The previous subsection
discussed the use of single characters or simple strings as the
value of FS.
More generally, the value of FS may be a string containing any
regular expression. In this case, each match in the record for the regular
expression separates fields. For example, the assignment:
FS = ", \t"
makes every area of an input line that consists of a comma followed by a space and a TAB into a field separator.
For a less trivial example of a regular expression, try using
single spaces to separate fields the way single commas are used.
FS can be set to "[ ]" (left bracket, space, right
bracket). This regular expression matches a single space and nothing else
(see Regexp).
There is an important difference between the two cases of `FS = " "'
(a single space) and `FS = "[ \t\n]+"'
(a regular expression matching one or more spaces, tabs, or newlines).
For both values of FS, fields are separated by runs
(multiple adjacent occurrences) of spaces, tabs,
and/or newlines. However, when the value of FS is " ",
awk first strips leading and trailing whitespace from
the record and then decides where the fields are.
For example, the following pipeline prints `b':
$ echo ' a b c d ' | awk '{ print $2 }'
-| b
However, this pipeline prints `a' (note the extra spaces around each letter):
$ echo ' a b c d ' | awk 'BEGIN { FS = "[ \t\n]+" }
> { print $2 }'
-| a
In this case, the first field is null or empty.
The stripping of leading and trailing whitespace also comes into
play whenever $0 is recomputed. For instance, study this pipeline:
$ echo ' a b c d' | awk '{ print; $2 = $2; print }'
-| a b c d
-| a b c d
The first print statement prints the record as it was read,
with leading whitespace intact. The assignment to $2 rebuilds
$0 by concatenating $1 through $NF together,
separated by the value of OFS. Because the leading whitespace
was ignored when finding $1, it is not part of the new $0.
Finally, the last print statement prints the new $0.