extract multiple directories using git-filter-branch

Question

I have a big repository which currently contains multiple projects in top level subfolders, say /a, /b, /c, and /d.

Answer 1

Use

git filter-branch -f --prune-empty --tree-filter 'bash preserve-only.sh a b' -- --all

where preserve-only.sh is:

IFS=':'
GLOBIGNORE="$*"
rm -rf *

This should remove everything but a and b from all commits of all branches, which should be the same as extracting exactly the given directories.

To complete the actual split you can use a filter like rm -rf a b to get all the changes not extracted in the first run.

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Update: While trying to speed things up using --index-filter I came to an even easier solution:

git filter-branch -f --prune-empty --index-filter \
  'git rm --cached -r -q -- . ; git reset -q $GIT_COMMIT -- a b' -- --all

This just removes everything and afterwards restores the given directories afterwards.