perl array for non-zero values - perl

Question

Im trying to code a "service" script based on "ps".
my code:
#!/usr/bin/perl
use strict;
use warnings;
die "usage: $0 <service name>\n" unless $ARGV[0];
my $service = $ARGV[0];
open(my $ps, "ps -aux |") || die "Uknown command\n";
my #A = <$ps>;
close $ps;
foreach my $i(grep /$service/, #A){
chomp $i;
if($i=~ /root/){
next
}
print "$i\n";
}
My problem: When running the script against undef arg like:
$0 blablabla
I want to return an output if there is no such service appears/when returns 0
Thanks

Answer 1

I assume what you are asking is: How to give a proper message when no matching lines are found?
Well, just store the result in an array instead:
my #lines = grep { !/root/ && /$service/ } #A;
if (#lines) { # if any lines are found
for my $line (#lines) {
...
}
} else {
print "No match for '$service'!\n";
}
Or you can print the number of matches regardless of their number:
my $found = #lines;
print "Matched found: $found\n";
Note also that you can add the check for root in your grep.
As a side note, this part:
die "usage: $0 <service name>\n" unless $ARGV[0];
my $service = $ARGV[0];
Is perhaps better written
my $service = shift;
die "usage ...." unless defined $service;
Which specifically checks if the argument is defined or not, as opposed to true or not.

Answer 2

If I understand you correctly, you want to inform the user if no such service was found? If so, you can modify the script as follows:
my $printed; # Will be used as a flag.
foreach my $i(grep /$service/, #A){
chomp $i;
if($i=~ /root/){
next
}
$printed = print "$i\n"; # Set the flag if the service was found.
}
warn "No service found\n" unless $printed;

Answer 3

You can try something like this:
my #processes = grep /$service/, #A;
if ( scalar #processes ) {
foreach my $i( #processes ){
chomp $i;
if($i=~ /root/){
next;
}
print "$i\n";
}
}
else {
print 'your message';
}

Answer 4

You could check the result of the grep command before traversing it in the for loop, like:
...
my #services = grep { m/$service/ } #A;
# Filter the perl process running this script and...
if ( ! #services ) {
print "No service found\n";
exit 0;
}
foreach my $i( #services ){
...
}
Take into account that the grep command will never give a false return because it is including the perl process, so you will have to filter it, but I hope you get the idea.