Make a commit on a branch in a Travis CI build, dealing with detached HEAD state safely

travis-ci-git-commit.bash

#!/bin/bash
# function to make a commit on a branch in a Travis CI build
# be sure to avoid creating a Travis CI fork bomb
# see https://github.com/travis-ci/travis-ci/issues/1701
function travis-branch-commit() {
    local head_ref branch_ref
    head_ref=$(git rev-parse HEAD)
    if [[ $? -ne 0 || ! $head_ref ]]; then
        err "failed to get HEAD reference"
        return 1
    fi
    branch_ref=$(git rev-parse "$TRAVIS_BRANCH")
    if [[ $? -ne 0 || ! $branch_ref ]]; then
        err "failed to get $TRAVIS_BRANCH reference"
        return 1
    fi
    if [[ $head_ref != $branch_ref ]]; then
        msg "HEAD ref ($head_ref) does not match $TRAVIS_BRANCH ref ($branch_ref)"
        msg "someone may have pushed new commits before this build cloned the repo"
        return 0
    fi
    if ! git checkout "$TRAVIS_BRANCH"; then
        err "failed to checkout $TRAVIS_BRANCH"
        return 1
    fi

    if ! git add --all .; then
        err "failed to add modified files to git index"
        return 1
    fi
    # make Travis CI skip this build
    if ! git commit -m "Travis CI update [ci skip]"; then
        err "failed to commit updates"
        return 1
    fi
    # add to your .travis.yml: `branches\n  except:\n  - "/\\+travis\\d+$/"\n`
    local git_tag=SOME_TAG_TRAVIS_WILL_NOT_BUILD+travis$TRAVIS_BUILD_NUMBER
    if ! git tag "$git_tag" -m "Generated tag from Travis CI build $TRAVIS_BUILD_NUMBER"; then
        err "failed to create git tag: $git_tag"
        return 1
    fi
    local remote=origin
    if [[ $GITHUB_TOKEN ]]; then
        remote=https://[email protected]/$TRAVIS_REPO_SLUG
    fi
    if [[ $TRAVIS_BRANCH != master ]]; then
        msg "not pushing updates to branch $TRAVIS_BRANCH"
        return 0
    fi
    if ! git push --quiet --follow-tags "$remote" "$TRAVIS_BRANCH" > /dev/null 2>&1; then
        err "failed to push git changes"
        return 1
    fi
}

function msg() {
    echo "travis-commit: $*"
}

function err() {
    msg "$*" 1>&2
}

Ah perfect, thank you, will try that.