Duplicate a section of XML in bash (via xmlstartlet, xmllint, xq, xslt, ...)

I have a valid XML file. I would like to duplicate a tag with contents via linux shell command(s).

By duplication I mean: copy <x>...</x> entirely, then paste right after the original.

I do not have any prior knowledge of the contents or structure inside of the tag being duplicated, but I do know XPath to that tag.

So far I have achieved required result with sed via regex. I would like to use appropriate tools like xmlstarlet or xmllint instead, tools from official debian repo are preferred.

It seems the task is achievable this way:

extract a segment of XML into another file
concatenate that file to the original file
find the inserted section and move it to the desired position

To me that procedure seems a little convoluted, is there any better way?

Example input XML:

<?xml version="1.0" encoding="UTF-8"?>
<root>
 <item>
  <someVariedContents/>
 </item>
</root>

Example output XML:

<?xml version="1.0" encoding="UTF-8"?>
<root>
 <item>
  <someVariedContents/>
 </item>
 <item>
  <someVariedContents/>
 </item>
</root>

Solution

To duplicate an element using xmlstarlet edit, for example:

# shellcheck shell=sh disable=SC2016

xmlstarlet edit \
  --var F 'root/item[1]' \
  -a '$F' -t 'elem' -n 'item' -v '' \
  -u '$prev' -x '$F/node() | $F/@*' \
file.xml

let the F variable refer to the source element (for brevity)
-a (aka --append) inserts a new empty item element just after $F
unlike --append's -v (--value) the -x (--expr) clause of the -u (--update) option takes an XPath argument, hence the two-step approach
-x '$F/node() | $F/@*' makes a deep copy of source's nodes on the child and attribute axes (using a union) -- since item doesn't actually have any attributes | $F/@* can be omitted

In an xmlstarlet edit command --var defines a named variable, and the back reference $prev variable (aka $xstar:prev) refers to the node(s) created by the most recent -s, -i, or -a option which all define or redefine it (see xmlstarlet.txt for a few examples of --var and $prev).

To process a multi-member node-set rather than a single element you can use the `-x` option with a relative XPath expression; for an example see this.