Auto-Attach or Start a Tmux Session at Shell Login (with a Smart Check)

Inspired by this helpful post on Coderwall. Full credit goes to the original author!

If you're a terminal-heavy developer like me, you probably live inside tmux. But there's always that small annoyance: having to manually attach to your session every time you open a new shell. What if your shell could do it for you—automatically?

The Goal

We want to auto-attach to a tmux session when a new terminal is opened—but only when:

• We're not already inside a tmux session.
• We're not connected via SSH (to avoid weird nested tmux-in-tmux behavior).
  

The Script

if [[ "$TERM" != "screen" ]] && 
        [[ "$SSH_CONNECTION" == "" ]]; then
    # Attempt to discover a detached session and attach 
    # it, else create a new session

    WHOAMI=$(whoami)
    if tmux has-session -t $WHOAMI 2>/dev/null; then
        tmux -2 attach-session -t $WHOAMI
    else
        tmux -2 new-session -s $WHOAMI
    fi
else

    # One might want to do other things in this case, 
    # here I print my motd, but only on servers where 
    # one exists

    # If inside tmux session then print MOTD
    MOTD=/etc/motd.tcl
    if [ -f $MOTD ]; then
        $MOTD
    fi
fi

This snippet checks:

• If you're not inside a tmux session ($TERM != screen)
• And you're not SSH'd into the machine ($SSH_CONNECTION == "")

If both conditions are true, it will try to attach to a tmux session named after your username (via whoami). If it can't find one, it will create a new session for you.

Want to enable it for SSH too?

if [[ "$TERM" != "screen" ]] && 
        [[ "$SSH_CONNECTION" == "" ]]; then

To this:

if [[ "$TERM" != "screen" ]]; then

And you’re good to go!

Why This is Handy

• Saves time (no more tmux attach every time).
• Prevents nested tmux sessions.
• You can still use SSH without getting tmux'd against your will.
  

Final Thoughts

A small shell tweak like this can make a big difference in your daily terminal workflow. Huge shoutout to Coderwall post by Justin Force for this tip—I just had to share it!