Classic Shell Scripting

Patterns and Actions

Patterns and actions form the heart of awk programming. It is awk's unconventional data-driven programming model that makes it so attractive and contributes to the brevity of many awk programs.

Patterns

Patterns are constructed from string and/or numeric expressions: when they evaluate to nonzero (true) for the current input record, the associated action is carried out. If a pattern is a bare regular expression, then it means to match the entire input record against that expression, as if you had written $0 ~ /regexp/ instead of just /regexp/. Here are some examples to give the general flavor of selection patterns:

NF =  = 0                                  Select empty records
NF > 3                                   Select records with more than 3 fields
NR < 5                                   Select records 1 through 4
(FNR =  = 3) && (FILENAME ~ /[.][ch]$/)    Select record 3 in C source files
$1 ~ /jones/                             Select records with "jones" in field 1
/[Xx][Mm][Ll]/                           Select records containing "XML", ignoring lettercase
$0 ~ /[Xx][Mm][Ll]/                      Same as preceding selection

awk adds even more power to the matching by permitting range expressions. Two expressions separated by a comma select records from one matching the left expression up to, and including, the record that matches the right expression. If both range expressions match a record, the selection consists of that single record. This behavior is different from that of sed, which looks for the range end only in records that follow the start-of-range record. Here are some examples:

(FNR =  = 3), (FNR =  = 10)                        Select records ...