the
product (i.e. `a + (b * c)').
You can overrule the precedence of the operators by using parentheses. You can think of the precedence rules as saying where the parentheses are assumed to be if you do not write parentheses yourself. In fact, it is wise to always use parentheses whenever you have an unusual combination of operators, because other people who read the program may not remember what the precedence is in this case. You might forget, too; then you could make a mistake. Explicit parentheses will help prevent any such mistake.
When operators of equal precedence are used together, the leftmost operator groups first, except for the assignment, conditional and exponentiation operators, which group in the opposite order. Thus, `a - b + c' groups as `(a - b) + c', and `a = b = c' groups as `a = (b = c)'.
The precedence of prefix unary operators does not matter as long as only unary operators are involved, because there is only one way to interpret them--innermost first. Thus, `$++i' means `$(++i)' and `++$x' means `++($x)'. However, when another operator follows the operand, then the precedence of the unary operators can matter. Thus, `$x^2' means `($x)^2', but `-x^2' means `-(x^2)', because `-' has lower precedence than `^' while `$' has higher precedence.
Here is a table of awk
's operators, in order from highest
precedence to lowest:
(...)
$
++ --
^ **
+ - !
* / %
+ -
Concatenation
< <= == !=
> >= >> |
print
and printf
statements belong to the statement level, not to expressions. The
redirection does not produce an expression which could be the operand of
another operator. As a result, it does not make sense to use a
redirection operator near another operator of lower precedence, without
parentheses. Such combinations, for example `print foo > a ? b : c',
result in syntax errors.
The correct way to write this statement is `print foo > (a ? b : c)'.
~ !~
in
&&
||
?:
= += -= *=
/= %= ^= **=
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