# Arch Linux Forum Thread — The $TMUX Guard Pattern

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | shell-multiplexer-auto-attach |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | bbs.archlinux.org forum thread (tmux auto-attach) |
| Source URL | https://bbs.archlinux.org/viewtopic.php?id=144544 |
| Source publisher / author | Arch Linux Forums (community) |
| Source type | forum thread |
| Source date (published) | Unknown — not stated |
| Date researched | 2026-10-09 |
| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
| Research method | web search synthesis |
| Confidence | Medium |
| Status | Active |

## Summary

Establishes the core, near-universal guard for auto-attaching tmux from a shell rc file: `[[ -z "$TMUX" ]]`, since tmux itself sets the `$TMUX` environment variable inside any pane it creates. A forum participant confirms a simple `if [[ -z "$TMUX" ]]; then tmux attach; fi` block worked for them in practice.

## Key findings

- `$TMUX` is tmux's own self-identifying environment variable, set automatically inside every pane — checking for its absence is the standard way to avoid launching tmux inside itself (nested sessions).
- This is community-tested, working advice, not a theoretical suggestion.

## Pros / strengths

- Simple, single-variable, directly tied to tmux's own behavior rather than an inferred heuristic — about as reliable a signal as exists for "am I already inside a tmux pane."

## Cons / limitations / caveats

- Only detects nesting on the *same host* — per the query D SSH-nesting dossier in this subject, `$TMUX` does not survive an SSH hop, so this guard alone is insufficient once remote sessions are involved.

## Relevance to tai

The mandatory first guard for the proposal's `.bashrc` hook: never attempt to auto-attach if `$TMUX` (or tai's own equivalent marker) is already set, to prevent runaway nested sessions — this is non-negotiable baseline safety, not an optional refinement.

## Changelog

| Date | Change |
|---|---|
| 2026-10-09 | Initial dossier created. |