Question

Given a file with the following contents:

<root>
<a></a>
<b></b>
</root>

The command should output:

<root>
<a></a>
<b></b>

Things I’ve tried using the GNU Win32 port of sed:

Remove the last two lines.

This is fast, but it assumes </root> is the second to last line and will cause a bug if it’s not.

sed -e '$d' test.xml | sed -e '$d'

Substituting all occurrences of </root> with an empty string.

This works, but is slower than the first solution, and will break if there are nested <root> elements (unlikely).

sed -e 's|</root>||' test.xml

The file I’m dealing with can be large so efficiency is important.

Is there a way to limit sed substitution to the last occurrence in the file? Or is there some other utility that would be faster?

Answer 1

Using Perl with File::Backwards should be very fast (relative, I know, but still…). Perlfaq5 has a topic on going through a file backwards and removing lines. You can check for your pattern using this topic’s code as a starting point.