diff --git a/git_sync_two_branches/git_sync_two_branches.sh b/git_sync_two_branches/git_sync_two_branches.sh
new file mode 100755
index 00000000..9bfdad93
--- /dev/null
+++ b/git_sync_two_branches/git_sync_two_branches.sh
@@ -0,0 +1,142 @@
+#!/usr/bin/env bash
+# WORKSPACE: Path to the workspace directory.
+# $gitcmd: Path to git executable.
+# $gitdir: Directory containing git repo.
+# $gitremote: Remote name where the branches are located. Default: origin.
+# $dryrun: If set to anything, the script will not perform any changes.
+# $source: Source branch to sync from.
+# $target: Target branch to sync to.
+
+# Want exit on error.
+set -e
+
+# This script will sync two branches in a git repo given the target branch is an ancestor
+# of the source branch. If the opposite is detected (source branch is an ancestor of the
+# target branch), the script will exit with an error.
+# Everything (compare, send changes...) to the "gitremote" remote, unconditionally.
+
+# Verify everything is set
+required="WORKSPACE gitdir gitcmd source target"
+for var in $required; do
+    if [ -z "${!var}" ]; then
+        echo "Error: ${var} environment variable is not defined. See the script comments."
+        exit 1
+    fi
+done
+
+# Calculate some variables
+mydir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+gitdir=${gitdir:-}
+gitremote=${gitremote:-origin}
+gitcmd=${gitcmd:-git}
+source=${source:-master}
+target=${target:-master}
+dryrun=${dryrun:-}
+
+# Load some functions
+source "${mydir}/lib.sh"
+
+# Verify that the git directory is valid and that the remote exits.
+cd "${gitdir}"
+
+if ! ${gitcmd} remote | grep -q "^${gitremote}$"; then
+    echo "Error: ${gitremote} is not a valid remote."
+    exit 1
+fi
+
+if ! ${gitcmd} rev-parse --git-dir > /dev/null 2>&1; then
+    echo "Error: ${gitdir} is not a valid git directory."
+    exit 1
+fi
+
+# Get the url of the gitremote remote.
+remote_url=$(${gitcmd} config --get "remote.${gitremote}.url")
+
+# Verify that the source and target branches exist in the remote.
+if ! ${gitcmd} ls-remote --exit-code --heads "${gitremote}" "${source}" > /dev/null 2>&1; then
+    echo "Error: ${source} branch does not exist in ${gitremote} remote (${remote_url})."
+    exit 1
+fi
+
+if ! ${gitcmd} ls-remote --exit-code --heads "${gitremote}" "${target}" > /dev/null 2>&1; then
+    echo "Error: ${target} branch does not exist in ${gitremote} remote (${remote_url})."
+    exit 1
+fi
+
+echo "Syncing ${gitremote} remote (${remote_url}): Set ${target} target branch to ${source} source branch..."
+
+if [ -n "${dryrun}" ]; then
+    echo "Dry-run enabled, no changes will be applied to the ${target} branch in ${gitremote} (${remote_url})."
+    dryrun="DRY-RUN: "
+fi
+
+# Ensure that both the source and target branches exist locally, creating them if necessary.
+if ! ${gitcmd} show-ref --verify --quiet "refs/heads/${source}"; then
+    echo "Creating local source branch ${source}..."
+    "${gitcmd}" fetch --quiet "${gitremote}" "${source}"
+    "${gitcmd}" branch --quiet --force "${source}" --track "${gitremote}/${source}"
+else
+    echo "Updating local source branch ${source}..."
+    "${gitcmd}" fetch --quiet "${gitremote}" "${source}:${source}"
+fi
+
+if ! ${gitcmd} show-ref --verify --quiet "refs/heads/${target}"; then
+    echo "Creating local target branch ${target}..."
+    "${gitcmd}" fetch --quiet "${gitremote}" "${target}"
+    "${gitcmd}" branch --quiet --force "${target}" --track "${gitremote}/${target}"
+else
+    echo "Updating local target branch ${target}..."
+    "${gitcmd}" fetch --quiet "${gitremote}" "${target}:${target}"
+fi
+
+# Let's calculate the commit that will be the HEAD at the end of the process. It's the one in the source branch.
+commit_outcome=$(${gitcmd} rev-parse --short=16 "${source}")
+
+# Verify if both branches are the same. If so, we are done.
+if is_ancestor "${source}" "${target}" && is_ancestor "${target}" "${source}"; then
+    echo "Branches ${source} and ${target} are the same. Nothing to do. Current HEAD: ${commit_outcome}"
+    exit 0
+fi
+
+# Verify that the source branch is not an ancestor of the target branch.
+if is_ancestor "${source}" "${target}"; then
+    echo "The target ${target} branch got new commits."
+    log "Error: target ${target} branch has some unexpected commits, not available in the source ${source} branch. Expected HEAD: ${commit_outcome}."
+    echo "Please, fix the target ${target} branch manually and try again."
+    exit 1
+fi
+
+# We are good to go with any of the remaining cases.
+
+# Verify that the target branch is an ancestor of the source branch. If so, we can fast-forward.
+if is_ancestor "${target}" "${source}"; then
+    echo "The source ${source} branch got new commits."
+    log "${dryrun}Fast-forwarding target branch ${target} to source branch ${source} at ${gitremote} remote (${remote_url}). New HEAD: ${commit_outcome}"
+    "${gitcmd}" fetch --quiet . "${source}:${target}"
+    # Self-assert that the operation happened.
+    new_commit=$(${gitcmd} rev-parse --short=16 "${target}")
+    if [[ "${new_commit}" != "${commit_outcome}" ]]; then
+        log "Error: fast-forwarding failed. Expected new HEAD: ${commit_outcome}. Actual new HEAD: ${new_commit}."
+        exit 1
+    fi
+    if [[ -z "${dryrun}" ]]; then
+        "${gitcmd}" push "${gitremote}" --quiet "${target}"
+    fi
+    exit 0
+fi
+
+# Arrived here, the source and target branches have diverged. We'll need to do a
+# hard reset of the target branch to the source branch.
+echo "Diverged branches (surely because of some rewrite in ${source})."
+log "${dryrun}Hard-resetting target branch ${target} to source branch ${source} at ${gitremote} remote (${remote_url}). New HEAD: ${commit_outcome}"
+"${gitcmd}" branch --quiet --force "${target}" "${source}"
+# Self-assert that the operation happened.
+new_commit=$(${gitcmd} rev-parse --short=16 "${target}")
+if [[ "${new_commit}" != "${commit_outcome}" ]]; then
+    log "Error: hard-resetting failed. Expected new HEAD: ${commit_outcome}. Actual new HEAD: ${new_commit}."
+    exit 1
+fi
+if [[ -z "${dryrun}" ]]; then
+  "${gitcmd}" push "${gitremote}" --force --quiet "${target}"
+fi
+exit 0
diff --git a/git_sync_two_branches/lib.sh b/git_sync_two_branches/lib.sh
new file mode 100755
index 00000000..eed4dd57
--- /dev/null
+++ b/git_sync_two_branches/lib.sh
@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+
+# Functions for git_sync_two_branches.sh
+
+# Let's go strict (exit on error)
+set -e
+
+# Apply some defaults in case nobody defined them.
+BUILD_NUMBER="${BUILD_NUMBER:-0}"
+BUILD_TIMESTAMP="$(date +'%Y-%m-%d_%H-%M-%S')"
+gitcmd="${gitcmd:-git}"
+logfile="${WORKSPACE}/git_sync_two_branches.log"
+
+# Utility function to check if a branch is an ancestor of another branch.
+# Returns 0 if $1 is an ancestor of $2, 1 otherwise.
+function is_ancestor() {
+    local branch1=$1
+    local branch2=$2
+    if ${gitcmd} merge-base --is-ancestor "${branch1}" "${branch2}"; then
+        return 0
+    else
+        return 1
+    fi
+}
+
+# Utility function to output something both to stdout and to a log file, with some extra information.
+function log() {
+    echo "$1"
+    echo "$BUILD_NUMBER $BUILD_TIMESTAMP $1" >> "${logfile}"
+}
diff --git a/tests/3-git_sync_two_branches.bats b/tests/3-git_sync_two_branches.bats
new file mode 100755
index 00000000..1deaf0f7
--- /dev/null
+++ b/tests/3-git_sync_two_branches.bats
@@ -0,0 +1,106 @@
+#!/usr/bin/env bats
+
+load libs/shared_setup
+
+setup_file() {
+    # All these tests need a moodle git clone with a remote available, called "local_ci_tests"
+    # pointing to the https://git.in.moodle.com/integration/prechecker.git repository.
+    cd "${gitdir}"
+    git remote add local_ci_tests https://git.in.moodle.com/integration/prechecker.git
+    cd $OLDPWD
+
+    # Note that we'll be removing this custom remote at the end of the tests in this file.
+}
+
+teardown_file() {
+    # Remove the custom remote we added in setup_file.
+    cd "${gitdir}"
+    git remote remove local_ci_tests
+    cd $OLDPWD
+}
+
+setup() {
+    # Always perform dry runs when testing. We don't want to change the fixture branches.
+    cd "${gitdir}"
+    export dryrun=1
+
+    # Set the rest of env variables needed for the script.
+    export gitremote=local_ci_tests
+
+    # Let's checkout master, so we test everything without any local checkout of the branches.
+    git checkout master --quiet
+    cd $OLDPWD
+}
+
+teardown() {
+    # Remove the source and target local branches created for the test.
+    # (all them begin with local_ci_git_sync_ prefix).
+    cd "${gitdir}"
+    git branch --list 'local_ci_git_sync_*' | xargs -r git branch -D --quiet
+    cd $OLDPWD
+}
+
+@test "git_sync_two_branches: Both branches are in sync" {
+
+    export source=local_ci_git_sync_master
+    export target=local_ci_git_sync_main
+
+    run git_sync_two_branches/git_sync_two_branches.sh
+
+    # Assert result.
+    assert_success
+
+    assert_output --partial "Syncing local_ci_tests remote"
+    assert_output --partial "Set ${target} target branch to ${source} source branch..."
+    assert_output --partial "Dry-run enabled"
+    assert_output --partial "Creating local source branch ${source}"
+    assert_output --partial "Creating local target branch ${target}"
+    assert_output --partial "Branches ${source} and ${target} are the same"
+    assert_output --partial "Current HEAD: d76e211be6ae65fe"
+}
+
+@test "git_sync_two_branches: Both branches have diverged (reset)" {
+
+    export source=local_ci_git_sync_master_diverged
+    export target=local_ci_git_sync_main
+
+    run git_sync_two_branches/git_sync_two_branches.sh
+
+    # Assert result.
+    assert_success
+
+    assert_output --partial "Diverged branches (surely because of some rewrite"
+    assert_output --partial "Hard-resetting target branch ${target} to source branch ${source}"
+    assert_output --partial "New HEAD: 150d134fa2d519cd"
+}
+
+@test "git_sync_two_branches: There are new commits in source branch (fast forward)" {
+
+    export source=local_ci_git_sync_master_fast_forward
+    export target=local_ci_git_sync_main
+
+    run git_sync_two_branches/git_sync_two_branches.sh
+
+    # Assert result.
+    assert_success
+
+    assert_output --partial "The source ${source} branch got new commits."
+    assert_output --partial "Fast-forwarding target branch ${target} to source branch ${source}"
+    assert_output --partial "New HEAD: 5f3a55f6f01142d8"
+}
+
+@test "git_sync_two_branches: There are new commits in target branch (error)" {
+
+    export source=local_ci_git_sync_master
+    export target=local_ci_git_sync_main_advanced
+
+    run git_sync_two_branches/git_sync_two_branches.sh
+
+    # Assert result.
+    assert_failure
+
+    assert_output --partial "The target ${target} branch got new commits."
+    assert_output --partial "Error: target ${target} branch has some unexpected commits, not available in the source"
+    assert_output --partial "fix the target ${target} branch manually"
+    assert_output --partial "Expected HEAD: d76e211be6ae65fe"
+}