Check a port: check

portcheck check tells you whether local ports are free and, if not, which process holds them.

$ portcheck check 3000 3001 80
PORT  PROTO  STATUS  PID    PROCESS  USER
80    tcp    denied  -      -        -
3000  tcp    in-use  42379  node     alice
3001  tcp    free    -      -        -

portcheck <port> is a shortcut for portcheck check <port>. Arguments are port specs, so portcheck check 8000-8010 works too (up to 4096 ports). Results are listed in ascending port order.

Statuses

Status	Meaning
free	No listener, and portcheck could bind the port on 127.0.0.1, 0.0.0.0, ::1 and :: (addresses your system doesn't support, e.g. IPv6, are skipped)
in-use	A process listens on the port, or binding failed because the address is in use (or for another unexpected reason, so a port is never wrongly reported free)
denied	Binding failed for lack of privileges: ports below 1024 without root on Linux (and on specific addresses on macOS), or a Windows excluded port range

Why both checks?

Some systems let you bind 0.0.0.0:3000 even though another program already listens on 127.0.0.1:3000. So portcheck looks for listeners and tries to bind every address, and reports free only when both agree.

An in-use port with no PID means the owner is hidden from your user; portcheck prints a hint on how to see it.

UDP

portcheck check 53 --proto udp

Exit status

Code	Meaning
0	every port is free
1	at least one port is in-use or denied
2	invalid arguments

portcheck check 5432 >/dev/null || { echo "Postgres port is taken"; exit 1; }

Races

A port reported free can be taken a moment later. When your own program needs a port, bind it directly, or bind port 0 and let the OS choose.