Rewrite #10 with refined interactive command design: bare-word REPL fallback instead of ! prefix, result table assigned to _, same shape as captured commands. Add #11 for Ctrl-C clearing the current line instead of exiting the REPL.
This commit is contained in:
Cormac Shannon
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# Issue #10 — Interactive command execution
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## Problem
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**Status:** open
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All backtick commands redirect stdout/stderr to pipes and capture output into a table `{code, stdout, stderr}`. Interactive programs lose their tty connection:
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## The two modes
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Lush has two distinct ways to run external commands:
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1. **Captured** — backtick syntax, pipes stdout/stderr, returns a table:
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```lua
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local r = `ls -l`
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r.code -- exit code
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r.stdout -- captured stdout
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r.stderr -- captured stderr
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```
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2. **Interactive** — bare command (no backticks), inherits the terminal, the process owns stdin/stdout/stderr directly:
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```
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> ls -l -- output goes straight to terminal
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> vim foo.lua -- full tty control, works properly
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> ssh user@host -- interactive session
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```
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This mirrors how traditional shells work: commands run interactively by default, and you explicitly capture output when you want it (in bash: `var=$(cmd)`; in lush: `` var = `cmd` ``).
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## Syntax — bare-word REPL fallback
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If a line fails to parse as valid Lua, try to interpret it as an interactive shell command:
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```
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> vim foo.lua -- not valid Lua → runs as shell command
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> ls -la -- not valid Lua → runs as shell command
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> local x = 1 -- valid Lua → runs as Lua
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> x = `ls`.stdout -- valid Lua → runs as Lua (captured)
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```
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This only applies in the REPL. In scripts, use backtick syntax for captured commands (interactive commands in scripts are a separate question — possibly via `!` prefix or `exec()`, deferred).
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## Return value and `_`
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Interactive commands can't capture stdout/stderr (the process owns the terminal). However, the exit code is still useful. After an interactive command completes, lush assigns a result table to the global `_`:
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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 has the same shape as a captured command result, but with empty stdout/stderr:
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```lua
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`bash` -- launches bash but it can't read/write the terminal
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`vim foo` -- screen is blank, no input works
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{code = <exit_code>, stdout = "", stderr = ""}
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```
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A real shell needs to hand the terminal to a child process and wait for it to finish.
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This keeps the interface consistent — `_` always has `.code`, `.stdout`, `.stderr`, whether the command was interactive or captured. Code that only checks `.code` works with both.
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## What interactive execution means
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### Why `_`?
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The child process inherits all three file descriptors (stdin, stdout, stderr) directly from the parent. The parent `waitpid()`s until the child exits. The terminal behaves as if lush isn't there — the child owns it.
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## Syntax
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### Option A: `!` prefix (recommended starting point)
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A `!` token marks a command as interactive. The command string extends to end-of-line (no closing delimiter needed):
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```lua
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!bash
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!vim foo.lua
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!ssh user@host
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```
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`!` doesn't conflict with Lua syntax (`not` is the logical-not keyword). Supports `${}` interpolation like backticks:
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```lua
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local f = "foo.lua"
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!vim ${f}
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```
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### Option B: Bare-word REPL fallback (separate issue?)
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If a line fails to parse as Lua, interpret it as an interactive command:
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```
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> vim foo.lua -- not valid Lua, runs interactively
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> ls -la -- same
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> local x = 1 -- valid Lua, runs as Lua
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```
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Only applies in the REPL, not scripts. Could be combined with Option A. Touches `lua.c` (REPL loop) rather than the lexer/parser, so may warrant its own issue.
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### Option C: Built-in function
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```lua
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exec("bash")
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exec("vim", "foo.lua")
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```
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No new syntax. Could serve as the low-level primitive that `!` compiles down to.
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## Return value
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Interactive commands return the exit code as a plain integer:
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```lua
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local code = !bash
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assert(type(code) == "number")
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```
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No stdout/stderr to capture — they go to the terminal. Returning an integer (not a table) clearly distinguishes interactive from captured commands.
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When used as a statement (no assignment), the return value is discarded.
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- `_` is largely unused in Lua beyond the community convention of throwaway variables in unpacking (`local _, b = f()`)
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- This convention is safe here: backtick commands return a hashmap table, not a sequence, so `local _, x = \`cmd\`` isn't meaningful anyway (you'd use `\`cmd\`.stdout`)
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- Bash uses `$?` for the last exit code — `_` serves a similar role
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## Runtime implementation
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Add `luaB_interactive` to `lcmd.c`, registered as `__interactive` global:
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- `fork()` without creating any pipes
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- Child: `execvp()` directly (inherits parent's stdin/stdout/stderr)
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- Parent: ignore `SIGINT`/`SIGQUIT` (so Ctrl-C goes to child, not lush), then `waitpid()` and return exit code as integer
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- Child: restore `SIG_DFL` for `SIGINT`/`SIGQUIT` before `execvp()`
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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()` and return result table
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- Reuses existing `parse_argv()` for command string tokenization
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## Lexer changes (if using `!` prefix)
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## REPL implementation
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- Add `TK_ICMD` and `TK_ICMD_INTERP` tokens to enum in `llex.h`
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- Add `in_icmd` flag to `LexState` (mirrors `in_command` for backticks)
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- `read_icmd_body()`: like `read_command_body()` but terminates on newline/EOF instead of closing backtick
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- `read_icmd()`: skips leading whitespace after `!`, then calls `read_icmd_body()`
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- `luaX_readicmdcont()`: continuation reader after `${}` interpolation (mirrors `luaX_readcommandcont()`)
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- Handle `!` case in `llex()` switch: when `!` is followed by a non-`=` character at statement position, read as interactive command
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In `lua.c`'s `loadline()`, after both `addreturn()` and `multiline()` fail:
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## Parser changes (if using `!` prefix)
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- Add `interactiveexp()` in `lparser.c` — mirrors `commandexp()` but compiles to `__interactive(cmdstring)` instead of `__command(cmdstring)`
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- Add `TK_ICMD` / `TK_ICMD_INTERP` cases in `simpleexp()`
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- Both simple and interpolated forms follow the same pattern as backtick commands
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1. Get the raw input line
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2. Pass it to `__interactive(line)`
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3. Assign the result to `_`
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4. Continue the REPL loop (don't report a Lua syntax error)
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## Open questions
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- Which syntax option(s) to pursue? Could start with `!` prefix and add others later.
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- Job control: should `Ctrl-Z` suspend the child and return to lush? Requires `tcsetpgrp()` / process group work. Probably defer to a later issue.
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- Should bare-word REPL fallback (Option B) be a separate issue?
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- `PATH` lookup: `execvp` already searches `PATH`, so this should just work.
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- Should `!` support multi-line commands (e.g. with `\` continuation)? Probably not — keep it simple.
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- Should `_` be set after captured (backtick) commands too? (So `_.code` always reflects the most recent command regardless of mode.)
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- Should the bare-word fallback support `${}` interpolation? (Probably not in the REPL fallback — the line isn't parsed by the lexer at all, it's just a raw string.)
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- Job control: should Ctrl-Z suspend the child and return to lush? Requires `tcsetpgrp()` / process group work. Defer to a later issue.
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- How to handle bare commands in scripts (not the REPL)? Defer — `!` prefix or `exec()` builtin are options for later.
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## Files touched
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| File | Description |
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|------|-------------|
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| `lcmd.c` | Add `luaB_interactive` — fork/exec without pipes, return exit code |
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| `lcmd.c` | Add `luaB_interactive` — fork/exec without pipes, return result table |
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| `lcmd.h` | Declare `luaB_interactive` |
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| `linit.c` | Register `__interactive` global in `opencommand()` |
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| `llex.h` | Add `TK_ICMD`, `TK_ICMD_INTERP` tokens; `in_icmd` flag on `LexState`; declare `luaX_readicmdcont()` |
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| `llex.c` | Add `read_icmd_body()`, `read_icmd()`, `luaX_readicmdcont()`; handle `!` in `llex()` |
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| `lparser.c` | Add `interactiveexp()`; handle `TK_ICMD`/`TK_ICMD_INTERP` in `simpleexp()` |
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| `lua.c` | *(Only if doing Option B)* REPL fallback logic in `loadline()` |
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| `lua.c` | REPL fallback: on parse failure, try as interactive command, assign result to `_` |
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