The number of a field does not need to 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, etc. 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 $(2*2) }' BBS-list
awk evaluates the expression `(2*2)' and uses its value as the number of the field to print. The `*' sign represents multiplication, so the expression `2*2' evaluates to four. The parentheses are used so that the multiplication is done before the `$' operation; they are necessary whenever there is a binary operator in the field-number expression. This example, then, prints the hours of operation (the fourth field) for every line of the file BBS-list. (All of the awk operators are listed, in order of decreasing precedence, in Precedence.)
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 Fields,
awk stores the current record's number of fields in the built-in
variable NF (also see Built-in Variables). The expression
$NF is not a special feature—it is the direct consequence of
evaluating NF and using its value as a field number.