Is it possible to replace the tree of a git branch without first checking it out?

Question

For pages on github.io, it is common to use a branch called gh-pages on the main repository to publish a website associated with a particular repository.

In my current project, I perform the following steps to update my documentation which is hosted on github.io.

1. Build API documentation on the main branch using Doxygen.
2. Move the html directory to a temporary directory.
3. Checkout the gh-pages branch.
4. Copy the contents of the temporary directory to the project directory.
5. git commit and git push the gh-pages branch.
6. Checkout master branch again.

Is it possible to commit the contents of the html directory to the gh-pages branch in a single step?

Note that I prefer not to add the contents of the html directory to the master branch because it is generated content rather than source.

I have already looked at this question and this question but they do not appear to solve this problem.

This is not actually a duplicate of this question because this question concerns whole-tree replacement rather than adding a single file, and I believe the former operation can be done in a cleaner way than the latter.

Answer 1

Clone your remote repo again in a new local repo. Checkout the branch gh_pages in this new repository.

Use the repo you already have for development, the same way you are using it now.

Build the API documentation on the main branch of the development repository using Doxygen as you do now. If possible, configure the process to produce the output in the directory where the second local repo is stored. Otherwise move the generated documentation to the second local repo after it is created.

Run git add .; git commit; git push in the second repository (the documentation repo).

Repeat the last two steps every time it's needed.

Answer 2

I have written the following script to avoid the use of an additional working directory, although @axiac's answer is a valid way to solve this problem.

#!/bin/bash

# This script will take a directory and replace the contents of a non-current
# branch with it.
if [ "$#" -ne 3 ]; then
    echo \
    "Usage:  replace-branch <top level directory> <branchname> <commit message>"
    exit
fi

root="$1"
branch="$2"
msg="$3"

# First check if the index is polluted, since we will be using it.
save=HEAD
indexDiverges=$(git diff-index --cached  $(git rev-parse HEAD))
if [[ -n "$indexDiverges" ]]; then
    save=$(git write-tree)
fi

# Create a new index
git read-tree --empty
GIT_WORK_TREE=$root git add .

# Check whether the index is actually different from the target.
isDuplicateCommit=$(git diff-index --cached  $(git rev-parse $branch))
if [[ ! -n "$isDuplicateCommit" ]]; then
    echo "The branch already matches the state we are trying to commit!"
    git read-tree $save
    exit
fi

# Write the index out to a new tree
tree=$(git write-tree)

# Commit with the given commit message to the other branch
commithash=$(echo "$msg" | git commit-tree $tree -p $(git rev-parse $branch))

# Move the branch pointer.
git update-ref $(git show-ref --heads "$branch"  | awk '{print $2}') \
    "$commithash"

# Restore the index to its previous state
git read-tree $save

I have also decided to upgrade this a bit and put it on GitHub with example usage.