A Bash script that runs a co-process and waits until any process is accepting connections on a specific local TCP port.

wait_for_listen_on_port.sh

#!/usr/bin/env bash
# A script that runs a co-process and waits until any process is accepting connections on a specific local TCP port.
#
# Upon successfully connecting to that given port, a SIGTERM is sent to the co-process.
# No timeout is applied while waiting, however this script can conveniently wrapped in a TIMEOUT(1) call.
#
# This script requires no external tools and will make use of Linux' /proc file system.

if [[ $# -lt 2 ]]; then
  printf 'Usage: %s PORT COMMAND [ARGS...]\n' "$0" >&2
  exit 1
fi

port=$1
shift
printf -v expected_local_address '%08X:%04X' 0 "$port"

coproc "$@"

# The COPROC_PID variable will be unset when the corresponding process exits
while [[ -n "$COPROC_PID" ]]; do
  while read -r _ local_address _ connection_state _; do
    if [[ "$connection_state" == 0A && "$local_address" == "$expected_local_address" ]]; then
      kill "$COPROC_PID"
      wait -f "$COPROC_PID"
      status=$?
      # Exit code 143 means "terminated by SIGTERM" which is expected here.
      # Let's return 0 in this case.
      if [[ "$status" -eq 143 ]]; then
        status=0
      fi
      break 2
    fi
  done </proc/"$COPROC_PID"/net/tcp
  sleep 1
done

exit "${status:-1}"