Implementing $Date$ substitution - solution using pre-commit

Hi!

So, my filter-based approach at doing RCS-like $Date$ expansion didn't work out. Since I wanted to have the Date expand on check-in, not on check-out, and be expanded in history, I had made it into a "clean" filter. But due to how Git internally checks if files are clean, that did not work properly.

The solution to this was instead to make the Date expansion into a pre-commit hook. Using .gitattributes, I set "datereplace=true" on all the files I want to have "$Date$" expand in, and then install the following file as .git/hooks/pre-commit:

===8< pre-commit >8=== #!/bin/bash -e

# Find base 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

# Set up Date substitution
export NOW=$(date +"%Y-%m-%d %H:%M:%S")
for file in $(git diff-index --cached --diff-filter=AM --name-only $against); do
   if git check-attr datereplace -- "$file" | grep 'datereplace: true' > /dev/null; then
     perl -w -i.bak -e 'my $now = $ENV{"NOW"};
$now =~ s/[^-:0-9 ]//g;
while (<>)
{
  if (/\$Date:?[^\$]*\$/)
  {
    s/\$Date:?[^\$]*\$/\$Date: ${now} \$/;
  }
  print
}' "$file"
     git update-index --add "$file"
   fi
done

exit 0 ===8< pre-commit >8===

This has the added bonus over a filter that it all files committed at the same time will have the same date stamp, whereas the filter would expand on the time "git add" is executed.