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A field number need not be a constant. Any expression in
the awk
language can be used after a ‘$’ to refer to a
field. The value of the expression specifies the field number. If the
value is a string, rather than a number, it is converted to a number.
Consider this example:
awk '{ print $NR }'
Recall that NR
is the number of records read so far: one in the
first record, two in the second, and so on. So this example prints the first
field of the first record, the second field of the second record, and so
on. For the twentieth record, field number 20 is printed; most likely,
the record has fewer than 20 fields, so this prints a blank line.
Here is another example of using expressions as field numbers:
awk '{ print $(2*2) }' mail-list
awk
evaluates the expression ‘(2*2)’ and uses
its value as the number of the field to print. The ‘*’
represents multiplication, so the expression ‘2*2’ evaluates to four.
The parentheses are used so that the multiplication is done before the
‘$’ operation; they are necessary whenever there is a binary
operator21
in the field-number expression. This example, then, prints the
type of relationship (the fourth field) for every line of the file
mail-list. (All of the awk
operators are listed, in
order of decreasing precedence, in
Operator Precedence (How Operators Nest).)
If the field number you compute is zero, you get the entire record.
Thus, ‘$(2-2)’ has the same value as $0
. Negative field
numbers are not allowed; trying to reference one usually terminates
the program. (The POSIX standard does not define
what happens when you reference a negative field number. gawk
notices this and terminates your program. Other awk
implementations may behave differently.)
As mentioned in Examining Fields,
awk
stores the current record’s number of fields in the built-in
variable NF
(also see section Predefined Variables). Thus, the expression
$NF
is not a special feature—it is the direct consequence of
evaluating NF
and using its value as a field number.
A binary operator, such as ‘*’ for
multiplication, is one that takes two operands. The distinction
is required because awk
also has unary (one-operand)
and ternary (three-operand) operators.
Next: Changing Fields, Previous: Fields, Up: Reading Files [Contents][Index]