initial release

.gitignore

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+.DS_Store

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [1.0.0] - 2021-03-16
+
+### Added
+* Initial release.

Dockerfile

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+FROM alpine
+
+RUN apk add --no-cache git openssh-client && \
+  echo "StrictHostKeyChecking no" >> /etc/ssh/ssh_config
+
+ADD *.sh /
+
+ENTRYPOINT ["/entrypoint.sh"]

README.md

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+# Sync Git Branch Action
+
+A GitHub Action for synchronizing a git branch to another location via SSH.
+
+## Inputs
+
+| Name                 | Description                               | Default                             | Required |
+| -------------------- | ----------------------------------------- | ----------------------------------- | -------- |
+| `source-branch`      | Branch name in the source repository      |                                     | Yes      |
+| `destination-repo`   | SSH URL of the destination repository     |                                     | Yes      |
+| `source-repo`        | SSH URL of the source repository          | The respository this action runs in | No       |
+| `destination-branch` | Branch name in the destination repository | Same as `source-branch`             | No       |
+
+## Environment variables
+
+`SSH_PRIVATE_KEY`: Create a [SSH key](https://help.github.com/en/github/authenticating-to-github/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent#generating-a-new-ssh-key) **without** a passphrase which has access to both repositories.
+
+On GitHub you should add the public key to repository "deploy keys". 
+
+Store [the private key as a secret](https://help.github.com/en/actions/configuring-and-managing-workflows/creating-and-storing-encrypted-secrets) and use it in your workflow as seen in the example usage below.
+
+## Example workflow
+
+```yml
+name: Sync main branch to Bitbucket
+
+on: [push, delete, create]
+
+jobs:
+  git-mirror:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: zent-contrib/git-branch-sync-action@v1
+        env:
+          SSH_PRIVATE_KEY: ${{ secrets.SSH_PRIVATE_KEY }}
+        with:
+          source-branch: main
+          destination-repo: "git@bitbucket.org:<org>/<repo>.git"
+```

action.yml

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+name: "Sync Git Branch Action"
+description: "Action for synchronizing a git branch to another location (Bitbucket, GitHub, GitLab, etc.) using SSH."
+branding:
+  icon: "copy"
+  color: "#155bd4"
+inputs:
+  source-repo:
+    description: "SSH URL of the source repository. Defaults to the repository this actions runs in."
+    required: false
+    default: 'git@github.com:${{ github.repository }}.git'
+  source-branch:
+    description: "Branch name in source repository."
+    required: true
+    default: ""
+  destination-repo:
+    description: "SSH URL of the destination repository."
+    required: true
+    default: ""
+  destination-branch:
+    description: "Branch name in destination repository. Defaults to source-branch if not present."
+    required: false
+    default: ""
+runs:
+  using: "docker"
+  image: "Dockerfile"
+  args:
+    - ${{ inputs.source-repo }}
+    - ${{ inputs.source-branch }}
+    - ${{ inputs.destination-repo }}
+    - ${{ inputs.destination-branch }}

entrypoint.sh

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+#!/bin/sh
+
+set -e
+
+if [[ -n "$SSH_PRIVATE_KEY" ]]
+then
+  mkdir -p /root/.ssh
+  echo "$SSH_PRIVATE_KEY" > /root/.ssh/id_rsa
+  chmod 600 /root/.ssh/id_rsa
+fi
+
+mkdir -p ~/.ssh
+cp /root/.ssh/* ~/.ssh/ 2> /dev/null || true
+
+sh -c "/sync-git-branch.sh $*"

sync-git-branch.sh

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+#!/bin/sh
+
+set -e
+
+SOURCE_REPO="$1"
+SOURCE_BRANCH="$2"
+DESTINATION_REPO="$3"
+DESTINATION_BRANCH="${4:-$2}"
+REPO_DIR="repo"
+
+echo "SOURCE=$SOURCE_REPO:$SOURCE_BRANCH"
+echo "DESTINATION=$DESTINATION_REPO:$DESTINATION_BRANCH"
+
+# Only clone the branch we want to sync
+git clone --single-branch --branch "$SOURCE_BRANCH" "$SOURCE_REPO" "$REPO_DIR"
+cd "$REPO_DIR"
+git remote add mirror "$DESTINATION_REPO"
+git push mirror "$SOURCE_BRANCH:$DESTINATION_BRANCH"