Friction-less login to a tmux session on a suspended host

The Skating Minister by Henry Raeburn – Friction-less! (public domain)

I recently found that more and more I’m using technical solutions only when they are friction-less (I guess that is not that unusual generally, but a bit more unusual for me). This includes my work on a remote machine, which is suspended most of the time, and for which I’ve enabled WOL (Wake-on-LAN, see my related blog post on how I accomplished this).

I wanted to create a way to log into this machine regardless if it is suspended or not, and continue working where I left off (i.e. not losing any state even when it goes to sleep or if the network is interrupted). I’m usually doing complex work in several shell sessions with several files open at once, several processes running at once, etc.

Below, you’ll find a shell script to accomplish this. You can customize the host name and the host’s MAC address directly in the script, and then call it without arguments.

What does the script do?

It will first wake the machine up using wakeonlan . If it is already awake, then wakeonlan will not have any effect, and all is fine. If it is sleeping, then it will come online in a few seconds. Using an exponential back-off strategy, I’m polling the availability of the SSH port 22. If the port comes online, we know that SSH is ready to receive the login. I’m then trying to attach to an already existing tmux session where I can continue to work where I left off after the last ssh disconnection (simulate this by typing sequentially Enter ~ .) or tmux detachment (Ctrl+b d), and if there is no running tmux session, then it will start a new tmux session, which will also keep me logged in using the shell configured for my user (bash or zsh).

And without further ado, here is the script:

https://gist.github.com/michaelfranzl/5a4dc6b6e450577c39522a7deca51358

#!/bin/bash

# Copyright 2020 Michael Karl Franzl

# Permission is hereby granted, free of charge, to any person obtaining a copy of this software and
# associated documentation files (the "Software"), to deal in the Software without restriction,
# including without limitation the rights to use, copy, modify, merge, publish, distribute,
# sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:

# The above copyright notice and this permission notice shall be included in all copies or
# substantial portions of the Software.

# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT
# NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
# DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT
# OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

set -e

host_name="example.com"
host_mac="00:00:00:00:00:00"

echoerr() { printf "%s\n" "$*" >&2; }

wakeonlan ${host_mac}

# exponential backoff strategy for the login
for i in 1 2 4 8 16; do
  if [ $i -ge 16 ]; then
    echoerr "Host ${host_name} did not wake up. Giving up."
    exit 1
  fi

  echo "Waiting until host ${host_name} is awake..."
  nc -z ${host_name} 22 && break
  sleep $i
done

echo "Host ${host_name} is now awake. Logging in..."
ssh -t ${host_name} "tmux a || tmux"

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