How can I run Perl system commands in the background?

Question

#!/usr/bin/env perl use warnings; use strict; use 5.012; use IPC::System::Simple qw(system);

system( 'xterm', '-geometry', '80x25-5-5', '-bg', 'green', '&' );

say "Hello";
say "World";

I tried this to run the xterm-command in the background, but it doesn't work:

No absolute path found for shell: &

What would be the right way to make it work?

Answer 1

Have you tried?

system('xterm -geometry 80x25-5-5 -bg green &');

http://www.rocketaware.com/perl/perlfaq8/How_do_I_start_a_process_in_the_.htm

Answer 2

Perl's system function has two modes:

1. taking a single string and passing it to the command shell to allow special characters to be processed
2. taking a list of strings, exec'ing the first and passing the remaining strings as arguments

In the first form you have to be careful to escape characters that might have a special meaning to the shell. The second form is generally safer since arguments are passed directly to the program being exec'd without the shell being involved.

In your case you seem to be mixing the two forms. The & character only has the meaning of "start this program in the background" if it is passed to the shell. In your program, the ampersand is being passed as the 5th argument to the xterm command.

As Jakob Kruse said the simple answer is to use the single string form of system. If any of the arguments came from an untrusted source you'd have to use quoting or escaping to make them safe.

If you prefer to use the multi-argument form then you'll need to call fork() and then probably use exec() rather than system().

Answer 3

Look in perlfaq8 for How do I start a process in the background.

Answer 4

Note that the list form of system is specifically there to not treat characters such as & as shell meta-characters.

From perlfaq8's answer to How do I start a process in the background?

---

(contributed by brian d foy)

There's not a single way to run code in the background so you don't have to wait for it to finish before your program moves on to other tasks. Process management depends on your particular operating system, and many of the techniques are in perlipc.

Several CPAN modules may be able to help, including IPC::Open2 or IPC::Open3, IPC::Run, Parallel::Jobs, Parallel::ForkManager, POE, Proc::Background, and Win32::Process. There are many other modules you might use, so check those namespaces for other options too.

If you are on a Unix-like system, you might be able to get away with a system call where you put an & on the end of the command:

system("cmd &")

You can also try using fork, as described in perlfunc (although this is the same thing that many of the modules will do for you).

STDIN, STDOUT, and STDERR are shared

Both the main process and the backgrounded one (the "child" process) share the same STDIN, STDOUT and STDERR filehandles. If both try to access them at once, strange things can happen. You may want to close or reopen these for the child. You can get around this with opening a pipe (see open in perlfunc) but on some systems this means that the child process cannot outlive the parent. Signals You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too. SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is sent when you write to a filehandle whose child process has closed (an untrapped SIGPIPE can cause your program to silently die). This is not an issue with system("cmd&").

Zombies

You have to be prepared to "reap" the child process when it finishes.

$SIG{CHLD} = sub { wait };

$SIG{CHLD} = 'IGNORE';

You can also use a double fork. You immediately wait() for your first child, and the init daemon will wait() for your grandchild once it exits.

unless ($pid = fork) {
    unless (fork) {
        exec "what you really wanna do";
        die "exec failed!";
    }
    exit 0;
}
waitpid($pid, 0);

See Signals in perlipc for other examples of code to do this. Zombies are not an issue with system("prog &").