How to add and/or subtract a constant from some XML element values with xmlstarlet

Question

I have an XML file with lots of entries like the following:

<object>
    <name>buf</name>
    <bndbox>
        <xmin>535</xmin>
        <ymin>725</ymin>
        <xmax>636</xmax>
        <ymax>821</ymax>
    </bndbox>
</object> 

Is there a way to use xmlstarlet (or another tool) to subtract the value of 10 from each xmin element's current value?

Answer 1

To subtract the value of 10 from all xmin elements, use the following xmlstarlet command:

xmlstarlet ed -u "//xmin" -x ".-10" input.xml

Output of the sample XML is:

<?xml version="1.0" encoding="UTF-8"?>
<ParentId>
  <object>
    <name>buf</name>
    <bndbox>
      <xmin>525</xmin>
      <ymin>725</ymin>
      <xmax>636</xmax>
      <ymax>821</ymax>
    </bndbox>
  </object>
</ParentId>