How to fork gnuplot, write an initial string to it then background the process. Mean while parent is fetching data and writing to data file.This will run but after one replot gnuplot does not update as if dead.Can run similar program putting gnuplot string(DATA section) in file of its own and spawn gnuplot with this file as its source.Why does this code seem to freeze.How to fork open pipe write to it,exit but leave process running.

#!/usr/bin/perl -w



my $pid;
$pid = fork;

if($pid == 0)
{
    my $pid1 = open(CHILD, "|-", "gnuplot -persist ") or die "Couldn't
+ fork: $!\n";
    local $/=undef;
    my $plot=<DATA>;
    print CHILD "$plot\n";    
    close(CHILD);
    print"forking\n";    
    exit;
}

use LWP::Simple;
use Fcntl qw(:DEFAULT :flock);
$|++;
$SIG{TERM}=sub{print "ok1\n";alarm(0);exit;};
my $url='https://www.cryptonator.com/api/ticker/btc-gbp';
print"ok2\n";
my $interval = 30; # seconds between calls
fetch();
sub fetch{
    
    print"ok3\n";
    my $replie = get($url);
    print"$replie \n";
    my($a,$b,$c,$d)= $replie  =~ /^.+?(\d+\.\d+).+?(\d+\.\d+)?.+?(\d+\
+.\d+).+?(\d+).*$/;
$a="not defined" unless defined($a);
$b="not defined" unless defined($b);
$c="not defined" unless defined($c);
$d="not defined" unless defined($d);
        open(OUT,">>","/home/philip/Desktop/PERLBITCOIN/TICKER/bitgbp1
+.dat") or die "BIT File open error $!\n";
        unless (flock(OUT, LOCK_EX|LOCK_NB)) 
        {
            warn "can't immediately write-lock the file ($!), blocking
+ ...";
            unless (flock(OUT, LOCK_EX))
            {
                die "can't get write-lock on numfile: $!";
            }
        }
        print OUT "$c,$a,$d\n";
        flock(OUT, LOCK_UN) or die  "cant unlock file\n";
        close(OUT);        
        #print  "$c,$a,$d\n";
    
    alarm($interval);
};
$SIG{ALRM}=\&fetch;

1 while 1;
__DATA__
reset
delta_v(x) = ( vD = x - old_v, old_v = x, vD)
old_v = NaN
unset key
set xtics border in scale 0,0 nomirror rotate by -90  offset character
+ 0,0  autojustify
set datafile separator ','
set bmargin at screen 0.15
#set palette defined (100 "red", 200 "green", 300 "blue")
set palette defined (-1 "red", 1 "#1B6821")
#set palette model RGB defined ( -1 'red', 1 '#006400', 0 'black' )
set cbrange [-1:1]
set grid
set yrange [*:*]
unset colorbox
set style fill solid noborder
#set xrange [1446061653-946684800:1446064653-946684800]
set xdata time
set timefmt "%s"
set format x "%H:%M"     # or anything else
set xlabel "time"  
set xtics autofreq
data=system("tail -n 1 '/home/philip/Desktop/PERLBITCOIN/TICKER/bitgbp
+1.dat' | awk '{print $0}'")
print delta_v(2)
plot '< tail -n 40 /home/philip/Desktop/PERLBITCOIN/TICKER/bitgbp1.dat
+'  using 3:2:((delta_v($2) > 0.0) ? 1 : -1 )with impulses lw 2 palett
+e   
pause 10
reread
replot;
[download]

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