add xml attribute value based on other attribute from same tag using bash and xmlstarlet

Question

I have following xml structure input.xml

<route order="1" xml:base="directory/filename1.xml">
    <name>name1</name>
</route> 
<route order="2" xml:base="directory/filename2.xml">
    <name>name2</name>
</route> 

I want as output.xml

<route order="1" xml:base="directory/filename1.xml" xml:timestamp="2017-08-30_12:00:00">
    <name>name1</name>
</route> 
<route order="2" xml:base="directory/filename2.xml" xml:timestamp="2017-05-30_12:30:00">
    <name>name2</name>
</route> 

where xml:timestamp is the result of the date function with the xml:base as input

date -r directory/filename1.xml +'%Y-%m-%d_%H:%M:%S'

I can add a xml:timestamp attribute like this

xmlstarlet ed --insert "/route" --type attr -n xml:timestamp -v 2017-08-31 input.xml

but how do I pass the result of the date function to the -v ?

Answer 1

You can refer to the result returned from the date command by placing the complete command in $() and so the follwoing should work:

xmlstarlet ed --insert "/route" --type attr -n xml:timestamp -v "$(date -r directory/filename1.xml +'%Y-%m-%d_%H:%M:%S')" input.xml