How to start tmux with attach if a session exists

If naming your session is okay, then it's easy to do with the new-session command:

tmux new-session -A -s main

where main is the session name that will be attached to or created if needed.

From man tmux:

The -A flag makes new-session behave like attach-session if session-name already exists; in this case, -D behaves like -d to attach-session.

This can be shortened to rely on the default session name (which is 0):

tmux new -As0

Please also note that the -A option was introduced in tmux version 1.8 on 26 March 2013. For earlier versions, use:

tmux attach || tmux

The answer is much simpler. Just put this in your ~/.tmux.conf file:

# if run as "tmux attach", create a session if one does not already exist
new-session -n $HOST

If you run tmux attach and there is a session, then it will attach to that session (whether it's already attached or not). If there is not a session already then it will create one for you.

This will start a new session if attach gives an error:

tmux attach || tmux new

So an alias will do the job:

tm="tmux attach || tmux new"