Field Separators (The GNU Awk User’s Guide)
Next: Constant Size, Previous: Changing Fields, Up: Reading Files [Contents][Index]
4.5 Specifying How Fields Are Separated
• 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. |
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 section 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 section 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.)
Next: Constant Size, Previous: Changing Fields, Up: Reading Files [Contents][Index]