Port

A port is a sixteen-bit number identifying an endpoint on a host. A server binds one and listens; a client connects to it. Only one process can hold a given port on a given interface at a time.

On a machine running several projects, ports become a genuinely contended resource, and the resulting failures are among the most disorienting in development.

Wrong-port binding. A start command that reads its port from the shell, with a fallback, will bind the fallback when the shell does not provide one — even if a local configuration file says otherwise, because the shell expanded the value before the program read its configuration. See Shell.

Cross-talk. When two working copies are in flight, whoever already has a browser open on the shared port starts talking to the other branch's code, and their writes land in that branch's database. Traffic appearing in a log that you did not generate is the tell, and nothing in that log is trustworthy until the binding is checked.

Derived collisions. Ports generated from a project name by hashing are not collision-free, and the resulting bind failure is reported by whichever process starts second.

Confirming which process holds a port — and what directory it is running from — takes one command and settles all three.