Creating a pre-receive hook script

Use pre-receive hook scripts to create requirements for accepting or rejecting a push based on the contents.

In this article

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.

ArgumentDescription
<old-value>Old object name stored in the ref.
When you create a new ref, the value is 40 zeroes.
<new-value>New object name to be stored in the ref.
When you delete a ref, the value is 40 zeroes.
<ref-name>The full name of the ref.

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.

Exit-status valueAction
0The push will be accepted.
non-zeroThe push will be rejected.

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

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

VariableDescriptionExample value
$GIT_DIR
Path to the remote repository on the instance/data/user/repositories/a/ab/
a1/b2/34/100001234/1234.git
$GIT_PUSH_OPTION_COUNT
The number of push options that were sent by the client with --push-option. For more information, see "git-push" in the Git documentation.1
$GIT_PUSH_OPTION_N
Where N is an integer starting at 0, this variable contains the push option string that was sent by the client. The first option that was sent is stored in GIT_PUSH_OPTION_0, the second option that was sent is stored in GIT_PUSH_OPTION_1, and so on. For more information about push options, see "git-push" in the Git documentation.abcd
$GIT_USER_AGENT
User-agent string sent by the Git client that pushed the changesgit/2.0.0
$GITHUB_REPO_NAME
Name of the repository being updated in NAME/OWNER formatocto-org/hello-enterprise
$GITHUB_REPO_PUBLIC
Boolean representing whether the repository being updated is public
  • true: Repository's visibility is public
  • false: Repository's visibility is private or internal
$GITHUB_USER_IP
IP address of client that initiated the push192.0.2.1
$GITHUB_USER_LOGIN
Username for account that initiated the pushoctocat

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.

ValueActionMore information
auto-merge deployment api
Automatic merge of the base branch via a deployment created with the API"Repositories" in the REST API documentation
blob edit
Change to a file's contents in the web interface"Editing files in your repository"
branch merge api
Merge of a branch via the API"Repositories" in the REST API documentation
branches page delete button
Deletion of a branch in the web interface"Creating and deleting branches within your repository"
git refs create api
Creation of a ref via the API"Git database" in the REST API documentation
git refs delete api
Deletion of a ref via the API"Git database" in the REST API documentation
git refs update api
Update of a ref via the API"Git database" in the REST API documentation
git repo contents api
Change to a file's contents via the API"Repositories" in the REST API documentation
merge base into head
Update of the topic branch from the base branch when the base branch requires strict status checks (via Update branch in a pull request, for example)"About protected branches"
pull request branch delete button
Deletion of a topic branch from a pull request in the web interface"Deleting and restoring branches in a pull request"
pull request branch undo button
Restoration of a topic branch from a pull request in the web interface"Deleting and restoring branches in a pull request"
pull request merge api
Merge of a pull request via the API"Pulls" in the REST API documentation
pull request merge button
Merge of a pull request in the web interface"Merging a pull request"
pull request revert button
Revert of a pull request"Reverting a pull request"
releases delete button
Deletion of a release"Managing releases in a repository"
stafftools branch restore
Restoration of a branch from the site admin dashboard"Site admin dashboard"
tag create api
Creation of a tag via the API"Git database" in the REST API documentation
slumlord (#SHA)
Commit via Subversion"Support for Subversion clients"
web branch create
Creation of a branch via the web interface"Creating and deleting branches within your repository"

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.

VariableDescriptionExample value
$GITHUB_PULL_REQUEST_AUTHOR_LOGIN
Username of account that authored the pull requestoctocat
$GITHUB_PULL_REQUEST_HEAD
The name of the pull request's topic branch, in the format USERNAME:BRANCHoctocat:fix-bug
$GITHUB_PULL_REQUEST_BASE
The name of the pull request's base branch, in the format USERNAME:BRANCHoctocat:main

Available for pushes using SSH authentication

VariableDescriptionExample value
$GITHUB_PUBLIC_KEY_FINGERPRINT
The public key fingerprint for the user who pushed the changesa1:b2:c3:d4:e5:f6:g7:h8:i9:j0:k1:l2:m3:n4:o5:p6

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.

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