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Process Management

We can manage process that we own easily through Perl. Being able to manage your own process is important in creating Perl "scripts" that can accomplish your tasks in a more efficient manner.

Process Management :: Launching New Processes

One way to launch a new process is by using the system function. What if we wanted to get the current time and add who is currently logged into the system?
```
  system("date");  # pretty useless by itself, what if...
  $filename = "who.out";
  system "(date; who) >$filename &";
```
This would run the date command followed by the who command and redirect standard output to who.out. A sample output:
```
  Wed Aug  1 13:24:41 EDT 2001
  username     pts/0    Aug  1 12:10
```
The problem with typical system calls is that it invokes a shell. We can prevent ourselves from wasting a shell (not always) by giving system a list rather than one argument. For example, if we wanted to compile some .c files:
```
  @gccoptions = ("-ansi", "-pedantic", "-Wall", "-O2");
  system "gcc -o binexe @options @cfiles";           # shell used
  system "gcc", "-o", "binexe", @options, @cfiles;   # shell avoided
```
You can also launch a new process by using backquotes:
```
  $now = "time now is ".`date`;
```

Process Management :: Processes as Filehandles

We can launch a process in yet another way by creating a handle on the process. For a process that has output to be captured:
```
  open(WHOPROC, "who|");
  @whosaid = <WHOPROC>;
```
Note the vertical bar on the right side of who. If a command expects input, we could (you guessed it) put the vertical bar on the left side of who:
```
  open(LPR, "|lpr -pqueue");   # CUPS print to queue
  print LPR @somedata;
  close(LPR);
```

Process Management :: Using `fork()`

We can clone current processes by issuing the fork command. Taken from Schwartz pg. 148:
```
  if (!defined($kidpid = fork())) {
    # fork returned undef, so failed
    die "Cannot fork: $!";
  } elsif ($pid == 0) {
    # fork returned 0, so this branch is child
    exec("date");
    # if exec fails, fall through to the next statement
    die "can't exec date: $!";
  } else {
    # fork returned 0 nor undef
    # so this branch is parent
    waitpid($kidpid, 0);
  }
```
Of course, you would never fork a date process because that's just dumb. But for illustrative purposes, it gets the job done. If you do not understand what is happening, you should consult the fork(2) and exec(2) system calls and Schwartz/Wall. This is somewhat of a more "advanced" topic.

Process Management :: Sending and Receiving Signals

Interprocess communication can be acheived by sending and receiving signals. One can create a signal catcher and provide appropriate actions in response to signals received from a process. This is discussed in detail on pgs. 150-152 in Schwartz.
You can send signals to processes through the kill command:
```
  kill('INT', 234, 237);
```
Would send SIGINT to processes 234 and 237.
You can learn more about this in Wall, Ch. 6.

Manipulating Files and Directories Review

With all of the beyond Perl sections together, you can now use Perl to do most everyday tasks for you, as well as create powerful programs quickly and sometimes more efficiently than if you were to do it in other languages. That is the power of Perl, simple yet powerful.
We have not talked about the Object-Oriented side of Perl, if you wish to learn more about this, consult the Larry Wall, Programming Perl book.