## DESCRIPTION

g.extension downloads and installs, removes or updates extensions (addons) from the official GRASS GIS Addons repository or user-specified source into the local GRASS GIS installation.

Two types of extensions are supported:

- Python scripts: they are installed without the need of compilation or (usually) the need of special dependencies.
- Source code (mostly written in C programming language; may also be written in C++, Fortran or other languages): while on MS-Windows systems the requested GRASS GIS extension is downloaded pre-compiled from the GRASS GIS site, on Unix based systems the installation is preceded by the automated download of the extension's source code along with subsequent compilation and installation. This requires a compiler environment to be present on the user's computer.

### Managing installed extensions

Re-running g.extension on an installed GRASS GIS Addon extension re-installs the requested extension which may include updates.

To bulk-update all locally installed GRASS GIS extensions, g.extension.all module is available.

### Where the extensions are installed

GRASS GIS extensions are installed by g.extension into a dedicated directory. The default is a directory for application data and settings inside the user's home directory. On GNU/Linux it is `$HOME/.grass7/addons`, on MS Windows it is `$APPDATA\GRASS7\addons`. The directory is stored in `GRASS_ADDON_BASE` environmental variable.

The flag -s changes this install target directory to the GRASS GIS installation directory (determined by `GISBASE` environmental variable, e.g. `/usr/`) rather than the default GRASS_ADDON_BASE directory (see also documentation for variables). g.extension checks if the user has permission to write to GISBASE or GRASS_ADDON_BASE.

The place where the extensions are installed can be customized by the option prefix. Ensuring that these extensions will be accessible in GRASS GIS is then responsibility of the user.

### Compilation and installation

On MS Windows systems, where compilation tools not readily available, g.extension downloads a compiled executable from the GRASS GIS project server. On all other operating systems where it is not difficult to install compilation tools, g.extension downloads the source code of the requested extension (addon) and compiles it locally. This applies for both C and Python modules as well as any other extensions. The reason is that more things such as manual page are compiled, not only the source code (which is really necessary to compile just in case of C).

## EXAMPLES

### Download and install of an extension

Download and install r.stream.distance into current GRASS installation

    g.extension extension=r.stream.distance

This installs the extension from the official repository. For convenience, a shorter syntax can be used:

    g.extension r.stream.distance

### Download and install of an extension when behind a proxy

Example for an open http proxy:

    # syntax: http://proxyurl:proxyport
    g.extension extension=r.stream.distance proxy="http://proxy.example.com:8080"

Example for a proxy with proxy authentication:

    # syntax: http://username:password@proxyurl:proxyport
    g.extension extension=r.stream.distance proxy="http://username:password@proxy.example.com:8080"

### Managing the extensions

List all available extensions in the official GRASS GIS Addons repository:

    g.extension -l

List all locally installed extensions:

    g.extension -a

Removal of a locally installed extension:

    g.extension extension=r.stream.distance operation=remove

## TROUBLESHOOTING

Since extensions have to be compiled on Unix based systems (Linux, Mac OSX etc.) unless a Python extension is installed, a full compiler environment must be present on the user's computer.

### ERROR: Please install GRASS development package

While GRASS GIS is available on the user's computer, the respective development package is lacking. If GRASS was installed from a (Linux) repository, also the grass-dev* package (commonly named "grass-dev" or "grass-devel", sometimes along with the version number) must be installed.

## SEE ALSO

g.extension.all

GRASS GIS 7 Addons Manual pages
GRASS Addons wiki page.

## AUTHORS

Markus Neteler (original shell script)
Martin Landa, Czech Technical University in Prague, Czech Republic (Python rewrite)

Last changed: $Date: 2015-11-20 12:34:12 +0100 (Fri, 20 Nov 2015) $