Node:Multiple Line, Next:Getline, Previous:Constant Size, Up:Reading Files
In some databases, a single line cannot conveniently hold all the information in one entry. In such cases, you can use multiline records. The first step in doing this is to choose your data format.
One technique is to use an unusual character or string to separate
records. For example, you could use the formfeed character (written
\f in awk, as in C) to separate them, making each record
a page of the file. To do this, just set the variable RS to
"\f" (a string containing the formfeed character). Any
other character could equally well be used, as long as it won't be part
of the data in a record.
Another technique is to have blank lines separate records. By a special
dispensation, an empty string as the value of RS indicates that
records are separated by one or more blank lines. When RS is set
to the empty string, each record always ends at the first blank line
encountered. The next record doesn't start until the first nonblank
line that follows. No matter how many blank lines appear in a row, they
all act as one record separator.
(Blank lines must be completely empty; lines that contain only
whitespace do not count.)
You can achieve the same effect as RS = "" by assigning the
string "\n\n+" to RS. This regexp matches the newline
at the end of the record and one or more blank lines after the record.
In addition, a regular expression always matches the longest possible
sequence when there is a choice
(see How Much Text Matches?).
So the next record doesn't start until
the first nonblank line that follows--no matter how many blank lines
appear in a row, they are considered one record separator.
There is an important difference between RS = "" and
RS = "\n\n+". In the first case, leading newlines in the input
data file are ignored, and if a file ends without extra blank lines
after the last record, the final newline is removed from the record.
In the second case, this special processing is not done.
(d.c.)
Now that the input is separated into records, the second step is to
separate the fields in the record. One way to do this is to divide each
of the lines into fields in the normal manner. This happens by default
as the result of a special feature. When RS is set to the empty
string, the newline character always acts as a field separator.
This is in addition to whatever field separations result from FS.
The original motivation for this special exception was probably to provide
useful behavior in the default case (i.e., FS is equal
to " "). This feature can be a problem if you really don't
want the newline character to separate fields, because there is no way to
prevent it. However, you can work around this by using the split
function to break up the record manually
(see String Manipulation Functions).
Another way to separate fields is to
put each field on a separate line: to do this, just set the
variable FS to the string "\n". (This simple regular
expression matches a single newline.)
A practical example of a data file organized this way might be a mailing
list, where each entry is separated by blank lines. Consider a mailing
list in a file named addresses, which looks like this:
Jane Doe 123 Main Street Anywhere, SE 12345-6789 John Smith 456 Tree-lined Avenue Smallville, MW 98765-4321 ...
A simple program to process this file is as follows:
# addrs.awk --- simple mailing list program
# Records are separated by blank lines.
# Each line is one field.
BEGIN { RS = "" ; FS = "\n" }
{
print "Name is:", $1
print "Address is:", $2
print "City and State are:", $3
print ""
}
Running the program produces the following output:
$ awk -f addrs.awk addresses -| Name is: Jane Doe -| Address is: 123 Main Street -| City and State are: Anywhere, SE 12345-6789 -| -| Name is: John Smith -| Address is: 456 Tree-lined Avenue -| City and State are: Smallville, MW 98765-4321 -| ...
See Printing Mailing Labels, for a more realistic
program that deals with address lists.
The following
table
summarizes how records are split, based on the
value of
RS:
RS == "\n"
\n). In effect,
every line in the data file is a separate record, including blank lines.
This is the default.
RS == any single character
RS == ""
FS may have. Leading and trailing newlines in a file are ignored.
RS == regexp
gawk extension; it is not specified by the
POSIX standard.)
In all cases, gawk sets RT to the input text that matched the
value specified by RS.