GitHub - jobs : what is : use actions/checkout - github

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

Answer 2

Understanding terminologies made things clearer
Remote repo - It can also be referred to as the origin
Origin - the default name of the remote repo or the source repo being cloned
Head - a reference to human-friendly names for branches
git checkout - switch to a particular branch and displaying the changes currently on that branch
origin/name_of_branch - branch name created when fetching changes from a particular branch on the remote repo
Side Note: When git fetch is used, a custom branch is created locally in the form "origin/name_of_branch", changes on this branch can be viewed locally. These changes are the updated version of the files, not the specific change in that file as seen when commits are being inspected on GitHub.
Back to the question
When the action is executed
jobs:
myjob:
steps:
- name: checkout
uses: "actions/checkout#something"
- ...
The default steps being executed are:
The current repo in which the workflow is being triggered gets cloned.
Depending on the defined events such as a push or pull request:
For a push event, it runs the command below, where $GITHUB_REF points to the latest commit on the specified branch for the push event in the workflow.
git fetch --depth 1 $GITHUB_REF
For pull requests, it checks $GITHUB_REF points to the latest commit on the pull request source branch. This means it points to the would-be code/result from merging the pull request. This is the code/result other steps within the job are executed on such as running builds or tests. (Not completely sure of the command which runs under the hood)
Environment variables being referenced in the commands are explained here.
Additional options can be added to implement specific processes or scenarios such as checking out a different branch. This can be found in the official repo readme.