Automatically launch `tmux` on boot

[ezio-melotti]

In case of power outage, the RPis might reboot. Currently, if that happens, we have to SSH into each one of them and manually relaunch tmux. This should be automated, however:

1. launching tmux from rc.local doesn't seem to work;
2. launching the processes directly works, but it doesn't provide us the ability to see their output conveniently like tmux does.

It should however be possible to define a systemd service that starts the tmux session on boot.

[ezio-melotti]

I looked into this but couldn't find a way to make it work. Here are some notes.

I created this file:

# tmux/sam-tmux.service
[Unit]
Description=Start tmux session at boot
After=network.target

[Service]
User=pi
Type=oneshot
ExecStart=python3 /home/pi/simoc-sam/simoc-sam.py run-tmux
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

I also tried Type=forking instead of oneshot

I set it as executable with chmod +x tmux/sam-tmux.service. I symlinked it with sudo ln -s /home/pi/simoc-sam/tmux/sam-tmux.service /etc/systemd/system/sam-tmux.service. I enabled the service with sudo systemctl enable sam-tmux.service and started it with sudo systemctl start sam-tmux.service. I used restart after making changes. I used status or journalctl -u sam-tmux.service -b to see the logs.

I made the following changes in simoc-sam.py:

@@ -40,10 +40,11 @@ def cmd(func):
 def run(args):
     print('>>', ' '.join(args))
     print('-'*80)
-    result = subprocess.run(args)
+    result = subprocess.run(args, capture_output=True)
     print('-'*80)
     print('<<', result)
     print()
+    print(result.stderr.decode())
     return not result.returncode

This shows the stderr output of the executed commands.

@@ -287,10 +288,10 @@ def run_server():
 def run_tmux(file='mqtt'):
     """Launch a tmux script (or attach to an existing session)."""
     if run(['tmux', 'has-session', '-t', TMUX_SNAME]):
-        run(['tmux', 'attach-session', '-t', TMUX_SNAME])  # attach to sessions
+        return run(['tmux', 'attach-session', '-t', TMUX_SNAME])  # attach to sessions
     else:
         tmux_path = SIMOC_SAM_DIR / 'tmux' / f'{file}.sh'
-        run([str(tmux_path), TMUX_SNAME])  # start new sessions
+        return run([str(tmux_path), TMUX_SNAME])  # start new sessions

This ensures that the exit status of simoc-sam.py is correct (should be done to the other commands too).

I also updated tmux/mqtt.sh to correctly start the venv:

• 111

In tmux/mqtt.sh:

-tmux new-session -s $SNAME -d -x "$(tput cols)" -y "$(tput lines)"
-# The first pane is just a shell
+#tmux new-session -s $SNAME -d -x "$(tput cols)" -y "$(tput lines)"
+echo 'starting new session' >&2
+tmux new-session -s $NAME -d
+# The first pane is just a shell
+echo 'sending keys' >&2

This adds debug prints to stderr and removes tputs (that don't work).

The command still fails with:

starting new session
open terminal failed: not a terminal
sending keys
no server running on /tmp/tmux-1000/default

The last message is repeated for any other command in mqtt.sh.

[ezio-melotti]

In #112 I went for a different approach that doesn't require tmux. However it should be possible to create two systemd services: one that runs the tmux server, and the other that connects to it.

I haven't tried this approach, since the solution proposed in #112 is cleaner.