How can I set my default shell to start up tmux

@StarNamer's answer is generally accurate, though I typically include the following tests to make sure that (1) tmux exists on the system, (2) we're in an interactive shell, and (3) tmux doesn't try to run within itself:

if command -v tmux &> /dev/null && [ -n "$PS1" ] && [[ ! "$TERM" =~ screen ]] && [[ ! "$TERM" =~ tmux ]] && [ -z "$TMUX" ]; then
  exec tmux
fi

---

References

• Using bash's command to check for existence of a command - http://man7.org/linux/man-pages/man1/bash.1.html#SHELL_BUILTIN_COMMANDS
• Why to use command instead of which to check for the existence of commands - https://unix.stackexchange.com/a/85250
• Using $PS1 to check for interactive shell - https://www.gnu.org/software/bash/manual/html_node/Is-this-Shell-Interactive_003f.html
• Expected state of $TERM environment variable "for all programs running inside tmux" - http://man7.org/linux/man-pages/man1/tmux.1.html#WINDOWS_AND_PANES

Start tmux on every shell login, from Arch wiki, seems to work. Simply add the following line of bash code to your .bashrc before your aliases; the code for other shells is very similar:

[[ $TERM != "screen" ]] && exec tmux

Adding a line like

[ -z "$TMUX"  ] && { tmux attach || exec tmux new-session && exit;}

in your bashrc file will probably do the job. Note this line will exit ssh and terminate the connection once you detach or exit tmux. I like this configuration as it saves key strokes to terminate the connection. But if you don't love this(which I think is very unlikely) and would rather remain in the login shell after termination, just remove the exit part:

[ -z "$TMUX"  ] && { tmux attach || exec tmux new-session;}

Also note you shouldn't wrap tmux attach with exec, as this would cause the connection to be closed when there are no tmux sessions to attach to.