A range pattern is made of two patterns separated by a comma, in the form ‘begpat, endpat’. It is used to match ranges of consecutive input records. The first pattern, begpat, controls where the range begins, while endpat controls where the pattern ends. For example, the following:
awk '$1 == "on", $1 == "off"' myfile
prints every record in myfile between ‘on’/‘off’ pairs, inclusive.
A range pattern starts out by matching begpat against every input record. When a record matches begpat, the range pattern is turned on, and the range pattern matches this record as well. As long as the range pattern stays turned on, it automatically matches every input record read. The range pattern also matches endpat against every input record; when this succeeds, the range pattern is turned off again for the following record. Then the range pattern goes back to checking begpat against each record.
The record that turns on the range pattern and the one that turns it
off both match the range pattern. If you don’t want to operate on
these records, you can write if statements in the rule’s action
to distinguish them from the records you are interested in.
It is possible for a pattern to be turned on and off by the same
record. If the record satisfies both conditions, then the action is
executed for just that record.
For example, suppose there is text between two identical markers (e.g.,
the ‘%’ symbol), each on its own line, that should be ignored.
A first attempt would be to
combine a range pattern that describes the delimited text with the
next statement
(not discussed yet, see The next Statement).
This causes awk to skip any further processing of the current
record and start over again with the next input record. Such a program
looks like this:
/^%$/,/^%$/ { next }
{ print }
This program fails because the range pattern is both turned on and turned off by the first line, which just has a ‘%’ on it. To accomplish this task, write the program in the following manner, using a flag:
/^%$/ { skip = ! skip; next }
skip == 1 { next } # skip lines with `skip' set
In a range pattern, the comma (‘,’) has the lowest precedence of all the operators (i.e., it is evaluated last). Thus, the following program attempts to combine a range pattern with another, simpler test:
echo Yes | awk '/1/,/2/ || /Yes/'
The intent of this program is ‘(/1/,/2/) || /Yes/’.
However, awk interprets this as ‘/1/, (/2/ || /Yes/)’.
This cannot be changed or worked around; range patterns do not combine
with other patterns:
$ echo Yes | gawk '(/1/,/2/) || /Yes/' error→ gawk: cmd. line:1: (/1/,/2/) || /Yes/ error→ gawk: cmd. line:1: ^ syntax error
As a minor point of interest, although it is poor style, POSIX allows you to put a newline after the comma in a range pattern. (d.c.)