1 1 4.5 Specifying How Fields Are Separated 1 ======================================= 1
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· Default Field Splitting How fields are normally 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.
· Full Line Fields Making the full line be a single field.
· Field Splitting Summary Some final points and a summary table.
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1 The "field separator", which is either a single character or a regular
1 expression, controls the way 'awk' splits an input record into fields.
1 'awk' scans the input record for character sequences that match the
1 separator; the fields themselves are the text between the matches.
1
1 In the examples that follow, we use the bullet symbol (*) to
1 represent spaces in the output. If the field separator is 'oo', then
1 the following line:
1
1 moo goo gai pan
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1 is split into three fields: 'm', '*g', and '*gai*pan'. Note the leading
1 spaces in the values of the second and third fields.
1
1 The field separator is represented by the predefined variable 'FS'.
1 Shell programmers take note: 'awk' does _not_ use the name 'IFS' that is
1 used by the POSIX-compliant shells (such as the Unix Bourne shell, 'sh',
1 or Bash).
1
1 The value of 'FS' can be changed in the 'awk' program with the
1 assignment operator, '=' (⇒Assignment Ops). Often, the right
1 time to do this is at the beginning of execution before any input has
1 been processed, so that the very first record is read with the proper
11 separator. To do this, use the special 'BEGIN' pattern (⇒
BEGIN/END). For example, here we set the value of 'FS' to the string
1 '","':
1
1 awk 'BEGIN { FS = "," } ; { print $2 }'
1
1 Given the input line:
1
1 John Q. Smith, 29 Oak St., Walamazoo, MI 42139
1
1 this 'awk' program extracts and prints the string '*29*Oak*St.'.
1
1 Sometimes the input data contains separator characters that don't
1 separate fields the way you thought they would. For instance, the
1 person's name in the example we just used might have a title or suffix
1 attached, such as:
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1 John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139
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1 The same program would extract '*LXIX' instead of '*29*Oak*St.'. If you
1 were expecting the program to print the address, you would be surprised.
1 The moral is to choose your data layout and separator characters
1 carefully to prevent such problems. (If the data is not in a form that
1 is easy to process, perhaps you can massage it first with a separate
1 'awk' program.)
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