Perl

09. Process Control

system

Using the system command we can cause a Bourne Shell process to start and execute an arbitrary command for us.

system "date";              # causes date to run, output to stdout

$status = system("date");   # get exit status of command

$command = "who | wc -l";
system $command;

$status = system "date &";  # send to bg, keep going, status indicates
                            #   whether launch went well

Perl may avoid using the shell and call the program directly in some cases if it feels it can. Will always do if you call with a list of args:

system "grep glong phonelist";          # calls shell which calls grep
system "grep", "glong", "phonelist";    # calls grep directly

Backticks

Can use backticks for command substitution-like form, just like sh.

$timestamp = "today is " . `date`;

foreach (`who`) {        # print everyone logged in
   @people = split;
   print "logged in: $people[0]\n";
}

Processes as Filehandles

Can "open" a command to read from/write to.

#!/usr/bin/perl -w

open(FINGER, "finger |")         or die "can't finger";

while ($line = <FINGER>) {
   chomp($line);
   print "found: $line\n";            # prints out each line in finger output
}

close(FINGER);


open(PRINT, "| lpr -Pprinter")       or die "can't print";
print PRINT "this goes to printer", " along with this";
close(PRINT);

Can open arbitrary pipes full of stuff. Can use to prefilter input with some other command or postfilter data with something else.