gitsis

Always use the correct SSH identity file depending on your current repository.

Why

Working with multiple SSH identities and Git can be hard. You might be working as a contractor for multiple companies or simply work on personal projects on your company device. In such cases, you probably deal with multiple SSH identities. In a normal workflow, all these SSH keys are loaded into an SSH agent instance and then tried one after another during the authentication phase with a remote host. In case you want to control which key is preferred for which host, the ↗ SSH config file is a great place to get started.

This pattern however quickly breaks down in the context of Git where a remote's host is often the same (e.g. github.com) regardless of the repository you are working on. It is still possible to control the used key used by SSH but this requires a workaround that either has to be set on the ↗ repository or ↗ remote level. I personally find both options unsatisfying because each one of them requires additional steps in my Git workflow when I set up a repository locally. This ⭐ should just work 🌟 out of the box every time.

Using the wrong identity is especially a problem when only a subset of your identities is authorized for SSO access access to a repository on GitHub. In such a case, a key which is not authorized for a specific organisation will fail the whole interaction instead of proceeding with the next registered public key.

Usage

After the installation and configuration using the config file, you do not have to do anything. Simply perform any Git operation that requires a remote and see gitsis work its magic.

In case you want to debug what identity file gitsis is using, run

export GITSIS_DEBUG=1

before executing your Git command. This prompts gitsis to write a debug log to /tmp/gitsis.log.

Installation

Install the required dependencies:
- Bash (>= v4.2)
- jq
ℹ️ In case you run MacOS and want to install gitsis using ↗ brew, jump straight to step 2. Brew will also handle the installation of dependencies.

On MacOS you can install both dependencies using brew by running the following command:
```
brew install bash jq
```
On Linux you can install them using e.g. apt-get by running the following command:
```
apt-get install bash jq
```
To install the dependencies on any other operating system/using any other package manager, please consult the respective documentation.
Download and install gitsis. In case you run MacOS and use brew, you can do this by executing the following command:
```
brew install mariuskiessling/gitsis/gitsis
```
Alternatively, you can directly download gitsis into a directory that is inside your PATH and mark it as executable. A good default is /usr/local/bin. Depending on your system setup, you need root privileges to write to this directory. In such a case, prefix the following commands with sudo or consult the documentation of your operating system.
```
curl -o /usr/local/bin/gitsis https://raw.githubusercontent.com/mariuskiessling/gitsis/main/gitsis
chmod +x /usr/local/bin/gitsis
```
Set gitsis as the ↗ SSH command for Git.

ℹ️ Replace /usr/local/bin/gitsis with your installation path in case you chose a different one in the previous step.

You can either do this using an environment variable by setting
```
export GIT_SSH_COMMAND="/usr/local/bin/gitsis"
```
inside your shell's init file (e.g. ~/.bashrc for Bash or ~/.config/fish/config.fish for fish). If you use an alternative shell, please consult its documentation to find out how to set the environment variable.

Alternatively, you can set the command directly in Git's configuration by running the following command:
```
git config --global core.sshCommand /usr/local/bin/gitsis
```
Finally, finish the installation by creating your configuration file.

Configuration

gitsis uses a JSON configuration file. The order in which the file is discovered is:

$XDG_CONFIG_HOME/gitsis/config.json
$HOME/.config/gitsis/config.json

The following properties may be set inside the configuration file:

.owners: A map of repository owners and their respective private identity files. The public keys are automatically derived by SSH.

You may use ~ to refer to your home directory when defining an identity file. Other substitutions are not allowed.

In case no identity is defined inside gitsis' configuration file for a specific repository owner that is currently being operated on, SSH will try the ↗ default keys in addition to those that may be registered in an SSH agent instance.

Example

Let's suppose you work for Mega Corp and have two identity files; one for your personal work and one for your work at Mega Corp. In such a case, your config file can look like this:

{
  "owners": {
    "MY-GITHUB-USERNAME": "~/.ssh/id_personal",
    "mega-corp": "~/.ssh/id_mega_corp"
  }
}

(Un)supported Use Cases

gitsis operates on the assumption that your repositories are hosted under a path that looks like this:

owner/my-repo.git

owner can either be your personal username or an organisation name. This assumption is true for most Git hosting solutions (e.g. GitHub or GitLab). In case you are operating on a Git host that uses a different path schema, feel free to open an issue.