Using qx or backticks will only capture the STDOUT. I'm wondering if the calls where you are not capturing anything could be related to the "output" of the external command is going to the STDERR instead.

Whenever I have used backticks and I don't get what I expect, a common debugging technique that I use is to store the external command in a variable and print that out. That has helped me find mistakes in constructing the external command that I wanted to run.

One suggestion that you could try is to look at using Capture::Tiny, which will let you capture STDOUT, STDERR and the exit code of an external command.

I don't know about the "cmd.exe" (windows command-line shell), but in any *n*x shell (e.g. bash), you can redirect stdout and stderr independently.

When you use perl to run a shell command via backticks or qx, you could (if you want) include redirection, to have both stdout and stderr print to stream, like this:

my $major_minor = qx($Bin/$get_version_prog $filer 2>&1)
[download]

The redirection works for Windows too.

Premature optimization is the root of all job security

I have been trying to write a test program that will re-create the issue. So far, no luck. Everything I've done to call the external in my test programs works as expected. I've been trying to make the test programs more complex, adding in more modules, etc., but so far nothing exhibits the symptoms but the main daemon. I will continue to try and replicate.

My next plan of attack is to un-daemonize the daemon and try running it through the debugger.

#!/usr/bin/perl --
use strict;
use warnings;
use Capture::Tiny qw/ capture /;
my @cmd = ( "$Bin/$get_version_prog", $filer );
my( $stdout, $stderr, $exit ) = capture {
     system { $cmd[0] } @cmd;
};;
$exit and die "it failed with $exit and $stderr ";
[download]

`...` is just short for qx`...`, which is short for readpipe(qq`...`).

$ perl -MO=Concise,-exec -e'`...`'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <$> const[PV "..."] s
4  <1> backtick[t1] vK
5  <@> leave[1 ref] vKP/REFC
-e syntax OK

$ perl -MO=Concise,-exec -e'qx`...`'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <$> const[PV "..."] s
4  <1> backtick[t1] vK
5  <@> leave[1 ref] vKP/REFC
-e syntax OK

$ perl -MO=Concise,-exec -e'readpipe(qq`...`)'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <$> const[PV "..."] s
4  <1> backtick[t2] vK/1
5  <@> leave[1 ref] vKP/REFC
-e syntax OK
[download]

This is by no means new; this has been the case for at least 18 years. (The oldest documentation I could find is perl5.005_53 from Oct 31st, 1998.)

I'm not sure what your definition of relatively new is, but qx has existed since at least 5.8.8 (oldest Perldoc readily available on perldoc.perl.org) and 5.8.8 was released over 10 years ago (2006-Jan-31).

No change, same output. Or rather lack thereof.

Found an example, where the daemonization closes STDOUT and STDIN in order to not create files. This could confuse the child process (which has no stdout). So either, maybe this works:

use Daemon::Simple;

open(SAVEIN, "<&STDIN");
open(SAVEOUT, "<&STDOUT");

Daemon::Simple::init("daemon");

open(STDIN, "<&SAVEIN");
open(STDOUT, "<&SAVEOUT");

print qx("/external/command");

close(STDIN);
close(STDOUT);
[download]

or you will have to redirect your output to a file

qx("/external/command > /tmp/file")

and read it back using open(), diamond and close()