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# Command Line Interface
The `@mozilla/glean` package exposes the `glean` CLI.
This utility installs `glean_parser` in a virtual environment
and allows users to execute `glean_parser` command through it.
In order to get a list of all commands available run `npx glean --help`.
## Customizing virtual environment
The `glean` CLI will look for / create a `.venv` virtual environment
at the directory it is called from and install `glean_parser` there by default.
However, it is possible to customize the directory in which it looks for a virtual environment,
by setting the `VIRTUAL_ENV` environment variable. Example:
```bash
VIRTUAL_ENV="my/venv/path" npx glean --help
```
By default Glean will try and use specific binaries based on the platform: `python.exe` on windows and `python3` everywhere else. You can customize the Python binary used through environment variables. You are able to define both `GLEAN_PYTHON` and `GLEAN_PIP`. Example:
Mac:
```bash
export GLEAN_PYTHON=<path_to_python_binary>
export GLEAN_PIP=<path_to_pip_binary>
```
Windows:
```bat
SET GLEAN_PYTHON=<path_to_python_binary>
SET GLEAN_PIP=<path_to_pip_binary>
```
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### What if the `glean` command is called from inside an active virtual environment?
> The `VIRTUAL_ENV` environment variable [is automatically set while a virtual environment in active](https://docs.python.org/3/library/venv.html), in which case Glean will not create a new environment, but use the currently active one.
## Preventing automatic installation of `glean_parser`
In order to prevent the `glean` CLI from installing `glean_parser` itself, it must be provided
with the path to a virtual environment that already has `glean_parser` installed.
When doing that it is important to keep the separate installation of `glean_parser` in sync with
the version expected by the Glean SDK version in use. The `glean` CLI exposes the `--glean-parser-version` command that returns the expected version of `glean_parser`
for such use cases.
With this command it is possible to something like:
```bash
pip install -U glean_parser==$(npx glean --glean-parser-version)
```