When awk reads an input record, the record is
automatically separated or parsed by the interpreter into chunks
called fields. By default, fields are separated by whitespace,
like words in a line.
Whitespace in awk means any string of one or more spaces,
tabs or newlines;(5) other characters such as
formfeed, and so on, that are
considered whitespace by other languages are not considered
whitespace by awk.
The purpose of fields is to make it more convenient for you to refer to
these pieces of the record. You don't have to use them--you can
operate on the whole record if you wish--but fields are what make
simple awk programs so powerful.
To refer to a field in an awk program, you use a dollar-sign,
`$', followed by the number of the field you want. Thus, $1
refers to the first field, $2 to the second, and so on. For
example, suppose the following is a line of input:
This seems like a pretty nice example.
Here the first field, or $1, is `This'; the second field, or
$2, is `seems'; and so on. Note that the last field,
$7, is `example.'. Because there is no space between the
`e' and the `.', the period is considered part of the seventh
field.
NF is a built-in variable whose value
is the number of fields in the current record.
awk updates the value of NF automatically, each time
a record is read.
No matter how many fields there are, the last field in a record can be
represented by $NF. So, in the example above, $NF would
be the same as $7, which is `example.'. Why this works is
explained below (see section Non-constant Field Numbers).
If you try to reference a field beyond the last one, such as $8
when the record has only seven fields, you get the empty string.
$0, which looks like a reference to the "zeroth" field, is
a special case: it represents the whole input record. $0 is
used when you are not interested in fields.
Here are some more examples:
$ awk '$1 ~ /foo/ { print $0 }' BBS-list
-| fooey 555-1234 2400/1200/300 B
-| foot 555-6699 1200/300 B
-| macfoo 555-6480 1200/300 A
-| sabafoo 555-2127 1200/300 C
This example prints each record in the file `BBS-list' whose first
field contains the string `foo'. The operator `~' is called a
matching operator
(see section How to Use Regular Expressions);
it tests whether a string (here, the field $1) matches a given regular
expression.
By contrast, the following example looks for `foo' in the entire record and prints the first field and the last field for each input record containing a match.
$ awk '/foo/ { print $1, $NF }' BBS-list
-| fooey B
-| foot B
-| macfoo A
-| sabafoo C
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