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Closing Output Files and Pipes

When a file or pipe is opened, the file name or command associated with it is remembered by awk and subsequent writes to the same file or command are appended to the previous writes. The file or pipe stays open until awk exits. This is usually convenient.

Sometimes there is a reason to close an output file or pipe earlier than that. To do this, use the close function, as follows:

close(filename)

or

close(command)

The argument filename or command can be any expression. Its value must exactly equal the string used to open the file or pipe to begin with--for example, if you open a pipe with this:

print $1 | "sort -r > names.sorted"

then you must close it with this:

close("sort -r > names.sorted")

Here are some reasons why you might need to close an output file:

• To write a file and read it back later on in the same awk program. Close the file when you are finished writing it; then you can start reading it with getline (see section Explicit Input with getline).
• To write numerous files, successively, in the same awk program. If you don't close the files, eventually you may exceed a system limit on the number of open files in one process. So close each one when you are finished writing it.
• To make a command finish. When you redirect output through a pipe, the command reading the pipe normally continues to try to read input as long as the pipe is open. Often this means the command cannot really do its work until the pipe is closed. For example, if you redirect output to the mail program, the message is not actually sent until the pipe is closed.
• To run the same program a second time, with the same arguments. This is not the same thing as giving more input to the first run! For example, suppose you pipe output to the mail program. If you output several lines redirected to this pipe without closing it, they make a single message of several lines. By contrast, if you close the pipe after each line of output, then each line makes a separate message.

close returns a value of zero if the close succeeded. Otherwise, the value will be non-zero.

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