The textual nature of SVG makes it very easy to generate and edit SVG documents with simple scripts. However, no matter what your script is supposed to do, chances are that you will find it necessary to figure out the bounding boxes of some SVG objects (for example, to check if the text inserted into SVG from a database fits the provided space, or to create a background rectangle or frame for a specific object). Proper calculation of a bounding box, in a general case, is way too complex to do in a script—you would have to reimplement a lot of Inkscape’s geometry and rendering code if you want to take into account everything that may affect the bounding box.
Fortunately, instead of all that you can simply ask Inkscape. For example:
$ inkscape --query-width --query-id=text1256 file.svg 45.2916
Here, we asked Inkscape to tell us the width (in px units) of the object with the id="text1256". It loaded the document, found that object, printed its width back to the console, and then quit.
Similarly, you can --query-height, --query-x, and --query-y to find out the dimensions and coordinates of the object’s bounding box. Such Inkscape calls are reasonably fast because they don’t load the GUI and don’t render the document; however, if you need to query the bounding boxes of many objects, this may add up to quite a delay. In this case, it is better to use the --query-all parameter which returns all bounding box numbers for all objects in a document, as follows:
$ inkscape --query-all file.svg svg2,-55.11053,-29.90404,328.3131,608.6359 layer1,-55.11053,-29.90404,328.3131,608.6359 image2372,-8.917463,349.8089,282.12,212.6382 text2317,-39.85518,454.3014,20.40604,13.32647 tspan2319,-32.58402,454.3014,12.79618,4.989286 tspan2408,-39.85518,462.4921,20.40604,5.135838 path2406,-16.43702,386.617,6.34172,154.7896 text2410,-46.11609,376.8753,34.34841,5.135838 tspan2414,-46.11609,376.8753,34.34841,5.135838 text2418,-55.11053,365.9197,43.02429,5.135838
Here, each line is a comma-separated list of: object ID, x, y, width, and height. Parsing such a line in your script should be easy.