How can I keep running a unix program in the background even if I log out?

Question

I want to run a Perl script with some while(1) loop in the background on a unix machine until I kill it.

This is a remote computer to which I don't have administrative permissions (so for some reason, I can't use Daemon::Generic::While1), I log to it through SSH, and I want it to continue to run after I log out.

One way I found out is write something like this to bash:

nohup ./my_script.pl &

Is there some other, more preferable way to do it?

Editing the crontab is forbidden on that computer (while running background and long-lasting processes isn't).

Answer 1

I believe this should work from within your perl script.

$SIG{ HUP } = 'IGNORE';

Note the man page doc for nohup says:

To do this, nohup sets the SIGHUP signal(3) (``terminal line hangup'') to be ignored, then executes utility along with any arguments.

Answer 2

Running the job in the background from the shell is fine, depending on how much manual labor you wish to put into making sure it's running continuously.

I would use crontab's @reboot (and possibly a regular checkup as well). But then you need to take into consideration that the process might be started multiple times. Simplest way to handle that is having a lock-file. If the file is locked, exit, else continue to the task. Remember to use non-blocking checks, else you risk hogging unnecessary system resources with dead processes.

screen is also an option if it's available.

crontab + screen can be combined.

Answer 3

Snippet is in perldoc perlipc:

use POSIX 'setsid';

sub daemonize {
    chdir '/'               or die "Can't chdir to /: $!";
    open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
    open STDOUT, '>/dev/null'
                            or die "Can't write to /dev/null: $!";
    defined(my $pid = fork) or die "Can't fork: $!";
    exit if $pid;
    setsid                  or die "Can't start a new session: $!";
    open STDERR, '>&STDOUT' or die "Can't dup stdout: $!";
}

But I'm not sure whether setsid will work if you're not root. If not, try to double-fork after closing standard handles.

some links: howto, daemonize

Answer 4

use Proc::Daemon;
Proc::Daemon::Init;

That's what I use for my Sendmail filter program and probably the easiest way to go. The module is available via CPAN.

Answer 5

You need to close stdout, stderr, stdin, otherwise you are still bound to that specific TTY

./my_script.pl >/dev/null 2>&1 </dev/null &

This should do the trick.