This GitHub API page provides the full reference. The API endpoint for reading a file:

https://api.github.com/repos/{username}/{repository_name}/contents/{file_path}

{
  "encoding": "base64",
  "size": 5362,
  "name": "README.md",
  "content": "encoded content ...",
  "sha": "3d21ec53a331a6f037a91c368710b99387d012c1",
  ...
}

• Consider using a personal access token
  • Rate-limits (up to 60 per-hour for anonymous, up to 5,000 per-hour for authenticated) read more
  • Enable accessing files in private repos
• The file content in the response is base64 encoded string

Using curl

Reading https://github.com/airbnb/javascript/blob/master/package.json using GitHub's API via curl:

curl -H 'Accept: application/vnd.github.v3.raw' https://api.github.com/repos/airbnb/javascript/contents/package.json

• Make sure to pass header Accept: application/vnd.github.v3.raw to get raw file response (thanks jakub.g)

Using Python

Reading https://github.com/airbnb/javascript/blob/master/package.json using GitHub's API in Python:

import base64
import json
import requests
import os


def github_read_file(username, repository_name, file_path, github_token=None):
    headers = {}
    if github_token:
        headers['Authorization'] = f"token {github_token}"
        
    url = f'https://api.github.com/repos/{username}/{repository_name}/contents/{file_path}'
    r = requests.get(url, headers=headers)
    r.raise_for_status()
    data = r.json()
    file_content = data['content']
    file_content_encoding = data.get('encoding')
    if file_content_encoding == 'base64':
        file_content = base64.b64decode(file_content).decode()

    return file_content


def main():
    github_token = os.environ['GITHUB_TOKEN']
    username = 'airbnb'
    repository_name = 'javascript'
    file_path = 'package.json'
    file_content = github_read_file(username, repository_name, file_path, github_token=github_token)
    data = json.loads(file_content)
    print(data['name'])


if __name__ == '__main__':
    main()

• Define an environment variable GITHUB_TOKEN before running

This will have to be relative to a particular commit, as some files may be present in some commits and absent in others, so before you can look at files you'll need to use something like List commits on a repository:

GET /repos/:owner/:repo/commits

If you're just interested in the latest commit on a branch you can set the sha parameter to the branch name:

sha string SHA or branch to start listing commits from.

Once you have a commit hash, you can inspect that commit

GET /repos/:owner/:repo/git/commits/:sha

which should return something like this (truncated from GitHub's documentation):

{
  "sha": "...",
  "...",
  "tree": {
    "url": "https://api.github.com/repos/octocat/Hello-World/git/trees/691272480426f78a0138979dd3ce63b77f706feb",
    "sha": "691272480426f78a0138979dd3ce63b77f706feb"
  },
  "...": "..."
}

Look at the hash of its tree, which is essentially its directory contents. In this case, 691272480426f78a0138979dd3ce63b77f706feb. Now we can finally request the contents of that tree:

GET /repos/:owner/:repo/git/trees/:sha

The output from GitHub's example is

{
  "sha": "9fb037999f264ba9a7fc6274d15fa3ae2ab98312",
  "url": "https://api.github.com/repos/octocat/Hello-World/trees/9fb037999f264ba9a7fc6274d15fa3ae2ab98312",
  "tree": [
    {
      "path": "file.rb",
      "mode": "100644",
      "type": "blob",
      "size": 30,
      "sha": "44b4fc6d56897b048c772eb4087f854f46256132",
      "url": "https://api.github.com/repos/octocat/Hello-World/git/blobs/44b4fc6d56897b048c772eb4087f854f46256132"
    },
    {
      "path": "subdir",
      "mode": "040000",
      "type": "tree",
      "sha": "f484d249c660418515fb01c2b9662073663c242e",
      "url": "https://api.github.com/repos/octocat/Hello-World/git/blobs/f484d249c660418515fb01c2b9662073663c242e"
    },
    {
      "path": "exec_file",
      "mode": "100755",
      "type": "blob",
      "size": 75,
      "sha": "45b983be36b73c0788dc9cbcb76cbb80fc7bb057",
      "url": "https://api.github.com/repos/octocat/Hello-World/git/blobs/45b983be36b73c0788dc9cbcb76cbb80fc7bb057"
    }
  ]
}

As you can see, we have some blobs, which correspond to files, and some additional trees, which correspond to subdirectories. You may want to do this recursively.

If you want a raw file, you just need to set request header Accept with application/vnd.github.v3.raw. Just one step would be enough.

GitHub API: /repos/{owner}/{repo}/contents/{path}

Refer to:

Official way to access / download github files from scripts?

https://docs.github.com/en/rest/repos/contents