# 01 — Design and finalize syntax for all new features

**Status:** done
**Blocks:** #02, #04, #05

## Backtick command execution

Backtick expressions return a plain table with three fields:

```lua
local output = `ls -lha`
print(
  output.code,
  output.stdout,
  output.stderr
)
```

- `output.stdout` — captured stdout as a string
- `output.stderr` — captured stderr as a string
- `output.code` — integer exit code
- No `__tostring` metamethod — access `.stdout` explicitly

### String interpolation

Backticks support `${expr}` interpolation:

```lua
local dir = "/tmp"
local result = `ls -lha ${dir}`
local pattern = "*.lua"
local result2 = `find ${dir} -name ${pattern}`
```

`${expr}` evaluates the Lua expression, converts to string, and splices it into the command string before argv parsing.

## Environment variables — `$` sigil

```lua
local dir = $PWD              -- getenv("PWD") → string or nil
$PWD = "/"                    -- setenv("PWD", "/")
$MY_ENV_VAR = "data"          -- setenv("MY_ENV_VAR", "data")
```

- `$NAME` as an expression → `getenv("NAME")`, returns string or `nil`
- `$NAME = expr` as a statement → `setenv("NAME", tostring(expr))`
- `$` followed by an identifier is a new token type in the lexer
- Note: `$NAME` (env var) is distinct from `${expr}` inside backticks (interpolation)

## Piping (future)

```lua
local result = `ls -l` | `grep ".lua"`
```

Challenge: `|` conflicts with Lua's bitwise OR. Pipes only valid between command expressions.

## Redirection (future)

```lua
`ls -l` > "output.txt"
`ls -l` >> "output.txt"
`cmd` 2> "err.txt"
`cmd` 2>&1
`cmd` < "input.txt"
```

Challenge: `>` / `>>` conflict with Lua's comparison/shift operators. Only valid in command context.