lists

// Lists: build one, write under a key, add at the end, test and read a key that may be missing, count.
// The collector checks that the TCP ports expected on a Linux are listening.
need('protocol:ssh');

$expected = variable('example:ports:expected', 'multiline', array(
    'label' => 'Expected ports',
    'help'  => 'One port per line, for example 22.',
));

output('listening', 'number', array('label' => 'Ports listening'));
output('closed', 'text', array('label' => 'Expected ports closed'));

watch.expect('port-closed', 'Expected port closed', 'closed', array('critical' => array('none')));

// One line per listening socket, "LISTEN 0 128 0.0.0.0:22 0.0.0.0:*": the fourth word is address:port.
$read = ssh.run('ss -H -l -t -n');

// array(): an empty list. Writing under a key that is missing creates it, and what is missing above it:
// $ports['22']['0.0.0.0'] creates $ports['22'] too.
$ports = array();
foreach (text.lines($read['output']) as $line) {
    $words = text.words($line);
    $m = text.match($words[3], '^(.*):([0-9]+)$');
    $ports[$m[2]][$m[1]] = true;
}
// list.count: the number of elements, here the number of ports.
value('listening', list.count($ports));

// list.has: is there an element under this key? list.add: the list with one more element at its end.
$closed = array();
foreach (text.lines($expected) as $port) {
    if (!list.has($ports, $port)) {
        $closed = list.add($closed, $port);
    }
}
// list.get: the element under a key, or the default given when the key is missing; here, the
// first port closed, '' when there is none.
if (list.get($closed, 0, '') == '') {
    value('closed', 'none');
} else {
    value('closed', text.join($closed, ', '));
}