4.1.1 Record Splitting with Standard awk

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, ‘=’ (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 is processed, so that the very first record is 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 = "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 (see section Data files for the Examples), 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 (see section Other Command-Line 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.

gawk allows RS to be a full regular expression (discussed shortly; see section Record Splitting with gawk). Even so, using a regular expression metacharacter, such as ‘.’ as the single character in the value of RS has no special effect: it is treated literally. This is required for backwards compatibility with both Unix awk and with POSIX.

When using regular characters as the record separator, there is one unusual case that occurs when gawk is being fully POSIX-compliant (see section Command-Line 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. 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, 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.