Heard it explained like this once:

As far as the system is concerned, 0 is an "uninteresting" exit... i.e. everything went fine.

Non-zero exit codes are "interesting" b/c if something failed you'll (probably) want to know why.

Which is something like perl's interesting/uninteresting system of true and false.

But since since exceptions are also 'false' it's kind of counter-intuitive to have

if ( chdir '/var/tmp' ) {
   print "Changed dirs successfully.\n";
}

if ( system 'cd /var/tmp' ) {
   die "Couldn't change dirs: $!.\n";
}
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Somewhere LW says he wishes he would have reversed this.