git pre-commit hook to check XML files for well-formedness, using 'file' and 'xmllint' commands

pre-commit

#!/bin/bash
#
# An example hook script to verify what is about to be committed.
# Called by "git commit" with no arguments.  The hook should
# exit with non-zero status after issuing an appropriate message if
# it wants to stop the commit.
#
# To enable this hook, rename this file to "pre-commit".

if git rev-parse --verify HEAD >/dev/null 2>&1
then
	against=HEAD
else
	# Initial commit: diff against an empty tree object
	against=4b825dc642cb6eb9a060e54bf8d69288fbee4904
fi

check_xml_wellformed()
{
	for FILE in `git diff-index --name-only --cached $against -- ` ; do
		mimetype=`file --brief --mime-type $FILE`
		# if file is an xml file, check that it is well-formed
		if [ $mimetype == 'application/xml' ]; then
			if ! xmllint --noout $FILE; then
				echo "========================================================="
	                        echo "Unstaging $FILE for the commit -- XML but not well-formed"
	                        git reset -q HEAD $FILE
	                        retval=1
			else
				echo "XML check for $FILE: OK"
	                fi
		fi
	done
	
    if [[ $retval -gt 0 ]]; then
        exit $retval
    fi
}

check_xml_wellformed

I have found that you can sneak a file past this if you have the .xml extension but have it not be a valid XML file. The mime-type check fails so xmllint never runs.

$ cat bad-xml.xml
<xml>
Some text
<a>foobar</b>
</>
$ file --brief --mime-type bad-xml.xml
text/plain

Version that uses a file extension check rather than mime-type

https://gist.github.com/robilic/e86313fcb3ff7f763b43c43a43f508a0