4.2 Examining Fields

When awk reads an input record, the record is automatically parsed or separated by the awk utility 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;(20) other characters, such as formfeed, vertical tab, etc., 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 want—but fields are what make simple awk programs so powerful.

A dollar-sign (‘$’) is used to refer to a field in an awk program, followed by the number of the field you want. Thus, $1 refers to the first field, $2 to the second, and so on. (Unlike the Unix shells, the field numbers are not limited to single digits. $127 is the one hundred twenty-seventh field in the record.) 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 automatically updates the value of NF each time it reads a record. No matter how many fields there are, the last field in a record can be represented by $NF. So, $NF is the same as $7, which is ‘example.’. 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. (If used in a numeric operation, you get zero.)

The use of $0, which looks like a reference to the “zero-th” field, is a special case: it represents the whole input record when you are not interested in specific 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 matching input record:

$ awk '/foo/ { print $1, $NF }' BBS-list
-| fooey B
-| foot B
-| macfoo A
-| sabafoo C