git: amend-at-commit

git-amend-at-commit.sh

#!/bin/bash

: <<'HELP'

Amend at commit
===

If you stage some files to be committed with git
this script will then cycle back to a specific commit,
apply those changes to that commit.

I keep this file at ~/bin/git-amend-at-commit.sh, with +x set
and I have ~/bin in my path, which allows me to run the script directly.

Direct usage
---

To run the script directly, make sure you've staged the changes you
want to add to the amended commit, and that you don't have any
other changes in your working tree.

    # After staging your changes
    $ git-amend-at-commit.sh <commit-id-to-amend>

Git alias
---

You can add this script as a git alias by adding the following to your
`~/.gitconfig`:

    [alias]
        amend-at-commit = !~/bin/git-amend-at-commit.sh

Now you can run this as a git alias:

    # After staging your changes
    $ git amend-at-commit <commit-id-to-amend>

Recovering from failure
---

The script will output the current commit ID as one of the first things it does:

    # Current HEAD:
    5cd9050ae53f39aced2a1b80b78d8e9f65b2eec5

If things go wrong, you should be able to just jump back to this commit
and hopefully everything will be fine:

    $ git cherry-pick --abort  # Abort any existing cherry-pick commands
    $ git reset --keep 5cd9050ae53f39aced2a1b80b78d8e9f65b2eec5

The script will also back-up the entire git log to `/tmp`,
and tell you where it is:

    # Backing up log:
    at/tmp/.1413462171.gitlog

HELP

if [ -z "$1" ]; then
    echo "No commit ID specified"
    exit 1
fi

if ! git diff --exit-code > /dev/null 2>&1; then
     echo 'Unstaged changes, exiting.'
     exit 1
fi

if git diff --cached --exit-code > /dev/null 2>&1; then
     echo 'Nothing staged, exiting.'
     exit 1
fi

COMMIT_TO_AMEND=`git rev-parse ${1}`
OLD_HEAD=`git rev-parse HEAD`
BACKUP_PATH=/tmp/.$(date +%s).gitlog

echo -e "\n# Current HEAD:"
echo ${OLD_HEAD}

echo -e "\n# Backing up log at:"
echo "${BACKUP_PATH}"
git log >> ${BACKUP_PATH}

echo -e "\n# Resetting to ${COMMIT_TO_AMEND}"
git reset --keep ${COMMIT_TO_AMEND}

echo -e "\n# Staging files"
git add .

echo -e "\n# Status before amend:"
git s

echo -e "\n# Amend commit ${COMMIT_TO_AMEND}"
git commit --amend --no-edit

echo -e "\n# New commit ID:"
git rev-parse HEAD

echo -e "\n# Cherry-pick from ${COMMIT_TO_AMEND} back to ${OLD_HEAD}"
git cherry-pick --allow-empty ${COMMIT_TO_AMEND}..${OLD_HEAD}