How to check well-formedness of 4GB XML file

Since you specifically asked for sed, here is a sed/bash script that should do what you want, provided that each <Name> element is opened and closed on the same line:

(IFS='';
n=0;
while read line; do
    if echo "${line}" | grep -Pq "<Name>\w+</Name>"; then
        ((n++));
        echo "${line}" | sed "s/<Name>\(\w\+\)<\/Name>/<Name${n}>\1<\/Name${n}>/";
    else
        echo "${line}";
    fi;
done) < names.xml

I tested it with this input file:

<!-- names.xml -->
<Names>
    <Name>Luigi</Name>
    <Name>Mario</Name>
    <Name>Peach</Name>
</Names>

And it produced the following output:

<Names>
    <Name1>Luigi</Name1>
    <Name2>Mario</Name2>
    <Name3>Peach</Name3>
</Names>

That said, this seems like a good candidate for a language with an XML parsing library. Here is a Python script that does what you want:

#!/usr/bin/env python2
# -*- encoding: ascii -*-

# add_suffix.py

import sys
import xml.etree.ElementTree

# Load the data
tree = xml.etree.ElementTree.parse(sys.argv[1])
root = tree.getroot()

# Update the XML tree
suffix = 0
for name in root.iter("Name"):
    suffix += 1
    name.tag += str(suffix)

# Write out the updated data
tree.write(sys.argv[2])

Run it like this:

python add_suffix.py names.xml new_names.xml

You have found the element, but you need the value of an attribute @DTS:ConnnectionString rather than the value of the element itself. Without namespaces you would pick this out with a suffix of @ConnectionString, but since you have namespaces here, xmllint requires you to match in the same way you've matched all your element names, @*[local-name()='ConnectionString']. Furthermore, in order to strip the attribute name from the front of the result you need to postprocess it all with string(). Ugly.

xmllint --xpath "string(//*[local-name()='Executable']/*[local-name()='ConnectionManagers']/*[local-name()='ConnectionManager']/*[local-name()='ObjectData']/*[local-name()='ConnectionManager']/@*[local-name()='ConnectionString'])" file.xml

Output

 Data Source=ADMIN-8DF005D47;Initial Catalog=AdventureWorks;Provider=SQLNCLI11;Integrated Security=SSPI;Application Name=SSIS-HardestNestedWorkflow2-{A32A68DF-3D53-4057-AF80-1B8D524F82BC}ADMIN-8DF005D47.AdventureWorks;Auto Translate=False;

Personally I prefer xmlstarlet for these kind of operations as it understands namespaces more cleanly, but ultimately it's best to use the tool that's most convenient.

xmlstarlet sel xmlns:DTS="www.microsoft.com/SqlServer/Dts" -t -v '/DTS:Executable/DTS:Property' -n file.xml
8

xmlstarlet sel xmlns:DTS="www.microsoft.com/SqlServer/Dts" -t -v '/DTS:Executable//DTS:ConnectionManager/@DTS:ConnectionString' -n file.xml
Data Source=ADMIN-8DF005D47;Initial Catalog=AdventureWorks;Provider=SQLNCLI11;Integrated Security=SSPI;Application Name=SSIS-HardestNestedWorkflow2-{A32A68DF-3D53-4057-AF80-1B8D524F82BC}ADMIN-8DF005D47.AdventureWorks;Auto Translate=False;