filegdb2shp

get the FileGDB API http://www.esri.com/apps/products/download/
extract it somewhere on your system and remember the path :)

mkdir build   #directory where we are playing around
cd build
git clone https://github.com/OSGeo/gdal.git
cd gdal
./configure --with-fgdb=/path/to/your/FileGDB_API
make
(make install) optional, I'm just using it locally

note: you will need all the dependencies, development files as well as all the compile tools. I don't dig into this topic here, but if somebody has trouble, I might be able to help.

First test:
build/gdal/apps/ogrinfo --formats   #I have to use absolute paths, since I didn't install gdal system wide

if you are lucky you will see a output like this:

Supported Formats:
...
-> "FileGDB" (read/write)
...

Now you can use ogrinfo or ogr2ogr to access your FileGDB

build/gdal/apps/ogrinfo /path/to/your/database.gdb <layername> -summary

convert a FileGDB to Shapefile:

build/gdal/apps/ogrinfo -f "ESRI Shapefile" <out_directory> /path/to/your/database.gdb

A directory will be created and for each layer within the filegdb a shapefile set will be created.

You could also use a query to retrieve a subset of your filegdb. There is an example on the gdal site http://www.gdal.org/ogrinfo.html