How to create or attach to a tmux session in one command?

Short answer

tmux new -A -s name

If a session called name already exists, you attach to it; otherwise it is created. No “duplicate session” error either way.

Details

tmux new-session (aliased tmux new) normally fails if the session name is already taken. The -A flag changes that: it makes new-session behave like attach-session when the target already exists.

That single property makes it the right command for anything non-interactive:

• In a shell startup file (.zshrc, .bashrc) to always land in a named session.
• In a project script, so the first run creates the session and later runs just re-attach to it.

Add -d to create the session detached:

tmux new -A -d -s name

One trap: -d only applies when the session gets created. If it already exists, -A switches to attach behaviour and the command attaches anyway; in a script with no terminal it fails with open terminal failed: not a terminal. For a strict create-if-missing, guard the call instead:

tmux has-session -t name 2>/dev/null || tmux new -d -s name

Verified on tmux 3.2a.

Links & references

• Wiki: tmux