gawk: awk split records
4.1.1 Record Splitting with Standard 'awk'
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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. To use a different character for
the record separator, simply assign that character to the predefined
variable 'RS'.
Like any other variable, the value of 'RS' can be changed in the
'awk' program with the assignment operator, '=' (
Assignment
Ops). 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 is
processed, so that the very first record is read with the proper
separator. To do this, use the special 'BEGIN' pattern (
BEGIN/END). For example:
awk 'BEGIN { RS = "u" }
{ print $0 }' mail-list
changes the value of 'RS' to 'u', before reading any input. The new
value is a string whose first character is the letter "u"; as a result,
records are separated by the letter "u". Then the input file is read,
and the second rule in the 'awk' program (the action with no pattern)
prints each record. Because each 'print' statement adds a newline at
the end of its output, this 'awk' program copies the input with each 'u'
changed to a newline. Here are the results of running the program on
'mail-list':
$ awk 'BEGIN { RS = "u" }
> { print $0 }' mail-list
-| Amelia 555-5553 amelia.zodiac
-| sq
-| e@gmail.com F
-| Anthony 555-3412 anthony.assert
-| ro@hotmail.com A
-| Becky 555-7685 becky.algebrar
-| m@gmail.com A
-| Bill 555-1675 bill.drowning@hotmail.com A
-| Broderick 555-0542 broderick.aliq
-| otiens@yahoo.com R
-| Camilla 555-2912 camilla.inf
-| sar
-| m@skynet.be R
-| Fabi
-| s 555-1234 fabi
-| s.
-| ndevicesim
-| s@
-| cb.ed
-| F
-| J
-| lie 555-6699 j
-| lie.perscr
-| tabor@skeeve.com F
-| Martin 555-6480 martin.codicib
-| s@hotmail.com A
-| Sam
-| el 555-3430 sam
-| el.lanceolis@sh
-| .ed
-| A
-| Jean-Pa
-| l 555-2127 jeanpa
-| l.campanor
-| m@ny
-| .ed
-| R
-|
Note that the entry for the name 'Bill' is not split. In the original
data file (Sample Data Files), the line looks like this:
Bill 555-1675 bill.drowning@hotmail.com A
It contains no 'u', so there is no reason to split the record, unlike
the others, which each have one or more occurrences of the 'u'. In
fact, this record is treated as part of the previous record; the newline
separating them in the output is the original newline in the data file,
not the one added by 'awk' when it printed the record!
Another way to change the record separator is on the command line,
using the variable-assignment feature (Other Arguments):
awk '{ print $0 }' RS="u" mail-list
This sets 'RS' to 'u' before processing 'mail-list'.
Using an alphabetic character such as 'u' for the record separator is
highly likely to produce strange results. Using an unusual character
such as '/' is more likely to produce correct behavior in the majority
of cases, but there are no guarantees. The moral is: Know Your Data.
When using regular characters as the record separator, there is one
unusual case that occurs when 'gawk' is being fully POSIX-compliant
(Options). Then, the following (extreme) pipeline prints a
surprising '1':
$ echo | gawk --posix 'BEGIN { RS = "a" } ; { print NF }'
-| 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.
(In the normal case, 'gawk' treats the newline as whitespace, printing
'0' as the result. Most other versions of 'awk' also act this way.)
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 without any 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. Multiple Line 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, as well as 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'.
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gawk: Records
gawk: gawk split records