Programming Perl, 4th Edition

Talking to Yourself

Another approach to IPC is to make your program talk to itself, in a manner of speaking. Actually, your process talks over pipes to a forked copy of itself. It works much like the piped open we talked about in the last section, except that the child process continues executing your script instead of some other command.

To represent this to the open function, you use a pseudocommand consisting of a minus. So the second argument to open looks like either “–|” or “|–”, depending on whether you want to pipe from yourself or to yourself. As with an ordinary fork command, the open function returns the child’s process ID in the parent process but 0 in the child process. Another asymmetry is that the filehandle named by the open is used only in the parent process. The child’s end of the pipe is hooked to either STDIN or STDOUT as appropriate. That is, if you open a pipe to minus with |–, you can write to the filehandle you opened, and your kid will find this in STDIN:

if (open(TO, "|–")) {
    print TO $fromparent;
}
else {
    $tochild = <STDIN>;
    exit;
}

If you open a pipe from minus with –|, you can read from the filehandle you opened, which will return whatever your kid writes to STDOUT:

if (open(FROM, "–|")) {
    $toparent = <FROM>;
}
else {
    print STDOUT $fromchild;
    exit;
}

One common application of this construct is to bypass the shell when you want to open a pipe from a command. You might want to do this because you don’t want the shell to interpret any possible metacharacters ...