lush/issues/10-interactive-command-execution.md

# Issue #10 — Interactive command execution

**Status:** open

## The two modes

Lush has two distinct ways to run external commands:

1. **Captured** — backtick syntax, pipes stdout/stderr, returns a table:
   ```lua
   local r = `ls -l`
   r.code    -- exit code
   r.stdout  -- captured stdout
   r.stderr  -- captured stderr
   ```

2. **Interactive** — bare command (no backticks), inherits the terminal, the process owns stdin/stdout/stderr directly:
   ```
   > ls -l              -- output goes straight to terminal
   > vim foo.lua         -- full tty control, works properly
   > ssh user@host       -- interactive session
   ```

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` ``).

## Syntax — bare-word fallback

If a line fails to parse as valid Lua, try to interpret it as an interactive shell command. This works in both the REPL and scripts — bare commands can appear anywhere a Lua statement can, including inside `if`/`for`/`while`/`do`/`function` blocks.

```
> vim foo.lua        -- not valid Lua → runs as shell command
> ls -la             -- not valid Lua → runs as shell command
> local x = 1        -- valid Lua → runs as Lua
> x = `ls`.stdout    -- valid Lua → runs as Lua (captured)
```

```lua
-- deploy.lua
local env = os.getenv("ENV") or "staging"
print("deploying to " .. env)
ssh deploy@prod ./restart.sh
ls -la /var/log
if _.code ~= 0 then
  print("deploy failed")
  os.exit(1)
end
```

Bare commands also work inside Lua blocks:

```lua
do
  ls
  ssh site.com
  btop
  ls -lha /
  print("hello world")
end
ls
print("hello world again")
```

## Parser-level detection

The bare-word fallback is implemented in the parser, not as a post-hoc retry. This allows bare commands inside Lua blocks and preserves single-chunk compilation for scripts.

### The heuristic

When the parser sees an identifier (not a keyword) at statement position, it peeks at the next token. If the next token is in the **exception list**, the statement is parsed as Lua. If not, it's a shell command.

The **exception list** (tokens that can follow an identifier in a valid Lua statement):

| Token | Lua meaning |
|-------|-------------|
| `(` | function call: `f(...)` |
| string literal | function call: `f "..."` |
| `{` | function call: `f {...}` |
| `.` | field access: `t.field` |
| `:` | method call: `obj:method()` |
| `[` | index: `t[k]` |
| `=` | assignment: `x = ...` |
| `,` | multi-assignment: `x, y = ...` |

This is exhaustive — every valid Lua statement starting with an identifier must have one of these tokens second. Anything else (another identifier, `-`, `/`, `+`, a keyword, EOF) means the line cannot be valid Lua, so it's safe to treat as a shell command. No valid Lua syntax is stolen.

### No newline awareness needed for detection

The heuristic doesn't need to distinguish same-line vs different-line tokens. A bare identifier like `ls` on its own line is followed by whatever comes next — an identifier, keyword, or EOF — none of which are in the exception list. So `ls` is correctly detected as a shell command.

Multi-line Lua function calls are preserved because their continuation tokens are in the exception list:

```lua
f
("hello")    -- f + ( → exception list → Lua ✓

f
"hello"      -- f + string → exception list → Lua ✓

f
{1, 2, 3}   -- f + { → exception list → Lua ✓
```

### Newline awareness for argument capture

Once the parser detects a shell command, it needs to know where the arguments end. Shell commands are newline-terminated — the parser extracts the raw text of the rest of the line from the source buffer (rather than reconstructing from tokens). This preserves the exact argument string, including characters the Lua lexer can't tokenize (like `@` in `ssh user@host`).

The parser emits bytecode equivalent to `_ = __interactive("raw line text")`.

### Double-dash flags (`--`)

`--` starts a comment in Lua. This conflicts with double-dash flags in shell commands: `git --version`, `ls --color=auto`, `grep --include="*.lua"`.

The problem: when the parser detects a shell command and peeks at the next token, the lexer may encounter `--` and consume the rest of the line as a comment. The argument text is lost before the parser can capture it.

The solution: the parser must record the source buffer position **before** peeking at the next token. When the heuristic determines "shell command," the parser extracts the raw text starting from the saved position (the character right after the first identifier), scanning to the end of the line in the source buffer. This bypasses the lexer entirely for the argument portion — `--version` is captured as raw text, not interpreted as a comment.

This means the lexer's position may be ahead of the raw-captured text (it already skipped the "comment"). The parser must advance the lexer to the correct position (next line) after extracting the raw command.

## Classification examples

### Shell — identifier + non-exception token

| Input | Next token | Result |
|-------|-----------|--------|
| `ls -la` | `-` | shell |
| `ls` | next statement / EOF | shell |
| `git status` | `status` (identifier) | shell |
| `git commit -m "fix bug"` | `commit` (identifier) | shell |
| `cd /tmp` | `/` | shell |
| `grep -r "pattern" .` | `-` | shell |
| `docker compose up -d` | `compose` (identifier) | shell |
| `tar xzf archive.tar.gz` | `xzf` (identifier) | shell |
| `sudo ls` | `ls` (identifier) | shell |
| `ssh user@host` | lexer error on `@` → raw capture | shell |
| `curl https://example.com` | `https` (identifier) | shell |
| `git --version` | `--` (lexer sees comment) | shell (raw source capture) |
| `ls --color=auto /tmp` | `--` (lexer sees comment) | shell (raw source capture) |
| `nonexistent_cmd` | next statement / EOF | shell (runs, exits 127) |

### Lua — identifier + exception-list token

| Input | Next token | Result |
|-------|-----------|--------|
| `print("hello")` | `(` | Lua |
| `print "hello"` | string literal | Lua |
| `f {1, 2, 3}` | `{` | Lua |
| `io.write("x")` | `.` | Lua |
| `obj:method()` | `:` | Lua |
| `t[1] = 5` | `[` | Lua |
| `x = 5` | `=` | Lua |
| `x, y = 1, 2` | `,` | Lua |

### Lua — keywords (heuristic doesn't apply)

| Input | Result |
|-------|--------|
| `local x = 1` | Lua |
| `if x then ... end` | Lua |
| `for i = 1, 10 do ... end` | Lua |
| `return x` | Lua |
| `while true do ... end` | Lua |

### Lua syntax + runtime fallback to shell

Lua's syntactic sugar means `echo "hello"` parses as `echo("hello")` — the string literal is in the exception list, so the parser treats it as Lua. A **runtime fallback** catches the error:

1. Lua executes the call → "attempt to call a nil value"
2. Extract the undefined name, look it up in PATH
3. Found → run as interactive shell command, assign result to `_`
4. Not found → report the original Lua error

| Input | Parses as | Runtime |
|-------|----------|---------|
| `echo "hello"` | `echo("hello")` | `echo` nil → PATH → found → shell |
| `grep "pattern"` | `grep("pattern")` | `grep` nil → PATH → found → shell |
| `man "ls"` | `man("ls")` | `man` nil → PATH → found → shell |
| `pirnt "hello"` | `pirnt("hello")` | `pirnt` nil → PATH → not found → Lua error |

### Bare expressions (already invalid Lua)

| Input | Next token | Notes |
|-------|-----------|-------|
| `x -y` | `-` | shell — `x - y` as a bare statement is a syntax error in Lua anyway |
| `a + b` | `+` | shell — same; `a` probably not in PATH → exits 127 |

## The fallback chain

1. **Parser heuristic** — identifier + non-exception token at statement position → emit `_ = __interactive("line")` bytecode. Handles most bare commands during compilation. Works in both REPL and scripts, inside any block.

2. **Normal Lua execution** — if the parser accepted the line as Lua, execute it. If OK → done.

3. **Runtime "attempt to call a nil value"** — extract the undefined name, check PATH. Found → re-execute as `_ = __interactive("line")`. Not found → report the original Lua error. Catches the `echo "hello"` edge case.

4. **Post-hoc fallback (REPL only)** — if the lexer itself errors before the parser heuristic can run (e.g., unusual characters not after an identifier), try the raw input line as a shell command.

## Return value and `_`

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 `_`:

```
> ls -l
(output appears directly in terminal)
> _.code
0
```

The table has the same shape as a captured command result, but with empty stdout/stderr:

```lua
{code = <exit_code>, stdout = "", stderr = ""}
```

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.

### Why `_`?

- `_` is largely unused in Lua beyond the community convention of throwaway variables in unpacking (`local _, b = f()`)
- 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`)
- Bash uses `$?` for the last exit code — `_` serves a similar role

## Runtime implementation

Add `luaB_interactive` to `lcmd.c`, registered as `__interactive` global:

- `fork()` without creating any pipes
- Child: restore `SIG_DFL` for `SIGINT`/`SIGQUIT`, then `execvp()` (inherits parent's stdin/stdout/stderr)
- Parent: ignore `SIGINT`/`SIGQUIT` (so Ctrl-C goes to child, not lush), then `waitpid()` and return result table
- Reuses existing `parse_argv()` for command string tokenization

Add `luaB_command_exists` to `lcmd.c` — PATH lookup utility:

- Takes a command name, searches each directory in `PATH`
- Returns 1 (found) or 0 (not found)
- Used by the runtime fallback (step 3) to decide whether to attempt a shell command or report a Lua error

## Open questions

- Should `_` be set after captured (backtick) commands too? (So `_.code` always reflects the most recent command regardless of mode.)
- Should the bare-word fallback support `${}` interpolation? (Probably not — the line isn't parsed by the lexer at all, it's just a raw string.)
- Job control: should Ctrl-Z suspend the child and return to lush? Requires `tcsetpgrp()` / process group work. Defer to a later issue.
- Path-based commands (`./script.sh`, `/usr/bin/foo`, `~/bin/script`) — first token isn't an identifier, so the parser heuristic doesn't apply. See issue #12.

## Files touched

| File | Description |
|------|-------------|
| `lcmd.c` | Add `luaB_interactive` (fork/exec without pipes), `luaB_command_exists` (PATH lookup) |
| `lcmd.h` | Declare new functions |
| `linit.c` | Register `__interactive` and `__command_exists` globals in `opencommand()` |
| `llex.c` | Track line boundaries so parser can extract raw source text for shell command arguments |
| `lparser.c` | Bare-word heuristic in statement parsing: detect shell commands via exception list, emit `__interactive` calls with raw source text |
| `lua.c` | Runtime fallback: catch "attempt to call a nil value" in `docall`, check PATH, re-execute as shell |