How can I make TMUX be active whenever I start a new shell session?

warning this can now 'corrupt' (make it unable to open a terminal window - which is not good!) your Ubuntu logins. Use with extreme caution and make sure you have a second admin account on the computer that you can log into in case you have the same problems I did. See my other answer for more details and a different approach.

Given that warning, the simplest solution can be to append the tmux invocation to the end of your .bashrc, e.g.

alias g="grep"
alias ls="ls --color=auto"

# ...other stuff...

if [[ ! $TERM =~ screen ]]; then
    exec tmux
fi

Note that the exec means that the bash process which starts when you open the terminal is replaced by tmux, so Ctrl-B D (i.e. disconnect from tmux) actually closes the window, instead of returning to the original bash process, which is probably the behaviour you want?

Also, the if statement is required (it detects if the current bash window is in a tmux process already) otherwise each time you start tmux, the contained bash process will attempt to start its own tmux session, leading to an infinite number of nested tmuxen which can be, err, quite annoying (that said, it looks cool).

However, there is a very small risk this can make bash behave in a way that other programs don't expect, since running bash can possibly cause it to turn into a tmux process, so it might be better to modify how you start your terminal emulator.

I use a small executable shell script ~/bin/terminal (with ~/bin in $PATH, so it is found automatically) that looks a bit like:

#!/bin/sh
exec gnome-terminal -e tmux

(I don't use gnome-terminal, so you might have to remove the exec, I'm not sure.)

Now whenever you run the terminal scipt you have a terminal with tmux. You can add this to your menu/desktop/keyboard shortcuts to replace the default terminal.

(This approach also allows you to more easily customise other things about the terminal emulator later, if you ever desire.)

My original, accepted answer, stopped working on my Ubuntu14 system after a recent upgrade.

Using either

[ -z "$TMUX" ] && command -v tmux > /dev/null && TERM=xterm-256color && exec tmux

or

[ $TERM != "screen" ] && TERM=xterm-256color && exec tmux

would stop me from being able to even login. I was only able to resolve this due to having a second admin login on the computer.

The fix for me on Ubuntu (and in osx too) was to change my terminal program to actually run tmux instead, i.e.

I still have

[ `uname -s` != Linux ] && exec tmux

as my last .bashrc line but that his only for my Mac OSX systems now.

If you want to have a single tmux session, put the following in your ~/.bashrc for bash or ~/.zshrc for zsh:

tmux attach &> /dev/null

if [[ ! $TERM =~ screen ]]; then
    exec tmux
fi

The tmux attach line is to make sure if there is a session it attaches to and if there was no session you will not get the warning about "no session".

For me, I would love my tmux to be started every time I shell onto my remote machine, and when I detach or exit from tmux, the connection should be closed automatically. After digging into this issue for a while, the following code does exactly what I want and is believed to be the most optimized to the best of my knowledge.

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

Note this line should be the first line in you bashrc file to make sure it is loaded first. We can't put an "exec" call in front of "tmux attach" because after the exec replaces the bash process with the tmux one, the connection will be closed even if there are no sessions to attach to. Therefore we need an "exit" call to terminate the connection after we detach or exit from the attached sessions. But putting an "exec" in front the new-session command is fine as that's the last command to be executed.

Append following line of code to the end of .bashrc,

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