# 03 — Implement direct command execution runtime (no shell)

**Status:** done
**Blocked by:** #02
**Blocks:** #06, #07

Implement the C runtime that executes commands when a backtick expression is evaluated.

**Critical constraint:** do NOT spawn a shell (no `bash`, `dash`, `sh`, `system()`, `popen()`).

## Return value

Backtick expressions return a table:

```lua
local r = `ls -lha`
r.code    -- exit code (integer)
r.stdout  -- captured stdout (string)
r.stderr  -- captured stderr (string)
```

## Implementation approach

- `fork()` + `execvp()` on Unix/macOS
- Set up two pipes: one for stdout, one for stderr
- In child: `dup2()` pipes onto fd 1 and fd 2, then `execvp(argv[0], argv)`
- In parent: read both pipes into buffers, `waitpid()` for exit status
- Build a Lua table with `code`, `stdout`, `stderr` fields and push onto stack
- Depends on #04 (argv parsing) to tokenize the command string into argv[]

## Error handling

- Command not found → set `code` to an error value, `stderr` to error message
- Permission denied → same
- `fork()` failure → raise a Lua error

## Open questions

- New file (`lcmd.c` / `lcmd.h`) or add to an existing lib?
- Signal handling during child execution
- Binary output — return raw string bytes as-is?
- Set `__tostring` metamethod on result table?