The awk utility divides the input for your awk
program into records and fields.
Records are separated by a character called the record separator.
By default, the record separator is the newline character.
This is why records are, by default, single lines.
You can use a different character for the record separator by
assigning the character to the built-in variable RS.
You can change the value of RS in the awk program,
like any other variable, with the
assignment operator, `=' (see section Assignment Expressions).
The new record-separator character should be enclosed in quotation marks,
which indicate
a string constant. Often the right time to do this is at the beginning
of execution, before any input has been processed, so that the very
first record will be read with the proper separator. To do this, use
the special BEGIN pattern
(see section The BEGIN and END Special Patterns). For
example:
awk 'BEGIN { RS = "/" } ; { print $0 }' BBS-list
changes the value of RS to "/", before reading any input.
This is a string whose first character is a slash; as a result, records
are separated by slashes. Then the input file is read, and the second
rule in the awk program (the action with no pattern) prints each
record. Since each print statement adds a newline at the end of
its output, the effect of this awk program is to copy the input
with each slash changed to a newline. Here are the results of running
the program on `BBS-list':
$ awk 'BEGIN { RS = "/" } ; { print $0 }' BBS-list
-| aardvark 555-5553 1200
-| 300 B
-| alpo-net 555-3412 2400
-| 1200
-| 300 A
-| barfly 555-7685 1200
-| 300 A
-| bites 555-1675 2400
-| 1200
-| 300 A
-| camelot 555-0542 300 C
-| core 555-2912 1200
-| 300 C
-| fooey 555-1234 2400
-| 1200
-| 300 B
-| foot 555-6699 1200
-| 300 B
-| macfoo 555-6480 1200
-| 300 A
-| sdace 555-3430 2400
-| 1200
-| 300 A
-| sabafoo 555-2127 1200
-| 300 C
-|
Note that the entry for the `camelot' BBS is not split. In the original data file (see section Data Files for the Examples), the line looks like this:
camelot 555-0542 300 C
It only has one baud rate; there are no slashes in the record.
Another way to change the record separator is on the command line, using the variable-assignment feature (see section Other Command Line Arguments).
awk '{ print $0 }' RS="/" BBS-list
This sets RS to `/' before processing `BBS-list'.
Using an unusual character such as `/' for the record separator
produces correct behavior in the vast majority of cases. However,
the following (extreme) pipeline prints a surprising `1'. There
is one field, consisting of a newline. The value of the built-in
variable NF is the number of fields in the current record.
$ echo | awk 'BEGIN { RS = "a" } ; { print NF }'
-| 1
Reaching the end of an input file terminates the current input record,
even if the last character in the file is not the character in RS
(d.c.).
The empty string, "" (a string of no characters), has a special meaning
as the value of RS: it means that records are separated
by one or more blank lines, and nothing else.
See section Multiple-Line Records, for more details.
If you change the value of RS in the middle of an awk run,
the new value is used to delimit subsequent records, but the record
currently being processed (and records already processed) are not
affected.
After the end of the record has been determined, gawk
sets the variable RT to the text in the input that matched
RS.
The value of RS is in fact not limited to a one-character
string. It can be any regular expression
(see section Regular Expressions).
In general, each record
ends at the next string that matches the regular expression; the next
record starts at the end of the matching string. This general rule is
actually at work in the usual case, where RS contains just a
newline: a record ends at the beginning of the next matching string (the
next newline in the input) and the following record starts just after
the end of this string (at the first character of the following line).
The newline, since it matches RS, is not part of either record.
When RS is a single character, RT will
contain the same single character. However, when RS is a
regular expression, then RT becomes more useful; it contains
the actual input text that matched the regular expression.
The following example illustrates both of these features.
It sets RS equal to a regular expression that
matches either a newline, or a series of one or more upper-case letters
with optional leading and/or trailing white space
(see section Regular Expressions).
$ echo record 1 AAAA record 2 BBBB record 3 |
> gawk 'BEGIN { RS = "\n|( *[[:upper:]]+ *)" }
> { print "Record =", $0, "and RT =", RT }'
-| Record = record 1 and RT = AAAA
-| Record = record 2 and RT = BBBB
-| Record = record 3 and RT =
-|
The final line of output has an extra blank line. This is because the
value of RT is a newline, and then the print statement
supplies its own terminating newline.
See section A Simple Stream Editor, for a more useful example
of RS as a regexp and RT.
The use of RS as a regular expression and the RT
variable are gawk extensions; they are not available in
compatibility mode
(see section Command Line Options).
In compatibility mode, only the first character of the value of
RS is used to determine the end of the record.
The awk utility keeps track of the number of records that have
been read so far from the current input file. This value is stored in a
built-in variable called FNR. It is reset to zero when a new
file is started. Another built-in variable, NR, is the total
number of input records read so far from all data files. It starts at zero
but is never automatically reset to zero.
Go to the first, previous, next, last section, table of contents.