What is the use of "actions/checkout" in GitHub Actions?

Question

I saw a lot of uses of :

jobs:
  myjob:
    steps:
      - name: checkout
        uses: "actions/checkout@something"
      - ...

But i can not find what is the purpose of the line :

uses : "actions/checkout@something"

Is it similar to this ?

 run: git checkout something   

Answer 1

For this line: uses : "actions/checkout@something", it will use the actions/checkout github action (source here) with the ref something. This ref only refers to the github action version (nothing to do with your repo)

The uses statement refers to a github action that is being used in this step. From github documentation for jobs.<job_id>.steps[*].uses:

Selects an action to run as part of a step in your job. An action is a reusable unit of code. You can use an action defined in the same repository as the workflow, a public repository, or in a published Docker container image.

From actions/checkout readme :

This action checks-out your repository under $GITHUB_WORKSPACE, so your workflow can access it.

By default it checks out only one commit. My understanding is that it's doing something similar to:

git fetch --depth 1 origin $GITHUB_REF

This action also persists an auth token in git config. This way, your workflow can run authenticated git commands

By default, it clones your current repository ({{ github.repository }}) but you can also use this action to clone a different repository, and specify additionnal parameters like token, branch, path etc...

An example with additionnal input parameters: check out all git history by setting fetch-depth to 0 (default is 1), see usage doc:

- uses: actions/checkout@v2
  with:
    fetch-depth: 0