Replacing one XML element with another

Question

I'm interested in finding out what's the shortest script one can write to replace one XML element in a file with another one from a second file.

I can whip up a simple program to easily do this, but I'm wondering if it's easily do-able using a shell script. It's just a utility tool meant as a convenience. Can this be done using sed? awk? I'm not familiar with those. I suppose I can probably do it with a combination of grep and wc, but it seems likely that there's a much more direct way to do this.

Essentially, I have a large configuration file, say config.xml, which say looks like this:

<config>
    <element name="a">
        <subelement />
    </element>
    <element name="b">
        <subelement />
    </element>
    <element name="c">
        <subelement />
    </element>
    <!-- and so on... -->
</config>

Once in a while, changes require me to modify/add/delete one subelement. Now, it so happens that there's a sort of generator that will generate an up-to-date subconfig.xml, like following file:

<config>
    <element name="c">
        <subelement />
        <subelement />
    </element>
</config>

My thinking is that if I can take the element in subconfig.xml and replace the existing one in config.xml, then hey, that'd be great! Yea, it's not much a work-saver, since it's only needed rarely, but it just occurred to me that I could try to do it in a script, but I'm not sure how.

Any help appreciated (including pointing out that I'd be better off writing a program for this ^-^).

Answer 1

I wouldn't attempt to do this with command-line tools, you'll run into all sorts of difficulties. The way you should do this is with a proper XML parser. The logic would be: consume original file, parse it, consume update file, parse it, identify which node this guy is to replace, do the replace, write out the result.

I don't know what you are comfortable with coding-wise, but there's XML parsers available in most popular languages.

Answer 2

If your xml are consistent and your replacement requirement is simple, there's actually no need to use parsers. Just simple awk will do

$ subconfig=(<subconfig.xml)
$ awk -v subconf=$subconfig '/<config>/{print subconf}/<config>/,/<\/config>/{next}1' config.xml