split files out of commits to build a new branch

git-split

#!/bin/bash

# * 358aee4 (HEAD) fix css
# |  css/site.css | 13 ++---
# |
# * f869f40 init site
# |  html/index.html    | 12 ++--
# |  html/header.html   | 34 +-----
# |  css/site.css       | 61 +++++++------
# |
# * 82554be fix doc of Build
#
#
# # to split folder html into a separate branch:
# > git-split 82554be..HEAD -- html/
#
#
# *   948149a Merge commit 'aa76f4de5be5ddbb2908ba145fea5697c7baf728' into HEAD
# |\
# | * aa76f4d init site
# | |  html/index.html  | 12 ++--
# | |  html/header.html | 34 +-----
# | |
# * | 088e903 fix css
# | |  css/site.css | 13 ++---
# | |
# * | 0b606c2 init site
# | |  css/site.css | 61 +++++++------
# |/
# * 82554be fix doc of Build

usage()
{
    echo 'split files out of commits to build a new branch'
    echo 'git-split [ -b <to-new-branch> ] <from>..[<to>] -- <fns>'
    exit 1
}

mes()
{
    local Brown="$(tput setaf 3)"
    local NC="$(tput sgr0)"
    echo $Brown"$@"$NC
}

githash()
{
    git log -n1 --format="%H" $1
}

new_branch=
while getopts "b:" opname; do
    case $opname in
        b)
            new_branch=$OPTARG
            ;;
        [?])
            usage
            ;;
    esac
done
shift $(($OPTIND - 1))

if [ ".$2" != ".--" ]; then
    usage
fi

range=$1
shift
shift
fns="$@"

base=${range%..*}
if [ ".$base" = ".$range" ]; then
    usage
fi

tip=${range#*..}
if [ ".$tip" = "." ]; then
    tip=HEAD
fi

head=$(git symbolic-ref --short HEAD 2>/dev/null)
if [ ".$head" = "." ]; then
    head=$(git rev-parse HEAD)
fi

tip_hash=$(githash $tip)
new_hash=
merged_hash=


extract()
{
    for c in $(git log --reverse $base..$tip_hash --format="%H" -- $fns); do

        mes "== to pick $fns from:"
        git log --color --stat --decorate --pretty=oneline --abbrev-commit -M -n1 $c

        mes=$(git log -n1 --format="%B" $c)

        p=$(git diff $c~ $c -- $fns)
        git apply --index <<<"$p"

        git status

        git commit -am "$mes"
    done
    return 0
}

build_new_branch()
{
    git checkout --detach $base \
        && mes "== build new branch ==" \
        && extract \
        && new_hash=$(githash) \
        && \
        if [ ".$new_branch" != "." ]; then
            git branch "$new_branch"
        fi \
        && mes "== new branch $new_branch hash is $new_hash == "
}

discard_changes()
{
    local script="git rm -r -f $fns 2>/dev/null; git checkout $base -- $fns 2>/dev/null; git status >/dev/null"

    git checkout --detach $tip_hash \
        && mes "== discard from $tip ==" \
        && git filter-branch -f --prune-empty --tree-filter "$script" $base..HEAD \
        && [ $(git log $base..HEAD -- "$fns" | wc -l | awk '{print $1}') = 0 ] \
        && mes "== confirmed that no changes through $base..HEAD"
}

merge_them()
{
    mes "== merge splitted branch back to $tip ==" \
        && \
        if [ ".$new_branch" = "." ]; then
            git merge --commit --no-edit --no-ff $new_hash
        else
            git merge --commit --no-edit --no-ff $new_branch
        fi \
        && merged_hash=$(githash) \
        && mes "merged hash: $merged_hash"
}

is_same_as_original()
{
    mes "== diff with original $tip ==" \
        && git diff $tip
}

update_tip_ref()
{
    git update-ref -d refs/original/HEAD

    # if it is a reference(branch, tag), not hash or HEAD

    if [ ".$tip" = ".HEAD" ]; then
        git update-ref $tip $merged_hash \
            && mes "== $tip set to $merged_hash"
    fi

    if git rev-parse refs/heads/$tip >/dev/null 2>/dev/null;  then
        git update-ref refs/heads/$tip $merged_hash \
            && mes "== $tip set to $merged_hash"
    fi

}

mes "split $fns from $range" \
    && build_new_branch \
    && discard_changes \
    && merge_them \
    && is_same_as_original \
    && update_tip_ref \
    && git checkout $tip >/dev/null 2>/dev/null \
    && mes "== Done =="