Getline/Coprocess (The GNU Awk User’s Guide)
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4.10.7 Using getline from a Coprocess
Reading input into getline
from a pipe is a one-way operation. The command that is started with ‘command | getline
’ only sends data to your awk
program.
On occasion, you might want to send data to another program for processing and then read the results back. gawk
allows you to start a coprocess, with which two-way communications are possible. This is done with the ‘|&
’ operator. Typically, you write data to the coprocess first and then read the results back, as shown in the following:
print "some query" |& "db_server" "db_server" |& getline
which sends a query to db_server
and then reads the results.
The values of NR
and FNR
are not changed, because the main input stream is not used. However, the record is split into fields in the normal manner, thus changing the values of $0
, of the other fields, and of NF
and RT
.
Coprocesses are an advanced feature. They are discussed here only because this is the section on getline
. See section Two-Way Communications with Another Process, where coprocesses are discussed in more detail.