Node:Field Separators, Next:Constant Size, Previous:Changing Fields, Up:Reading Files

Specifying How Fields Are Separated

• Regexp Field Splitting: Using regexps as the field separator.
• Single Character Fields: Making each character a separate field.
• Command Line Field Separator: Setting FS from the command-line.
• Field Splitting Summary: Some final points and a summary table.

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, = (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.)

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.