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@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
  

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.

There is command chsh which changes login shell. Consult man chsh.

I'm successfully using

case $- in *i*)

    [ -z "$TMUX" ] && exec tmux

esac

in my .zshrc. If you're using bash, put it in your .bashrc instead.

I also just tried setting tmux as my default shell (chsh -s $(which tmux)) and it seems to break direct command execution via SSH, e.g. ssh $some_server echo foo will not produce any output.

Add this into your ~/.tmux.conf

set -g default-command /usr/local/bin/fish

None of the above responses worked for me - exec tmux prevents me closing tmux without quitting the shell (whether it's opened with Ctrl + T or from the application menu).

I use Linux Mint, which lets you map certain hotkeys to commands, and (un-kosher as it may be to some..) I've got bash shells with commands starting up there, e.g. Win+Alt+B does some sort of convoluted bash -exec(vim) statement to edit my .bashrc, so it acts like a regular shell.

tmux loaded rather than vim under that situation after placing the above at the top of my .bashrc. I've wrapped the tmux executing line in another if statement, checking that it's running in interactive mode.

if command -v tmux>/dev/null; then

        if [ ! -z "$PS1" ]; then # unless shell not loaded interactively, run tmux

                [[ ! $TERM =~ screen ]] && [ -z $TMUX ] && tmux

        fi

fi

You could wrap all that onto one line but for readability I think that's fine.

Go to terminal preferances.

And then click to "command" bar.

Check the "Run a custom command instead of my sell" and write whatever command you want to execute at the startup of your terminal.

This is in my ~/.profile (I'm using ksh93):

if [[ -z "$TMUX" ]]; then

    if tmux has-session 2>/dev/null; then

        exec tmux attach

    else

        exec tmux

    fi

fi

If the TMUX environment variable is unset/empty, then we're not already in a tmux session, so...

If tmux has-session returns with a zero exit status (true), there is an available session to attach to. Attach to it.

If not, create a new session.

It's also possible to use tmux as your login shell. If you do this, however, make sure to set default-shell to an actual shell in your ~/.tmux.conf file (see the tmux manual for more info about this).

You could (and as of now probably should) use chsh command to set tmux as default shell, then add the following to .tmux.conf:

set-option -g default-shell /path/to/your/favourite/shell

Just remember to modify the path at the end of the line.

Adding to @Louis Maddox 's answer, I would execute tmux part with;

(exec tmux attach || exec tmux new-session)

I combined the extensive checks, with the conditional session checking, and put my own spin on it, to create a default session that is connected to or else created. Unless you are inside of another tmux session, or other protective conditions are met.


if command -v tmux &> /dev/null && [ -n "$PS1" ] && [[ ! "$TERM" =~ screen ]] && [[ ! "$TERM" =~ tmux ]] && [ -z "$TMUX" ]; then
tmux a -t default || exec tmux new -s default && exit;
fi


Based on these answers:

https://unix.stackexchange.com/a/306165/198110

https://unix.stackexchange.com/a/113768/198110