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Expressions as Action Statements
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   Expressions are the basic building block of `awk' actions.  An
expression evaluates to a value, which you can print, test, store in a
variable or pass to a function.  But beyond that, an expression can
assign a new value to a variable or a field, with an assignment
operator.

   An expression can serve as a statement on its own.  Most other kinds
of statements contain one or more expressions which specify data to be
operated on.  As in other languages, expressions in `awk' include
variables, array references, constants, and function calls, as well as
combinations of these with various operators.

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Constants
String, numeric, and regexp constants.
Variables
Variables give names to values for later use.
Arithmetic Ops
Arithmetic operations (`+', `-', etc.)
Concatenation
Concatenating strings.
Comparison Ops
Comparison of numbers and strings with `<', etc.
Boolean Ops
Combining comparison expressions using boolean operators `||' ("or"), `&&' ("and") and `!' ("not").
Assignment Ops
Changing the value of a variable or a field.
Increment Ops
Incrementing the numeric value of a variable.
Conversion
The conversion of strings to numbers and vice versa.
Values
The whole truth about numbers and strings.
Conditional Exp
Conditional expressions select between two subexpressions under control of a third subexpression.
Function Calls
A function call is an expression.
Precedence
How various operators nest.