In the last days I was suddenly faced with an oddly persistent issue: New tmux sessions were started with sh instead of my default shell. Even changing my .tmux.conf did not seem to make a difference:
set -g default-shell /bin/zsh
Then the internet reminded me that tmux is driven by a server process. Killing the process and having it restart by creating a new session magically brought it back to my default shell zsh.
So something must have started this tmux session without my default shell.
I also noticed that this happened reliably in my new tmux session started at boot to control my Framework 16 LED Matrix. Realizing that also my whole environment (mainly populated by .zshenv) was missing, I started to put one and one together.
An inconspicuous crontab change
Apparently, the crontab session was the actual culprit. Through launching a tmux session in my user crontab with @reboot a tmux server process was launched without my environment. This seems to be a general issue with at least personal crontabs as a minutious cronjob to print out env showed me:
*/1 * * * * env
Observed via:
journalctl --no-hostname --follow
I could not find a way to have cron use the default user shell, so instead I put SHELL=/bin/zsh at the top of my crontab file, which also correctly initialized the whole environment.
Amendment: Graphical Environment
Not all environment is present though - unlike when I run tmux later, the graphical desktop environment is not yet present, so in case I want to run something that depends on this, I need to go back and pkill tmux to get a server with the whole environment.
Short of not using tmux in my crontab, I have not found a solution for that yet. I guess crontab might not be the right choice here, but an xdg autostarted .deskop-file.