4.4 Changing the Contents of a Field

The contents of a field, as seen by awk, can be changed within an awk program; this changes what awk perceives as the current input record. (The actual input is untouched; awk never modifies 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 value of field three minus ten: ‘$3 - 10’. (See section Arithmetic Operators.) Then it prints the original and new values for field three. (Someone in the warehouse made a consistent mistake while inventorying the red boxes.)

For this to work, the text in field $3 must make sense as a number; the string of characters must be converted to a number for the computer to do arithmetic on it. The number resulting from the subtraction is converted back to a string of characters that then becomes field three. See section Conversion of Strings and Numbers.

When the value of a field is changed (as perceived by awk), the text of the input record is recalculated to contain the new field where the old one was. In other words, $0 changes to reflect the altered field. Thus, this program prints a copy of the input file, with 10 subtracted 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 ‘+’ sign represents addition. For the file ‘inventory-shipped’, $6 represents the total number of parcels shipped for a particular month.

Creating a new field changes awk’s internal copy of the current input record, which is the value of $0. Thus, if you do ‘print $0’ after adding a field, the record printed includes the new field, with the appropriate number of field separators between it and the previously existing fields.

This recomputation affects and is affected by NF (the number of fields; see section Examining Fields). For example, the value of NF is set to the number of the highest field you create. The exact format of $0 is also affected by a feature that has not been discussed yet: the output field separator, OFS, used to separate the fields (see section Output Separators).

Note, however, that merely referencing an out-of-range field does not change the value of either $0 or NF. Referencing an out-of-range field only produces an empty string. For example:

if ($(NF+1) != "")
    print "can't happen"
else
    print "everything is normal"

should print ‘everything is normal’, because NF+1 is certain to be out of range. (See section The if-else Statement, for more information about awk’s if-else statements. See section Variable Typing and Comparison Expressions, for more information about the ‘!=’ operator.)

It is important to note that making an assignment to an existing field changes the value of $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
-| 4

The 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
-| 6

The intervening field, $5, is created with an empty value (indicated by the second pair of adjacent colons), and NF is updated with the value six.

Decrementing NF throws away the values of the fields after the new value of NF and recomputes $0. (d.c.) Here is an example:

$ echo a b c d e f | awk '{ print "NF =", NF;
>                            NF = 3; print $0 }'
-| NF = 6
-| a b c

CAUTION: 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 innocuous assignment:

$1 = $1   # force record to be reconstituted
print $0  # or whatever else with $0

This 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 applies to any built-in function that updates $0, such as sub() and gsub() (see section String-Manipulation Functions).

Advanced Notes: Understanding $0

It is important to remember that $0 is the full record, exactly as it was read from the input. This includes any leading or trailing whitespace, and the exact whitespace (or other characters) that separate the fields.

It is a not-uncommon error to try to change the field separators in a record simply by setting FS and OFS, and then expecting a plain ‘print’ or ‘print $0’ to print the modified record.

But this does not work, since nothing was done to change the record itself. Instead, you must force the record to be rebuilt, typically with a statement such as ‘$1 = $1’, as described earlier.