Thursday, March 07, 2013

Return codes from system commands in Perl

So say I'm running a Perl script which then calls some system command, let's say 'echo blah'.  Running that returns three things to us;
  1. the actual output (stdout) which would be "blah",
  2. any error output (stderr) which would be "" ie; NULL or empty,
  3. and the return code which would be "0";
So until now I had to choose which one of those i care about most and use the appropriate system executable handler to suit; let me explain;

my $result = `echo blah`; # captures "blah" into $result
my $result = system("echo blah"); # captures the return value of echo into $result
..and i stopped paying attention about there.

Suffice to say, no method i knew about allowed me to capture them all at the same time. Until i discovered IO::CaptureOutput.  Check out this foo;
$ cat ./ 
use strict;
use IO::CaptureOutput qw/capture_exec/;

my @cmd= "echo blah";

my ($stdout, $stderr, $success, $exit_code) = capture_exec( @cmd ); chomp($stdout);

print "running '@cmd'\n";
print "stdout = $stdout\n";
print "stderr = $stderr\n";
print "success = $success\n";
print "exit_code = $exit_code\n";

$ ./ 
running 'echo blah'
stdout = blah
stderr = 
success = 1
exit_code = 0

So now i can trap these in my code and handle them each as i see fit.  It means better error handling and software that runs (and stops!) how and when one might want it to.  And that's cool.

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