GAWK: Effective AWK Programming

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42 GAWK: Effective AWK Programminghighest field you create. The exact format of $0 is also affected by a feature that hasnot been discussed yet: the output field separator, OFS, used to separate the fields (seeSection 4.3 [Output Separators], page 60).Note, however, that merely referencing an out-of-range field does not change the valueof either $0 or NF. Referencing an out-of-range field only produces an empty string. Forexample:if ($(NF+1) != "")print "can’t happen"elseprint "everything is normal"should print ‘everything is normal’, because NF+1 is certain to be out of range. (SeeSection 6.4.1 [The if-else Statement], page 102, for more information about awk’s ifelsestatements. See Section 5.10 [Variable Typing and Comparison Expressions], page 87,for more information about the ‘!=’ operator.)It is important to note that making an assignment to an existing field changes the valueof $0 but does not change the value of NF, even when you assign the empty string to a field.For example:$ echo a b c d | awk ’{ OFS = ":"; $2 = ""> print $0; print NF }’⊣ a::c:d⊣ 4The field is still there; it just has an empty value, denoted by the two colons between ‘a’and ‘c’. This example shows what happens if you create a new field:$ echo a b c d | awk ’{ OFS = ":"; $2 = ""; $6 = "new"> print $0; print NF }’⊣ a::c:d::new⊣ 6The intervening field, $5, is created with an empty value (indicated by the second pair ofadjacent colons), and NF is updated with the value six.Decrementing NF throws away the values of the fields after the new value of NF andrecomputes $0. Here is an example:$ echo a b c d e f | awk ’{ print "NF =", NF;> NF = 3; print $0 }’⊣ NF = 6⊣ a b cCaution: Some versions of awk don’t rebuild $0 when NF is decremented. Caveat emptor.Finally, there are times when it is convenient to force awk to rebuild the entire record,using the current value of the fields and OFS. To do this, use the seemingly innocuousassignment:$1 = $1 # force record to be reconstitutedprint $0 # or whatever else with $0This forces awk rebuild the record. It does help to add a comment, as we’ve shown here.There is a flip side to the relationship between $0 and the fields. Any assignment to$0 causes the record to be reparsed into fields using the current value of FS. This also
Chapter 3: Reading Input Files 43applies to any built-in function that updates $0, such as sub and gsub (see Section 8.1.3[String-Manipulation Functions], page 132).3.5 Specifying How Fields Are SeparatedThe field separator, which is either a single character or a regular expression, controls theway awk splits an input record into fields. awk scans the input record for character sequencesthat 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 theoutput. If the field separator is ‘oo’, then the following line:moo goo gai panis split into three fields: ‘m’, ‘•g’, and ‘•gai•pan’. Note the leading spaces in the values ofthe second and third fields.The field separator is represented by the built-in variable FS. Shell programmers takenote: awk does not use the name IFS that is used by the POSIX-compliant shells (such asthe Unix Bourne shell, sh, or bash).The value of FS can be changed in the awk program with the assignment operator, ‘=’(see Section 5.7 [Assignment Expressions], page 83). Often the right time to do this is at thebeginning of execution before any input has been processed, so that the very first record isread with the proper separator. To do this, use the special BEGIN pattern (see Section 6.1.4[The BEGIN and END Special Patterns], page 99). For example, here we set the value of FSto the string ",":awk ’BEGIN { FS = "," } ; { print $2 }’Given the input line:John Q. Smith, 29 Oak St., Walamazoo, MI 42139this awk program extracts and prints the string ‘•29•Oak•St.’.Sometimes the input data contains separator characters that don’t separate fields theway you thought they would. For instance, the person’s name in the example we just usedmight have a title or suffix attached, such as:John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139The same program would extract ‘•LXIX’, instead of ‘•29•Oak•St.’. If you were expectingthe program to print the address, you would be surprised. The moral is to choose your datalayout and separator characters carefully to prevent such problems. (If the data is not in aform 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 defaultvalue of the field separator FS is a string containing a single space, " ". If awk interpretedthis value in the usual way, each space character would separate fields, so two spaces in arow would make an empty field between them. The reason this does not happen is that asingle space as the value of FS is a special case—it is taken to specify the default mannerof delimiting fields.If FS is any other single character, such as ",", then each occurrence of that characterseparates two fields. Two consecutive occurrences delimit an empty field. If the characteroccurs at the beginning or the end of the line, that too delimits an empty field. The spacecharacter is the only single character that does not follow these rules.
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