gawk: Fields
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; other characters that are considered whitespace by other
languages (such as formfeed, vertical tab, etc.) 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.
You use a dollar sign ('$') 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 in the Unix shells,
the field numbers are not limited to single digits. '$127' is the 127th
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, '$7', is 'example.'.
Because there is no space between the 'e' and the '.', the period is
considered part of the seventh field.
'NF' is a predefined variable whose value is the number of fields in
the current record. 'awk' automatically updates the value of 'NF' each
time it reads a record. No matter how many fields there are, the last
field in a record can be represented by '$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 "zeroth" field,
is a special case: it represents the whole input record. Use it when
you are not interested in specific fields. Here are some more examples:
$ awk '$1 ~ /li/ { print $0 }' mail-list
-| Amelia 555-5553 amelia.zodiacusque@gmail.com F
-| Julie 555-6699 julie.perscrutabor@skeeve.com F
This example prints each record in the file 'mail-list' whose first
field contains the string 'li'.
By contrast, the following example looks for 'li' in _the entire
record_ and prints the first and last fields for each matching input
record:
$ awk '/li/ { print $1, $NF }' mail-list
-| Amelia F
-| Broderick R
-| Julie F
-| Samuel A
Info Catalog
gawk: Records
gawk: Reading Files
gawk: Nonconstant Fields