Perl - Regular expression yielding strange results - regex

Question

I am running a Perl script that runs a Linux system command, namely service --status-all in order to list services which have stopped and services which are currently running. The code that I am using runs the system command, and then chomps the input into an array. Followed by this the code is supposed to check if the service that the user promts for is running or has stopped, this is done using a regex that does yield the correct result. The problem lies in the additional information that the program lists, namely lines such as dnsdomainname: Unknown host and sometimes prints several lines of the same result.
The code that I am running is as follows:
use warnings;
use strict;
#Looping variables
my $service_query = 1;
#Command
my $command_3 = "service --status-all";
#Input variables
my $service = 0;
while ($service_query == 1){
print "Please choose the service that you wish to analyze (Named service):\n";
$service = <>;
if ($service =~ /^[a-zA-Z0-9.:_-]+$/)
{
$service_query = 0;
}
else
{
print "Argument not allowed.\n";
}
}
chomp(my #service_data_1 = qx/$command_3/);
chomp($service);
foreach my $line4(#service_data_1) #Filter output according to "$service".
{
if ($line4 =~ /$service\s\Spid\s[0-9]+\S\s(\S+)/)
{
print "1 $service is running\n";
}
elsif ($line4 =~ /$service\sis\s(\S+)/)
{
print "0 $service has stopped\n";
}
}
The system command yields results such as these:
sandbox is stopped
rpc.svcgssd is stopped
rpcbind (pid 1284) is running...
saslauthd is stopped
openssh-daemon (pid 1593) is running...
wpa_supplicant (pid 1444) is running...
What I want to be listed:
User inputs sandbox, program should print: 0 sandbox has stopped
What I actually get:
User inputs sandbox, program prints:
dnsdomainname: Unknown host
0 sandbox has stopped
I appreciate any help on the matter, thank you.

Answer 1

Most likely the service command clears the environment and does other things to ensure consistent results from what are essentially shell scripts n your service.d directory. In fact service itself may be a shell script (it usually was when I used Linux long ago). You can inspect it to help figure out why you are seeing dnsdomainname: Unknown host (it may be due to an environment variable like $HOSTNAME being cleared, or a missing name in /etc/hosts that normally wouldn't be an issue for the service command, for example).
Following on #choroba's query the output you see on your console when you run service --status-all in your shell might not be the same output you get from qx/ / in your perl script because of the way service itselfs handles STDOUT and STDERR.
From perdoc -f qx (cf. #TLP's remarks):
To read both a command's STDOUT and its STDERR separately, it's
easiest to redirect them separately to files, and then read from
those files when the program is done.
Another approach, if you are willing to use a CPAN module, is Capture::Tiny which is a great tool for grabbing what you want from perl code or external commands.