50 lines
3.2 KiB
Markdown
50 lines
3.2 KiB
Markdown
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# isatty - test if a file descriptor is a terminal
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## Metadata
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| Field | Value |
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| Institute | Molodetz Institute — Research & Retrieval Division |
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| Subject | shell-multiplexer-auto-attach |
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| Document type | Source Dossier |
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| Tier | 1 |
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| Source title | isatty - test if a file descriptor is a terminal |
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| Source URL | https://fishshell.com/docs/3.7/cmds/isatty.html |
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| Source publisher / author | fish-shell project (fishshell.com) |
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| Source type | vendor docs |
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| Source date (published) | Unknown — not stated |
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| Date researched | 2026-10-10 |
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| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
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| Research method | web search synthesis |
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| Confidence | High |
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| Status | Active |
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## Summary
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fish ships `isatty` as its own standalone builtin command (distinct from a `test -t` primary), accepting either a numeric file descriptor or one of the names `stdin`/`stdout`/`stderr`, defaulting to fd 0 if no argument is given, and returning exit status 0 when the descriptor is a terminal. A dedicated search for whether fish's `test` builtin itself supports a `-t` file-descriptor primary (the POSIX `test -t` equivalent) came back empty across every doc mirror checked — `isatty` is the only confirmed mechanism for this check in fish.
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## Key findings
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- `isatty` takes a named stream (`stdin`, `stdout`, `stderr`) or a raw fd number; `isatty stdin`/`isatty stdout` is the idiomatic way to check both streams, directly mirroring what the bash/zsh plexer block does with `[ -t 0 ] && [ -t 1 ]`.
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- Exit status 0 means "is a terminal", non-zero means "is not" — same polarity as POSIX `test -t`, so no inverted-logic trap when porting the bash/zsh condition.
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- The same behavior is documented identically across every fish doc version mirror checked from 3.2 through 4.5 (the 3.2, 3.3, 3.4, 3.5, 3.6 version pages and the w3cub 4.5 mirror all state the same thing), so this is a long-stable, safe builtin to depend on rather than a recent or version-fragile addition.
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- fish's `test` builtin was separately confirmed (via the companion search for a changelog/version-introduction date) to not document a `-t` primary at all in any version checked; `isatty` is therefore not an optional shortcut but the only available mechanism for this check in fish.
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## Pros / strengths
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- Officially documented, stable across at least five minor versions (3.2–3.6, plus 4.5 via mirror) with word-for-word identical behavior description — a safe primitive to build the plexer hook's tty check on.
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## Cons / limitations / caveats
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- Exact version of introduction could not be pinned down (see the separate, explicitly inconclusive isatty-version-changelog search, Tier 1 dossier fishshell-blog-release-3-4-0.md); documented as early as 3.2 in this search, which is more than sufficient for this project's purposes (any fish worth supporting today postdates 3.2 by years) but is a documented gap, not a guess.
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## Relevance to tai
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Directly the basis for `FISH_PLEXER_BLOCK` in tai.py using `isatty stdin` and `isatty stdout` instead of attempting a `test -t 0`/`test -t 1` primary that fish's `test` builtin does not support.
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## Changelog
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| Date | Change |
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| 2026-10-10 | Initial dossier created. |
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