divide_repo_subdirs.sh

#!/bin/bash

# changing a single repo with subdirs like this:
#
#   origrepo
#   +-- subdir1
#   +-- subdir2
#
# into a different/new repo with rearranged dirs (and discarded files),
# editing the full history
#
#   newrepo
#   +-- (all files/dirs from subdir1, now at the root)
#   +-- renamed2
#       +-- (all files/dirs from subdir2)

ORIGREPO=/c/path/to/origrepo
SUBDIR1=inst/shiny/SomeApp
NEWDIR1=. # no change
SUBDIR2=tests/testthat
NEWDIR2=tests

# I don't really intend for this script to be run all-at-once ...
exit -1

tmpdir1=$(mktemp -d)
tmpdir2=$(mktemp -d)
finaldir=$(mktemp -d)

## ==================================================================
## copy the old repo into two locations
## (it might be cheaper to clone once and "cp -R")
git clone ${ORIGREPO} ${tmpdir1}
git clone ${ORIGREPO} ${tmpdir2}

## ==================================================================
## prune each one
cd ${tmpdir1}
git filter-branch --force --prune-empty --tag-name-filter cat \
    --subdirectory-filter ${SUBDIR1} -- --all
git gc --aggressive --prune
## we now have:
##   tmprepo1
##   +-- (all files/dirs from subdir1)

cd ${tmpdir2}
git filter-branch --force --prune-empty --tag-name-filter cat \
    --subdirectory-filter ${SUBDIR2} -- --all
# let's say I want to remove various files by-name/by-path
git filter-branch --force --prune-empty --tag-name-filter cat \
    --tree-filter 'rm -rf helper* README.md test-[0123]*' \
    -- --all
git gc --aggressive --prune
## we now have:
##   tmprepo2
##   +-- (all files/dirs from subdir2)

## ==================================================================
## make the new "real" repo
mkdir ${finaldir}
cd ${finaldir}
git init

touch a_file_and_make_a_commit # see user's feedback
git add a_file_and_make_a_commit
git commit -am "at least one commit is needed for it to work"

## ------------------------------------------------------------------
## pattern to use for each repo# that you want copied into the root
git remote add repo1 ${tmpdir1}
git fetch repo1
git merge -s ours --no-commit --allow-unrelated-histories repo1/master
# this '-m' does a 'merge', bring files/subdirs into the root dir
git read-tree -m -u repo1/master
git commit -m "package-subdir2-to-new: import ${SUBDIR1}"

## ------------------------------------------------------------------
## pattern to use for each repo# that you want copied into a new subdir
git remote add repo2 ${tmpdir2}
git fetch repo2
git merge -s ours --no-commit --allow-unrelated-histories repo2/master
# this is where we finally move it to a new subdir in the new repo
git read-tree --prefix=${NEWDIR2} -u repo2/master
git commit -m "package-subdir2-to-new: import ${SUBDIR2}"

## ==================================================================
## optionally clean up ... PLEASE don't put "/" in these :-)
rm -rf "${tmpdir1}" "${tmpdir2}"