16.4.1. Problem

16.4.2. Solution

$pid = open $readme, "-|", "program", "arguments"
                                            or die "Couldn't fork: $!\n";
while (<$readme>) {
    # ...
}
close $readme                               or die "Couldn't close: $!\n";

To write to the program, put the pipe at the beginning:

$pid = open $writeme, "|-", "program", "arguments"
                                            or die "Couldn't fork: $!\n";
print $writeme "data\n";
close $writeme                              or die "Couldn't close: $!\n";

16.4.3. Discussion

In the case of reading, this is similar to using backticks, except you have a process ID and a filehandle, and the shell is never involved. If you want Perl to use the shell when it sees shell-special characters in its argument—for example, to let the shell do filename wildcard expansion and I/O redirection—then you must use the two-argument form of open:

open($writeme, "| program args");
open($readme, "program args |");

However, sometimes this isn't desirable. Piped open s that include unchecked user data would be unsafe while running in taint mode or in untrustworthy situations.

Notice how we specifically call close on the filehandle. When you use open to connect a filehandle to a child process, Perl remembers this and automatically waits for the child when you close the filehandle. If the child hasn't exited by then, Perl waits until it does. This can be a very, very long wait if your child doesn't exit:

$pid = open $f, "-|", "sleep", "100000";  # child goes to sleep
close $f;                                 # and the parent goes to lala land

If you want to run another program and be able to supply its STDIN yourself, a similar construct is used:

$pid = open $writeme, "|-", "program", "args";
print $writeme "hello\n";            # program will get hello\n on STDIN
close $writeme;                      # program will get EOF on STDIN

The second argument to open ("|-") tells Perl to start another process instead. It connects the open ed filehandle to the process's STDIN. Anything you write to the filehandle can be read by the program through its STDIN. When you close the filehandle, the open ed process will get EOF when it next tries to read from STDIN.

You can also use this technique to change your program's normal output path. For example, to automatically run everything through a pager, use something like:

$pager = $ENV{PAGER} || '/usr/bin/less';  # XXX: might not exist
open(STDOUT, "|-", $pager);

Now, without changing the rest of your program, anything you print to standard output will go through the pager automatically.

As before, the parent should also be wary of close. If the parent closes the filehandle connecting it to the child, the parent will block while waiting for the child to finish. If the child doesn't finish, neither will the close. The workaround as before is to either kill your child process prematurely, or else use the low-level pipe-fork-exec scenario.

When using piped opens, always check return values of both open and close, not just of open. That's because the return value from open does not indicate whether the command was successfully launched. With a piped open, you fork a child to execute the command. Assuming the system hasn't run out of processes, the fork immediately returns the PID of the child it just created.

By the time the child process tries to exec the command, it's a separately scheduled process. So if the command can't be found, there's effectively no way to communicate this back to the open function, because that function is in a different process!

16.4.4. See Also