Git trick: Deleting non-ancestor tags

Today I cloned the git tree for the pandaboard kernel, only to find that it didn't include the various kernel version tags from upstream, so running things like git describe or git log v3.0.. didn't work.

My first thought was to fetch just the tags from an upstream copy of the Linux kernel I had on my local machine:

git fetch -t ~/linus

Unfortunately I hadn't thought that though very well, as that local tree also contained all the tags from the linux-next tree, the tip tree as well as a whole bunch more from various distro trees and several other random ones, which I didn't want cluttering up my copy of the pandaboard kernel tree.

This lead me to try to find a way to delete all the non-ancestor tags (compared to the current branch) to simplify the tree. This may be useful to others to remove unused objects and make the tree smaller after a git gc -- that didn't factor into my needs as I had specified ~/linus to git clone with --reference so the objects were being shared.

Anyway, this is the script I came up with, note that this only compares the tags with the ancestors of the *current HEAD*, so you should be careful that you are on a branch with all the tags you want to keep first. Alternatively you could modify this script to collate the ancestor tags of every local/remote branch first, though this is left as an exercise for the reader.


#!/bin/sh

ancestor_tags=$(mktemp)
echo -n Looking up ancestor tags...\ 
git log --simplify-by-decoration --pretty='%H' > $ancestor_tags
echo done.

for tag in $(git tag --list); do
 echo -n "$tag"
 commit=$(git show "$tag" | awk '/^commit [0-9a-f]+$/ {print $2}' | head -n 1)
 echo -n ...\ 
 if [ -z "$commit" ]; then
  echo has no commit, deleting...
  git tag -d "$tag"
  continue
 fi
 if grep $commit $ancestor_tags > /dev/null; then
  echo is an ancestor
 else
  echo is not an ancestor, deleting...
  git tag -d "$tag"
 fi
done

rm -fv $ancestor_tags


Also note that this may still leave unwanted tags in if they are a direct ancestor of the current HEAD - for instance, I found a bunch of tags from the tip tree had remained afterwards, but they were much more manageable to delete with a simple for loop and a pattern.