port-kill.sh

#!/usr/bin/env bash
#
# port-kill.sh - kill whatever is listening on a TCP port.

set -uo pipefail

usage() {
    cat <<'HELP'
----------------------------------------
Dimitry4Now :: port-kill.sh
----------------------------------------

USAGE
  port-kill.sh [OPTIONS] PORT [PORT...]

OPTIONS
  -n, --dry-run   list the listeners, kill nothing
  -9, --force     send SIGKILL instead of SIGTERM
  -h, --help      show this help

Only TCP sockets in LISTEN state are matched. A browser tab open on
localhost:3000 holds a socket whose *remote* port is 3000, so a plain
`lsof -i :3000` hits the browser too - this does not.

Processes belonging to another user are listed and skipped, never killed.
HELP
}

die() {
    printf '%s: %s\n' "${0##*/}" "$1" >&2
    exit "${2:-2}"
}

# ---------------------------------------------------------------------------
# OPTIONS
# ---------------------------------------------------------------------------

dry_run=0
signal=TERM

while [ $# -gt 0 ]; do
    case "$1" in
        -n|--dry-run) dry_run=1 ;;
        -9|--force)   signal=KILL ;;
        -h|--help)    usage; exit 0 ;;
        --)           shift; break ;;
        -*)           die "unknown option: $1 (try --help)" ;;
        *)            break ;;
    esac
    shift
done

[ $# -gt 0 ] || die "missing PORT (try --help)"

for port in "$@"; do
    case "$port" in
        ''|*[!0-9]*) die "not a port number: $port" ;;
    esac
    [ "$port" -ge 1 ] && [ "$port" -le 65535 ] || die "port out of range: $port"
done

command -v lsof >/dev/null 2>&1 || die "lsof not found" 1

me=$(id -un)
found=0
killed=0

# ---------------------------------------------------------------------------
# MAIN
# ---------------------------------------------------------------------------

for port in "$@"; do
    # -sTCP:LISTEN is the whole point; -nP keeps lsof from resolving names and
    # turning a local lookup into a DNS wait.
    rows=$(lsof -nP -iTCP:"$port" -sTCP:LISTEN 2>/dev/null | tail -n +2)

    if [ -z "$rows" ]; then
        printf 'port %s: nothing listening\n' "$port"
        continue
    fi

    # A server bound to both IPv4 and IPv6 shows up as two rows, same pid.
    seen=" "
    while read -r comm pid user _rest; do
        [ -n "${pid:-}" ] || continue
        case "$seen" in *" $pid "*) continue ;; esac
        seen="$seen$pid "
        found=$((found + 1))

        if [ "$pid" -eq $$ ] || [ "$pid" -eq 1 ]; then
            printf 'port %s: skipping pid %s (%s)\n' "$port" "$pid" "$comm"
            continue
        fi

        if [ "$user" != "$me" ]; then
            printf 'port %s: %s (pid %s) belongs to %s - skipped\n' \
                "$port" "$comm" "$pid" "$user"
            continue
        fi

        if [ "$dry_run" -eq 1 ]; then
            printf 'port %s: would SIG%s %s (pid %s)\n' "$port" "$signal" "$comm" "$pid"
            continue
        fi

        if ! kill -"$signal" "$pid" 2>/dev/null; then
            printf 'port %s: could not signal %s (pid %s)\n' "$port" "$comm" "$pid"
            continue
        fi
        printf 'port %s: SIG%s -> %s (pid %s)\n' "$port" "$signal" "$comm" "$pid"
        killed=$((killed + 1))

        # SIGTERM is a request. Give it a moment, then say so if it was ignored.
        if [ "$signal" = "TERM" ]; then
            waited=0
            while [ "$waited" -lt 6 ] && kill -0 "$pid" 2>/dev/null; do
                sleep 0.5
                waited=$((waited + 1))
            done
            if kill -0 "$pid" 2>/dev/null; then
                printf 'port %s: pid %s still alive after SIGTERM (retry with -9)\n' \
                    "$port" "$pid"
            fi
        fi
    done <<EOF
$rows
EOF
done

[ "$found" -gt 0 ] || exit 1
[ "$dry_run" -eq 0 ] || printf '\nNothing was signalled.\n'
exit 0