tmux/Q/Why does a tmux -t =session target fail in zsh?

• Why does a tmux -t =session target fail in Zsh?

  • AI Answer

    • Short answer: Zsh eats the = before tmux ever sees it. =word is zsh's equals expansion, which substitutes the full path of the command word — and errors when there is no such command. Quote it:
      • sh

        tmux new-window -d -t '=hayward-2026-08-24' -n build

    • What each side means by =

      • To tmux, a leading = on a -t target means exact match: -t =mysession matches the session named exactly mysession rather than the first session with that prefix. Useful whenever two sessions share a prefix — hayward-2026-08-24 and hayward-2026-08-24-scratch.
      • To Zsh, a leading = on an unquoted word means look up this command and substitute its path. The two conventions collide on exactly the argument where the tmux one is wanted.

    • Reproduced

      • sh

        zsh -c 'print -r -- =ls'
        # /bin/ls
        zsh -c 'print -r -- =hayward-2026-08-24'
        # zsh:1: hayward-2026-08-24 not found
        zsh -c "print -r -- '=hayward-2026-08-24'"
        # =hayward-2026-08-24

      • The failure is reported by zsh, before the command runs at all. Nothing named tmux appears in the message, which is what makes it read like a missing session rather than a shell expansion.

    • It is an option, and it is on by default

      • The behavior is the EQUALS option. setopt does not list it, because setopt prints only what differs from the default — so its absence from that listing is what tells you it is on.
      • The negative test is what settles it:
        • sh

          zsh -c 'unsetopt equals; print -r -- =nope'
          # =nope

      • Turning EQUALS off globally is the wrong fix. Quoting is local, needs no shell state, and is correct in every shell.

    • Where it bites

      • Any tmux command taking a target: new-window -t, split-window -t, send-keys -t, respawn-pane -t, has-session -t. Session, window, and pane targets are all affected, since the exact-match prefix is a property of the target syntax rather than of any one command.
      • Bash has no equals expansion, so a script that worked when run with bash fails when the same line is pasted into an interactive zsh prompt — measured on 2026/08/24 against -t =reeve-2026-08-24, which failed with reeve-2026-08-24 not found.

    • Sources

      • zshexpn(1) — Filename Expansion — "If a word begins with an unquoted = … the rest of the word is taken as the name of a command … replaced with the full pathname."
      • tmux(1) man page — COMMANDS — a target beginning with = must match exactly.