6.15. Pipes

If you open a pipe in an awk program, you must close it before opening another one. The command on the right-hand side of the pipe symbol is enclosed in double quotes. Only one pipe can be open at a time.

Example 6.102.

(The Database)

% cat names

john smith

alice cheba

george goldberg

susan goldberg

tony tram

barbara nguyen

elizabeth lone

dan savage

eliza goldberg

john goldenrod



(The Command Line)

% nawk '{print $1, $2 | "sort –r +1 –2 +0 –1 "}' names



(The Output)

tony tram

john smith

dan savage

barbara nguyen

elizabeth lone

john goldenrod

susan goldberg

george goldberg

eliza goldberg

alice cheba

EXPLANATION

Awk will pipe the output of the print statement as input to the UNIX/Linux sort command, which does a reversed sort using the second field as the primary key and the first field as the secondary key (i.e., sort by last name in reverse). The UNIX/Linux command must be enclosed in double quotes. (See "sort" in Appendix A.)

6.15.1 Closing Files and Pipes

If you plan to use a file or pipe in an awk program again for reading or writing, you may want to close it first, because it remains open until the script ends. Once opened, the pipe remains open until awk exits. Therefore, statements in the END block will also be affected by the pipe. The first line in the END block closes the pipe.

Example 6.103.

(In Script)

1   { print $1, $2, $3 | " sort -r +1 -2 +0 -1"}

    END{

2   close("sort –r +1 –2 +0 –1")

    <rest of statements>  }

EXPLANATION

The system Function

The built-in system function takes a UNIX/Linux system command as its argument, executes the command, and returns the exit status to the awk program. It is similar to the C standard library function, also called system( ). The UNIX/Linux command must be enclosed in double quotes.

FORMAT

system( "UNIX/Linux Command")

Example 6.104.

(In Script)

    {

1   system ( "cat" $1 )

2   system ( "clear" )

    }

EXPLANATION