git-squash-against

#!/usr/bin/env bash
# Simulate a merge from the current branch into the target.  Squash that merge
# into a single commit.  Make that commit the new HEAD of the current branch.
#
# Based on work from the following sites:
#   https://gist.github.com/patik/b8a9dc5cd356f9f6f980
#   http://stackoverflow.com/questions/3697178/git-merge-all-changes-from-another-branch-as-a-single-commit
#   https://makandracards.com/makandra/527-squash-several-git-commits-into-a-single-commit

against=$1

if [[ -z "$against" ]]; then
    echo "Missing branch or commit we squash against" >&2
    exit 1
fi

against=$(git rev-parse "$against" 2> /dev/null)
current=$(git symbolic-ref HEAD 2> /dev/null)
undoCommit=$(git rev-parse HEAD 2> /dev/null)

if [[ -z "$against" ]]; then
    echo "Unable to find commit to squash against" >&2
    exit 1
fi

if [[ -z "$current" ]]; then
    echo "Unable to find reference to current branch" >&2
    exit 1
fi

current=${current#refs/heads/}
echo "Squashing $current against $against"

# Check if this is a clean repository
if git status | grep "^nothing to commit, working" -q; then
    stashed=false
else
    stashed=true
    git stash save -a -u
fi

git checkout -b "squash-$against" "$against"
git merge --squash "$current"
git commit
squashedCommit=$(git rev-parse HEAD)
git checkout "$current"
git reset --hard "$squashedCommit"
git branch -D "squash-$against"

if $stashed; then
    git stash pop
fi

echo "To undo this action, run this command:"
echo "    git reset --hard $undoCommit"