GAWK: Effective AWK Programming

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40 GAWK: Effective AWK Programming$ awk ’$1 ~ /foo/ { print $0 }’ BBS-list⊣ fooey 555-1234 2400/1200/300 B⊣ foot 555-6699 1200/300 B⊣ macfoo 555-6480 1200/300 A⊣ sabafoo 555-2127 1200/300 CThis example prints each record in the file ‘BBS-list’ whose first field contains the string‘foo’. The operator ‘~’ is called a matching operator (see Section 2.1 [How to Use RegularExpressions], page 24); it tests whether a string (here, the field $1) matches a given regularexpression.By contrast, the following example looks for ‘foo’ in the entire record and prints thefirst field and the last field for each matching input record:$ awk ’/foo/ { print $1, $NF }’ BBS-list⊣ fooey B⊣ foot B⊣ macfoo A⊣ sabafoo C3.3 Nonconstant Field NumbersThe number of a field does not need to be a constant. Any expression in the awk languagecan be used after a ‘$’ to refer to a field. The value of the expression specifies the fieldnumber. If the value is a string, rather than a number, it is converted to a number. Considerthis example:awk ’{ print $NR }’Recall that NR is the number of records read so far: one in the first record, two in thesecond, etc. So this example prints the first field of the first record, the second field of thesecond record, and so on. For the twentieth record, field number 20 is printed; most likely,the record has fewer than 20 fields, so this prints a blank line. Here is another example ofusing expressions as field numbers:awk ’{ print $(2*2) }’ BBS-listawk evaluates the expression ‘(2*2)’ and uses its value as the number of the field toprint. The ‘*’ sign represents multiplication, so the expression ‘2*2’ evaluates to four. Theparentheses are used so that the multiplication is done before the ‘$’ operation; they arenecessary whenever there is a binary operator in the field-number expression. This example,then, prints the hours of operation (the fourth field) for every line of the file ‘BBS-list’. (Allof the awk operators are listed, in order of decreasing precedence, in Section 5.14 [OperatorPrecedence (How Operators Nest)], page 94.)If the field number you compute is zero, you get the entire record. Thus, ‘$(2-2)’ has thesame value as $0. Negative field numbers are not allowed; trying to reference one usuallyterminates the program. (The POSIX standard does not define what happens when youreference a negative field number. gawk notices this and terminates your program. Otherawk implementations may behave differently.)As mentioned in Section 3.2 [Examining Fields], page 39, awk stores the current record’snumber of fields in the built-in variable NF (also see Section 6.5 [Built-in Variables],page 110). The expression $NF is not a special feature—it is the direct consequence ofevaluating NF and using its value as a field number.
Chapter 3: Reading Input Files 413.4 Changing the Contents of a FieldThe contents of a field, as seen by awk, can be changed within an awk program; this changeswhat awk perceives as the current input record. (The actual input is untouched; awk nevermodifies the input file.) Consider the following example and its output:$ awk ’{ nboxes = $3 ; $3 = $3 - 10> print nboxes, $3 }’ inventory-shipped⊣ 25 15⊣ 32 22⊣ 24 14...The program first saves the original value of field three in the variable nboxes. The ‘-’sign represents subtraction, so this program reassigns field three, $3, as the original valueof field three minus ten: ‘$3 - 10’. (See Section 5.5 [Arithmetic Operators], page 81.) Thenit prints the original and new values for field three. (Someone in the warehouse made aconsistent mistake while inventorying the red boxes.)For this to work, the text in field $3 must make sense as a number; the string of charactersmust be converted to a number for the computer to do arithmetic on it. The numberresulting from the subtraction is converted back to a string of characters that then becomesfield three. See Section 5.4 [Conversion of Strings and Numbers], page 79.When the value of a field is changed (as perceived by awk), the text of the input recordis recalculated to contain the new field where the old one was. In other words, $0 changesto reflect the altered field. Thus, this program prints a copy of the input file, with 10subtracted from the second field of each line:$ awk ’{ $2 = $2 - 10; print $0 }’ inventory-shipped⊣ Jan 3 25 15 115⊣ Feb 5 32 24 226⊣ Mar 5 24 34 228...It is also possible to also assign contents to fields that are out of range. For example:$ awk ’{ $6 = ($5 + $4 + $3 + $2)> print $6 }’ inventory-shipped⊣ 168⊣ 297⊣ 301...We’ve just created $6, whose value is the sum of fields $2, $3, $4, and $5. The ‘+’ signrepresents addition. For the file ‘inventory-shipped’, $6 represents the total number ofparcels shipped for a particular month.Creating a new field changes awk’s internal copy of the current input record, which isthe value of $0. Thus, if you do ‘print $0’ after adding a field, the record printed includesthe new field, with the appropriate number of field separators between it and the previouslyexisting fields.This recomputation affects and is affected by NF (the number of fields; see Section 3.2[Examining Fields], page 39). For example, the value of NF is set to the number of the
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