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 predefined 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, ‘=’ (see 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
(see 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.)