Development record

kill-port given no literal operand is allowed: a port read from standard input (echo 3000 | xargs npx kill-port, xargs -n1 npx kill-port < ports) or given as an attached flag (npx kill-port --port=3000, -p=3000) passes the guard, because the kill-port entry asks for at least one operand and xargs is not read as handing it one. Each line stops whatever holds the port, as npx kill-port 3000 does, which blocks.