Two ways to check if a SSH control socket is open in BASH

test.sh

#!/bin/bash

# How can I fully log all bash scripts actions?
# https://serverfault.com/a/103569/210437

if test -S "/absolute/path/to/ssh_control_socket"; then
  echo 'It exists'
fi

# Why can't bash recognize the existence of a socket file
# https://stackoverflow.com/a/37618542/2110294

TEST=`ssh -o ControlPath=ssh_control_socket -O check anything_but_required`
if [ "$TEST" = *"Master running"* ]; then
  echo 'It exists'
fi