Add issue #14: prefix-based interactive commands via !
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Cormac Shannon
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issues/14-prefix-interactive-commands.md
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issues/14-prefix-interactive-commands.md
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# Issue #14 — Prefix-based interactive command execution
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**Status:** open
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**Alternative to:** #10, #12
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## Motivation
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Issue #10 describes bare-word interactive commands via a parser heuristic. The architecture is complex:
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- An 8-entry exception list to distinguish `ls -la` (shell) from `print("hello")` (Lua)
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- Raw source buffer position tracking to work around `--` being consumed as a Lua comment
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- A runtime fallback chain for edge cases like `echo "hello"` parsing as `echo("hello")`
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- Issue #12 adding further parser rules for path-based commands (`./script`, `/bin/ls`)
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- Four distinct fallback stages (parser heuristic → normal Lua → runtime retry → post-hoc REPL)
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This issue proposes a simpler alternative: a `!` prefix to explicitly mark interactive commands.
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## Syntax
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A `!` at statement position introduces an interactive command. Everything after `!` to end-of-line is the command string.
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```
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> !ls -la
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> !vim foo.lua
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> !git commit -m "fix bug"
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> !ssh user@host
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> !./script.sh --deploy
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> !/usr/bin/env python3
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> !docker compose up -d
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```
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### Why `!`?
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- `!` is not used in Lua syntax (`not` for logical negation, `~` for bitwise NOT) — zero conflict at statement position
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- Familiar precedent: vim (`:!cmd`), IPython/Jupyter (`!cmd`), ed/ex (`!cmd`)
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- Single character, minimal friction
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## Comparison with bare-word approach (#10)
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| | Bare-word (#10) | Prefix (#14) |
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| --- | --- | --- |
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| Typing | `ls -la` | `!ls -la` |
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| Parser complexity | High (exception list, raw source tracking, `--` workaround) | Minimal (`!` token → capture rest of line) |
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| Runtime fallback | Required (`echo "hello"` edge case) | Not needed |
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| Path commands | Requires issue #12 | Works naturally (`!./script.sh`) |
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| `--` flags | Special handling (conflicts with Lua comments) | No conflict (raw capture after `!`) |
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| Ambiguity | Subtle cases exist | None |
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| Works in scripts | Yes (inside blocks) | Yes (inside blocks) |
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The trade-off: one extra character per command, but dramatically simpler implementation with no edge cases.
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## Parser implementation
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When the lexer sees `!` at statement position:
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1. Record the current source position (character after `!`)
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2. Extract raw text to end-of-line from the source buffer
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3. The parser emits bytecode equivalent to `_ = __interactive("raw line text")`
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4. Advance the lexer past the consumed line
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No heuristic, no exception list, no runtime fallback needed.
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## Return value and `_`
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Interactive commands inherit the terminal — stdout/stderr are not captured. After the command completes, the result is assigned to `_`:
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```
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> !ls -l
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(output appears directly in terminal)
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> _.code
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0
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```
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The table shape matches captured commands, but with blank stdout/stderr:
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```lua
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{code = <exit_code>, stdout = "", stderr = ""}
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```
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This keeps the interface consistent — code that checks `.code` works with both interactive and captured commands.
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## Runtime implementation
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Shares core logic with backtick commands in `lcmd.c`:
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- Reuses `parse_argv()` for command string tokenization
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- `fork()` without creating any pipes
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- Child: restore `SIG_DFL` for `SIGINT`/`SIGQUIT`, then `execvp()` (inherits parent's stdin/stdout/stderr)
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- Parent: ignore `SIGINT`/`SIGQUIT` (so Ctrl-C goes to child, not lush), then `waitpid()`
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- Build result table `{code=N, stdout="", stderr=""}` and assign to `_`
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## Examples
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```
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-- basic usage
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> !ls -la
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> !git status
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> !vim config.lua
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> !make -j8
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-- path-based commands (no special handling needed)
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> !./configure --prefix=/usr
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> !/usr/bin/env python3
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-- double-dash flags (no comment conflict)
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> !git --version
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> !grep --include="*.lua" -r "pattern" .
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-- check exit code
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> !make test
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> if _.code ~= 0 then print("tests failed") end
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-- in scripts
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!cargo build --release
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if _.code ~= 0 then
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print("build failed: " .. _.code)
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os.exit(1)
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end
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```
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## What comes for free
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- **Interpolation**: the command string passes through the same lexer pipeline as backtick strings, so `${expr}` interpolation works without any additional handling.
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- **Piping**: the command string goes through `split_pipeline()` (issue #06) before execution, so `!ls -l | grep foo` works — earlier stages inherit the terminal for stderr, the last stage inherits it for stdout too.
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## Open questions
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- Should `_` also be set after captured (backtick) commands?
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- Could both approaches coexist? `!` for the simple/reliable path, bare-word (#10) added later as sugar on top.
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## Files touched
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| File | Change |
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|------|--------|
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| `lcmd.c` | Add `luaB_interactive` (fork/exec without pipes) |
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| `lcmd.h` | Declare new function |
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| `linit.c` | Register `__interactive` global in `opencommand()` |
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| `llex.c` | Recognise `!` at statement position, extract raw line text |
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| `lparser.c` | Emit `_ = __interactive("raw text")` when `!` detected at statement position |
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