It is quite a three-pipe problem, and I beg that you won't speak to me for fifty minutes.
- Sherlock Holmes The Red-Headed League
Perl may be many things to many people, but to most of us it is the glue that connects diverse components. This chapter is about launching commands and connecting separate processes together. It's about managing their creation, communication, and ultimate demise. It's about systems programming.
When it comes to systems programming, Perl, as usual, makes easy things easy and hard things possible. If you want to use it as you would the shell, Perl is happy to assist you. If you want to roll up your sleeves for low-level hacking like a hardcore C programmer, you can do that, too.
Because Perl lets you get so close to the system, portability issues can sneak in. This chapter is the most Unix-centric chapter of the book. It will be tremendously useful to those on Unix systems, but only of limited use to others. We deal with features that aren't as universal as strings and numbers and basic arithmetic. Most basic operations work more or less the same everywhere. But if you're not using some kind of Unix or other POSIX conformant system, most of the interesting features in this chapter may work differently for you - or not at all. Check the documentation that came with your Perl port if you aren't sure.
In this chapter, we cover the proper care and feeding of your own child processes. Sometimes this means launching a stand-alone command and letting it have its own way with the world (using
We first show how to use the most portable and commonly used operations for managing processes: backticks,
For example, you might want to interrupt your program while it's running a different program. Maybe you need to process a child program's standard error separately from its standard output. Perhaps you need to control both the input and output of a program simultaneously. When you tire of having just one thread of control and begin to take advantage of multitasking, you'll want to learn how to split your current program into several, simultaneously running processes that all talk to each other.
For tasks like these, you have to drop back to the underlying system calls:
When a child process dies, its memory is returned to the operating system, but its entry in the process table isn't freed. This lets a parent check the exit status of its child processes. Processes that have died but haven't been removed from the process table are called
s, and you should clean them up lest they fill the whole process table. Backticks and the
Your process is notified of the death of a child it created with a signal . Signals are a kind of notification delivered by the operating system. They are used for errors (when the kernel says, "Hey, you can't touch that area of memory!") and for events (death of a child, expiration of a per-process timer, interrupt with Ctrl-C). If you're launching processes manually, you normally arrange for a subroutine of your choosing to be called whenever one of your children exits.
Each process has a default disposition for each possible signal. You may install your own handler or otherwise change the disposition of most signals. Only SIGKILL and SIGSTOP cannot be changed. The rest you can ignore, trap, or block.
Briefly, here's a rundown of the more important signals:
Signal names are a convenience for humans. Each signal has an associated number that the operating system uses instead of names. Although we talk about SIGCHLD, your operating system only knows it as a number, like 20 (these numbers vary across operating systems). Perl translates between signal names and numbers for you, so you can think in terms of signal names.