(gawk.info)Field Separators

   The "field separator", which is either a single character or a
regular expression, controls the way `awk' splits an input record into
fields.  `awk' scans the input record for character sequences that
match the separator; the fields themselves are the text between the
matches.

   In the examples that follow, we use the bullet symbol (*) to
represent spaces in the output.  If the field separator is `oo', then
the following line:

     moo goo gai pan

is split into three fields: `m', `*g', and `*gai*pan'.  Note the
leading spaces in the values of the second and third fields.

   The field separator is represented by the built-in variable `FS'.
Shell programmers take note:  `awk' does _not_ use the name `IFS' that
is used by the POSIX-compliant shells (such as the Unix Bourne shell,
`sh', or `bash').

   The value of `FS' can be changed in the `awk' program with the
assignment operator, `=' (Note: Assignment Expressions.
).  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 is
read with the proper separator.  To do this, use the special `BEGIN'
pattern (Note: The `BEGIN' and `END' Special Patterns.).
For example, here we set the value of `FS' to the string `","':

     awk 'BEGIN { FS = "," } ; { print $2 }'

Given the input line:

     John Q. Smith, 29 Oak St., Walamazoo, MI 42139

this `awk' program extracts and prints the string `*29*Oak*St.'.

   Sometimes the input data contains separator characters that don't
separate fields the way you thought they would.  For instance, the
person's name in the example we just used might have a title or suffix
attached, such as:

     John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139

The same program would extract `*LXIX', instead of `*29*Oak*St.'.  If
you were expecting the program to print the address, you would be
surprised.  The moral is to choose your data layout and separator
characters carefully to prevent such problems.  (If the data is not in
a form that is easy to process, perhaps you can massage it first with a
separate `awk' program.)

   Fields are normally separated by whitespace sequences (spaces, tabs,
and newlines), not by single spaces.  Two spaces in a row do not
delimit an empty field.  The default value of the field separator `FS'
is a string containing a single space, `" "'.  If `awk' interpreted
this value in the usual way, each space character would separate
fields, so two spaces in a row would make an empty field between them.
The reason this does not happen is that a single space as the value of
`FS' is a special case--it is taken to specify the default manner of
delimiting fields.

   If `FS' is any other single character, such as `","', then each
occurrence of that character separates two fields.  Two consecutive
occurrences delimit an empty field.  If the character occurs at the
beginning or the end of the line, that too delimits an empty field.  The
space character is the only single character that does not follow these
rules.