How to ignore empty selections in XMLStarlet? - suppress-warnings

Question

I have a script that performs many XML edit operations with XMLStarlet.
For instance, it removes all foo nodes if any are present:
xmlstarlet ed -d '//foo'
(except that in my script, the name of the element is not foo).
When no foo node is present, the following message is printed:
None of the XPaths matched; to match a node in the default namespace
use '_' as the prefix (see section 5.1 in the manual).
For instance, use /_:node instead of /node
But there is nothing wrong if no foo nodes are present in the input document.
So for this particular operation, I do want to avoid this particular warning,
while I do not want to disable such warnings in general.
How can I achieve this?

Answer 1

At the moment, deleting a node that does not exist is categorized by xmlstarlet as an error. The return code will be 1 which you will have to understand as "no nodes were removed".
The error message appears if you happen to use a document which has a default namespace:
No namespace:
echo '<foo />' | xmlstarlet sel -t -m foo -v joe
With a default namespace xmlstarlet prints the error message
echo '<foo xmlns="urn:foo" />' | xmlstarlet sel -t -m foo -v joe
None of the XPaths matched; to match a node in the default namespace
use '_' as the prefix (see section 5.1 in the manual).
For instance, use /_:node instead of /node
No default namespace:
echo '<ns:foo xmlns:ns="urn:foo" />' | xmlstarlet sel -t -m foo -v joe
In all of these cases, no nodes are found, and xmlstarlet thus exit with return code 1 (i.e. an error). The error message was to explain the error in the case where the user forgot that the document had a default namespace. I have discussed this with the author and more recently changes have been introduced to reduce the chances of these messages, and a way to inhibit them.
xmlstarlet has (yet undocumented) support for using the document's namespaces, without needing to declare them up-front:
Compare:
echo '<ns:foo xmlns:ns="urn:foo" />' | xmlstarlet sel -t -m ns:foo -v 'count(.)'
echo '<ns:foo xmlns:ns="urn:foo" />' | xmlstarlet sel -N xx=urn:foo -t -m xx:foo -v 'count(.)'
Technically, the commands are identical, except the first one depends on the document binding to the 'ns' prefix, while the second does not.
To inhibit the message, you have to redirecting stderr to null:
echo '<foo xmlns="urn:xxx"/>' |
xmlstarlet sel -t -m foo -v joe 2> /dev/null
The downside of this is that it suppresses legitimate error messages, not just this bogus one which is caused by the fact that the source document uses namespaces.
In recent builds --no-doc-namespace has been added to inhibit this behaviour
The change was introduced in this changeset and the changeset contains a long exchange regarding this error message, all caused by this StackOverflow question!