Creating a pre-receive hook script

You can see examples of pre-receive hooks for GitHub Enterprise Server in the github/platform-samples repository.

Writing a pre-receive hook script

A pre-receive hook script executes in a pre-receive hook environment on your GitHub Enterprise Server instance. When you create a pre-receive hook script, consider the available input, output, exit status, and environment variables.

Input (stdin)

After a push occurs and before any refs are updated for the remote repository, the git-receive-pack process on your GitHub Enterprise Server instance invokes the pre-receive hook script. Standard input for the script, stdin, is a string containing a line for each ref to update. Each line contains the old object name for the ref, the new object name for the ref, and the full name of the ref.

<old-value> SP <new-value> SP <ref-name> LF

This string represents the following arguments.

For more information about git-receive-pack, see "git-receive-pack" in the Git documentation. For more information about refs, see "Git References" in Pro Git.

Output (stdout)

The standard output for the script, stdout, is passed back to the client. Any echo statements will be visible to the user on the command line or in the user interface.

Exit status

The exit status of a pre-receive script determines if the push will be accepted.

Environment variables

In addition to the standard input for your pre-receive hook script, stdin, GitHub Enterprise Server makes the following variables available in the Bash environment for your script's execution. For more information about stdin for your pre-receive hook script, see "Input (stdin)."

Different environment variables are available to your pre-receive hook script depending on what triggers the script to run.

• Always available
• Available for pushes from the web interface or API
• Available for pull request merges
• Available for pushes using SSH authentication

Always available

The following variables are always available in the pre-receive hook environment.

$GIT_DIR

$GIT_PUSH_OPTION_COUNT

$GIT_PUSH_OPTION_N

$GIT_USER_AGENT

$GITHUB_REPO_NAME

$GITHUB_REPO_PUBLIC

$GITHUB_USER_IP

$GITHUB_USER_LOGIN

Available for pushes from the web interface or API

The $GITHUB_VIA variable is available in the pre-receive hook environment when the ref update that triggers the hook occurs via either the web interface or the API for GitHub Enterprise Server. The value describes the action that updated the ref.

auto-merge deployment api

blob#save

branch merge api

branches page delete button

git refs create api

git refs delete api

git refs update api

git repo contents api

merge base into head

pull request branch delete button

pull request branch undo button

pull request merge api

pull request merge button

pull request revert button

releases delete button

stafftools branch restore

tag create api

slumlord (#SHA)

web branch create

Available for pull request merges

The following variables are available in the pre-receive hook environment when the push that triggers the hook is a push due to the merge of a pull request.

$GITHUB_PULL_REQUEST_AUTHOR_LOGIN

$GITHUB_PULL_REQUEST_HEAD

$GITHUB_PULL_REQUEST_BASE

Available for pushes using SSH authentication

$GITHUB_PUBLIC_KEY_FINGERPRINT

Setting permissions and pushing a pre-receive hook to GitHub Enterprise Server

A pre-receive hook script is contained in a repository on your GitHub Enterprise Server instance. A site administrator must take into consideration the repository permissions and ensure that only the appropriate users have access.

We recommend consolidating hooks to a single repository. If the consolidated hook repository is public, the README.md can be used to explain policy enforcements. Also, contributions can be accepted via pull requests. However, pre-receive hooks can only be added from the default branch. For a testing workflow, forks of the repository with configuration should be used.

1. For Mac users, ensure the scripts have execute permissions:

   $ sudo chmod +x SCRIPT_FILE.sh

   For Windows users, ensure the scripts have execute permissions:

   git update-index --chmod=+x SCRIPT_FILE.sh

2. Commit and push to the designated repository for pre-receive hooks on your GitHub Enterprise Server instance.

   $ git commit -m "YOUR COMMIT MESSAGE"
   $ git push

3. Create the pre-receive hook on the GitHub Enterprise Server instance.

Testing pre-receive scripts locally

You can test a pre-receive hook script locally before you create or update it on your GitHub Enterprise Server instance. One method is to create a local Docker environment to act as a remote repository that can execute the pre-receive hook.

1. Ensure Docker is installed locally.

2. Create a file called Dockerfile.dev containing:

   FROM gliderlabs/alpine:3.3
   RUN \
     apk add --no-cache git openssh bash && \
     ssh-keygen -A && \
     sed -i "s/#AuthorizedKeysFile/AuthorizedKeysFile/g" /etc/ssh/sshd_config && \
     adduser git -D -G root -h /home/git -s /bin/bash && \
     passwd -d git && \
     su git -c "mkdir /home/git/.ssh && \
     ssh-keygen -t ed25519 -f /home/git/.ssh/id_ed25519 -P '' && \
     mv /home/git/.ssh/id_ed25519.pub /home/git/.ssh/authorized_keys && \
     mkdir /home/git/test.git && \
     git --bare init /home/git/test.git"
   
   VOLUME ["/home/git/.ssh", "/home/git/test.git/hooks"]
   WORKDIR /home/git
   
   CMD ["/usr/sbin/sshd", "-D"]
   

3. Create a test pre-receive script called always_reject.sh. This example script will reject all pushes, which is useful for locking a repository:

   #!/usr/bin/env bash
   
   echo "error: rejecting all pushes"
   exit 1
   

4. Ensure the always_reject.sh scripts has execute permissions:

   $ chmod +x always_reject.sh

5. From the directory containing Dockerfile.dev, build an image:

   $ docker build -f Dockerfile.dev -t pre-receive.dev .
   > Sending build context to Docker daemon 3.584 kB
   > Step 1 : FROM gliderlabs/alpine:3.3
   >  ---> 8944964f99f4
   > Step 2 : RUN apk add --no-cache git openssh bash && ssh-keygen -A && sed -i "s/#AuthorizedKeysFile/AuthorizedKeysFile/g"  /etc/ssh/sshd_config && adduser git -D -G root -h /home/git -s /bin/bash && passwd -d git && su git -c "mkdir /home/git/.ssh && ssh-keygen -t ed25519 -f /home/git/.ssh/id_ed25519 -P ' && mv /home/git/.ssh/id_ed25519.pub /home/git/.ssh/authorized_keys && mkdir /home/git/test.git && git --bare init /home/git/test.git"
   >  ---> Running in e9d79ab3b92c
   > fetch http://alpine.gliderlabs.com/alpine/v3.3/main/x86_64/APKINDEX.tar.gz
   > fetch http://alpine.gliderlabs.com/alpine/v3.3/community/x86_64/APKINDEX.tar.gz
   ....truncated output....
   > OK: 34 MiB in 26 packages
   > ssh-keygen: generating new host keys: RSA DSA ECDSA ED25519
   > Password for git changed by root
   > Generating public/private ed25519 key pair.
   > Your identification has been saved in /home/git/.ssh/id_ed25519.
   > Your public key has been saved in /home/git/.ssh/id_ed25519.pub.
   ....truncated output....
   > Initialized empty Git repository in /home/git/test.git/
   > Successfully built dd8610c24f82

6. Run a data container that contains a generated SSH key:

   $ docker run --name data pre-receive.dev /bin/true

7. Copy the test pre-receive hook always_reject.sh into the data container:

   $ docker cp always_reject.sh data:/home/git/test.git/hooks/pre-receive

8. Run an application container that runs sshd and executes the hook. Take note of the container id that is returned:

   $ docker run -d -p 52311:22 --volumes-from data pre-receive.dev
   > 7f888bc700b8d23405dbcaf039e6c71d486793cad7d8ae4dd184f4a47000bc58

9. Copy the generated SSH key from the data container to the local machine:

   $ docker cp data:/home/git/.ssh/id_ed25519 .

10. Modify the remote of a test repository and push to the test.git repo within the Docker container. This example uses git@github.com:octocat/Hello-World.git but you can use any repository you want. This example assumes your local machine (127.0.0.1) is binding port 52311, but you can use a different IP address if docker is running on a remote machine.

    $ git clone git@github.com:octocat/Hello-World.git
    $ cd Hello-World
    $ git remote add test git@127.0.0.1:test.git
    $ GIT_SSH_COMMAND="ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -p 52311 -i ../id_ed25519" git push -u test main
    > Warning: Permanently added '[192.168.99.100]:52311' (ECDSA) to the list of known hosts.
    > Counting objects: 7, done.
    > Delta compression using up to 4 threads.
    > Compressing objects: 100% (3/3), done.
    > Writing objects: 100% (7/7), 700 bytes | 0 bytes/s, done.
    > Total 7 (delta 0), reused 7 (delta 0)
    > remote: error: rejecting all pushes
    > To git@192.168.99.100:test.git
    >  ! [remote rejected] main -> main (pre-receive hook declined)
    > error: failed to push some refs to 'git@192.168.99.100:test.git'

    Notice that the push was rejected after executing the pre-receive hook and echoing the output from the script.

Further reading

• "Customizing Git - An Example Git-Enforced Policy" from the Pro Git website