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Git

Merging multiple repositories into one

With these instructions, you can combine the histories of multiple Git repositories into a single one. Adapted after this guide by SaintGimp.

# Go into the directory of the new repository and initialize it.
git init

# Make sure there is an initial commit.
echo "# Title" > README.md && git add . && git commit -m "Initial commit"

# Repeat the following steps for every repository that should be merged into the new one.

# If you want to move the contents of the old repository into a subfolder, do that first (including commiting and pushing that change to the remote).
# Now, add the old repository as a remote to the new one.
git remote add -f [remote_name] [remote_url]

# Do a rebase.
git rebase [remote_name]/master

# Remove the remote, we don't need it anymore.
git remote remove [remote_name]

Editing older commits

Using git-rebase

Older Git commits can easily be edited using git rebase -i [ref]. [ref] needs to be a commit older than the one you want to edit. For example, specify HEAD~5 to be able to edit the last five commits.
This will open an editor with a list of the commits you specified. Edit the word in front of every commit (pick by default) to choose what to do with that commit. At the bottom, there is also a list of possible commands. Useful ones include:

  • p: Use commit as-is
  • r: Change only the commit message
  • e: Edit the commit (You can then make changes, commit them using git commit --amend and continue using git rebase --continue.)
  • s: Squash commit into the previous one
  • d: Remove commit
  • Commits can also be re-ordered, they are in order from top (oldest) to bottom (newest).

Using git-fixup (alias)

If you only want to edit one older commit, you can also use the git-fixup alias by Filippo Valsorda. It works similar to using git commit --amend for the oldest commit.

Simply make the desired changes, add them using git add and run git fixup [ref], where [ref] is the commit you want to edit.

Installation

The alias needs to be added to ~/.gitconfig:

[alias]
    # Taken from: https://blog.filippo.io/git-fixup-amending-an-older-commit/ (2019-06-22)
    fixup = "!f() { TARGET=$(git rev-parse "$1"); git commit --fixup=$TARGET ${@:2} && EDITOR=true git rebase -i --autostash --autosquash $TARGET^; }; f"

This version however unfortunately doesn't work on Ubuntu because they use the dash shell as /bin/sh (which doesn't support the ${@:2} expansion). There, you can instead use this simplified version that doesn't support passing additional arguments:

[alias]
    # Adapted after: https://blog.filippo.io/git-fixup-amending-an-older-commit/ (2019-06-22)
    # I had to remove the `${@:2}` after `commit` which would have forwarded all remaining args to `commit`
    # but unfortunately, Ubuntu uses `/bin/dash` as `/bin/sh` and that doesn't support that expansion. :(
    fixup = "!f() { TARGET=$(git rev-parse "$1"); git commit --fixup=$TARGET && EDITOR=true git rebase -i --autostash --autosquash $TARGET^; }; f"

Staging only parts of a file

Sometimes, you don't want to git add a whole file but only a part of it. This can be done using git add -p which will then interactively ask you which parts to stage.

Possible commands for each hunk are (1):

  • y: Add this hunk
  • n: Don't this hunk
  • d: Skip the rest of the file
  • s: Split into smaller hunks (only works if there are unchanged lines in-between)
  • e: Edit hunk in editor
    When editing, make sure that each line starts with one of these characters: (unchanged line), - (remove line), + (add line).