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Zachary Bennett

Understanding and Using Git's index.lock File

Zachary Bennett

  • Oct 21, 2020
  • 4 Min read
  • 236 Views
  • Oct 21, 2020
  • 4 Min read
  • 236 Views
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Introduction

Git is the de facto standard when it comes to source control. Boasting a powerful API, git is simple enough for most users and yet has a rich set of commands to help manage complex workflows. If you have been a user of git, you have probably seen an error message that looks something like this:

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fatal: Unable to create '.git/index.lock': File exists.

This error alludes to the creation of an index.lock file within your hidden .git directory. In this guide, you will learn about the index.lock file, the purpose that it serves, and how you can resolve the error above.

Let's dive in!

Note: This guide assumes you have a basic working knowledge of git and that you already have git installed on your machine.

What is the Index.Lock File?

On your local machine, the working internals of a git repository live inside of a hidden .git directory. Inside this directory lives all sorts of information that the git command-line tool interfaces with when you use it. Branch, commit, and sub-module information are just a few of the things that live within this repo.

Here's an example of what a .git directory might look like:

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.git/
    hooks/
    info/
    logs/
    objects/
    refs/
    HEAD
    config
    description
    index
    packed-refs

A little known fact is that whenever you run a git process, git creates an index.lock file within the .git directory. For example, if you run the command git add . to stage all local changes within your current repository, git will create this index.lock file while the git add command is running:

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.git/
    ...
    index.lock

Upon successful completion of the git add process, the index.lock file is removed. What this does is ensure that simultaneous changes to your local git repo do not occur, as this would cause your git repo to be in an indeterminate state. The index.lock file prevents changes to your local repository from happening from outside of the currently running git process so as to ensure multiple git processes are not altering or changing the same repository internals at the same time.

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fatal: Unable to create '.git/index.lock': File exists.

So it makes sense that the error above is essentially telling you, "Hey, a git process is running in this git repo—you will have to wait to run the command you are attempting to run." However, it may be that there is no git process running! Sometimes, for whatever reason, a git process will not end gracefully. There are a number of reasons why this would happen. In this case, you can resolve the problem simply by removing the index.lock file manually via this command or a similar command on your operating system: rm .git/index.lock.

Conclusion

Git is a ubiquitous tool for source control and contains many powerful mechanisms for helping to facilitate developer workflows. Git uses the index.lock file to ensure transactional transparency within local repositories. By creating an index.lock file when a git process starts and failing the process if this file already exists, git can ensure that multiple git processes are not altering/reading the same internal repository information at the same time. This leads to a great safety guarantee when using git!

One more thing to think about is that you may find a use for creating an index.lock file yourself. If you have your own CLI tooling around your source code, it may make sense to lock down the internals within a local git repository.

For more information, check out the git documentation.

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