How to wrap a string around large gzip file?

Question

I have a large (several GB) xml file that is missing a root element. I might use cat on ubuntu to wrap around a <root> element around the content.

How can I achieve this without having to extract and repack the content?

It works if I create separate prefix and postfix gz files, and concatenate them with cat. But could I do better, without having to create explicit pre-/postfix gz files? Can I achieve the same on the fly?

echo "<root>" | gzip > prefix.gz
echo "</root>" | gzip > postfix.gz
cat prefix.gz input.xml.gz postfix.gz > newfile.gz

since you're dealing with a multi-GB file, this question might be interesting to take a look at regarding the performance. — myrdd, Apr 25 '18 at 09:27

Stephen Kitt · Accepted Answer · 2018-04-25T09:04:23.237

3

You can use here strings and a succession of commands:

(gzip <<<"<root>"; cat input.xml.gz; gzip <<<"</root>") > newfile.gz

Here strings are described in What does <<< mean? and in the Bash manual.

edited Apr 25 '18 at 09:04

answered Apr 25 '18 at 08:17

Stephen Kitt

434,908

gzip may refuse to compress short strings like that without -f. EDIT: No it doesn't, since it's from stdin. – Kusalananda Apr 25 '18 at 08:30
While I don't understand exactly, especially the <<<, it works as expected! – membersound Apr 25 '18 at 08:41
See this question or the Bash manual for a description of <<<. – Stephen Kitt Apr 25 '18 at 09:03

How to wrap a string around large gzip file?

1 Answers1