How can I update an XML attribute node with the maximum of its sub-attributes?

Question

Consider the following XML:

<?xml version="1.0"?>
<sportsClass>
    <pupils>
        <pupil name="Adam" highestJump="">
            <jump height="4"/>
            <jump height="1"/>
        </pupil>
        <pupil name="Berta" highestJump="">
            <jump height="4"/>
            <jump height="7"/>
        </pupil>
        <pupil name="Caesar" highestJump="">
            <jump height="1"/>
            <jump height="2"/>
        </pupil>
        <pupil name="Doris" highestJump="">
            <jump height="5"/>
            <jump height="5"/>
        </pupil>
    </pupils>
</sportsClass>

How can I fill the highestJump attribute nodes with the respective maximum height value, using xmlstarlet?

Answer 1

This problem consists of two sub-problems:

Finding the maximum

xmlstarlet does not have the max() function, so we have to find a way around:

cat jumps.xml | \
xmlstarlet select -t -v "//pupil/jump[not(@height <= following-sibling::jump/@height) and not(@height < preceding-sibling::jump/@height)]/@height"

Note the <= and < – if there are more than one maximum values, only the last one will be taken.

Result:

4
7
2
5

Updating the attribute

Constant value for practice

cat jumps.xml | xmlstarlet edit --update //pupil/@highestJump -v "Hahahaha"

...writes Hahahaha to every highestJump attribute.

Simple XPath

Take care: The XPath you use for replacing

• is relative to the selected attribute (so . is the attribute itself)
• has to be wrapped with eg. string() to have an effect

So:

cat jumps.xml | xmlstarlet edit --update //pupil/@highestJump -x "string(../@name)"

...gives (shortened):

<pupil name="Adam" highestJump="Adam">
<pupil name="Berta" highestJump="Berta">
<pupil name="Caesar" highestJump="Caesar">
<pupil name="Doris" highestJump="Doris">

Combining the two

cat jumps.xml | xmlstarlet edit --update //pupil/@highestJump -x "string(../jump[not(@height <= following-sibling::jump/@height) and not(@height < preceding-sibling::jump/@height)]/@height)"

...gives...

<?xml version="1.0"?>
<sportsClass>
  <pupils>
    <pupil name="Adam" highestJump="4">
      <jump height="4"/>
      <jump height="1"/>
    </pupil>
    <pupil name="Berta" highestJump="7">
      <jump height="4"/>
      <jump height="7"/>
    </pupil>
    <pupil name="Caesar" highestJump="2">
      <jump height="1"/>
      <jump height="2"/>
    </pupil>
    <pupil name="Doris" highestJump="5">
      <jump height="5"/>
      <jump height="5"/>
    </pupil>
  </pupils>
</sportsClass>