# Run BT programs

## Function

The BT interpreter executes `.bt` files. When started without a command or source argument, it executes `main.bt` if that file exists in the current directory; otherwise, it opens the interactive prompt. Ordinary startup does not install BT or check installed versions.

## Run and debug

```bt

// Output: hello world
print 'hello world'
```

 After installing the editor extension `bt-lang`, you can create `main.bt` in the project directory and run it directly.

## Command line execution

```text
bt demo.bt
bt -c demo.bt
```

Both commands execute the script and exit normally, without adding a pause. Use a full interpreter path if BT is not on `PATH`. To run a script at the `bt>` interactive prompt, enter `-c demo.bt`.

## Installation commands

Pending the next release: run `bt install` to install the current interpreter for your user account. The interactive prompt accepts `install` and `update` too. Use `bt update` to update an existing interpreter installation from the official website, or `bt update sqlite` to update an installed SQLite extension. `bt install sqlite` still installs the SQLite extension. See [environment setup](/en/docs/build) for initial installation commands, fixed paths, version selection, and platform requirements.

## Double-click a script

Pending the next release: after installation and selection of BT as the `.bt` default application, double-clicking `demo.bt` opens a terminal using the fixed installed interpreter. The launcher waits for Enter after success, an error, or `exit()`, so output remains visible. It runs the original script without inserting `pause()` or changing its return behavior. This waiting behavior belongs to file-association launches; normal script calls from a terminal or another application do not use it.

## Windows

You can keep `bt.exe` in the project directory. Opening the interpreter without arguments still executes the current directory's `main.bt` when present; otherwise it opens the interactive prompt. Double-clicking the interpreter itself does not trigger installation or the script-association pause.

## Linux and macOS

```shell
cd /home/project
chmod +x bt
./bt
```

## Notes

- Script files use the `.bt` suffix.
- Web services, desktop applications and normal CLI scripts all reuse the same BT language syntax.