06 Remotes in GitHub.ipynb

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    "<div id=\"body\">\n",
    "    <center>\n",
    "    <a href=\"05 Ignore Things.ipynb\">  <font size=\"6\"> &lt; </font></a>\n",
    "    <a href=\"index.ipynb\">  <font size=\"6\"> Version Control with Git </font> </a>\n",
    "    <a href=\"07 Collaborating.ipynb\">  <font size=\"6\"> &gt; </font></a>\n",
    "    </center>\n",
    "</div>"
   ]
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   "source": [
    "# **Remotes in GitHub**"
   ]
  },
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   "source": [
    "## **Questions**\n",
    "\n",
    "* How do I share my changes with others on the web?"
   ]
  },
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   "metadata": {},
   "source": [
    "## **Objectives**\n",
    "\n",
    "* Explain what remote repositories are and why they are useful.\n",
    "* Push to or pull from a remote repository."
   ]
  },
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   "source": [
    "Version control really comes into its own when we begin to collaborate with other people. We already have most of the machinery we need to do this; the only thing missing is to copy changes from one repository to another.\n",
    "\n",
    "Systems like Git allow us to move work between any two repositories. In practice, though, it’s easiest to use one copy as a **central hub**, and to keep it on the web rather than on someone’s laptop. Most programmers use hosting services like [GitHub](https://github.com/), [Bitbucket](https://bitbucket.org/) or [GitLab](https://gitlab.com/) to hold those master copies; we’ll explore the pros and cons of this in the final section of this lesson.\n",
    "\n",
    "Let’s start by sharing the changes we’ve made to our current project with the world. Log in to GitHub, then click on the icon in the top right corner to create a new repository called `git_tutorial`:\n",
    "\n",
    "&nbsp;\n",
    "\n",
    "<img src=\"static/images/github-create-repo-01.png\">\n",
    "\n",
    "&nbsp;\n",
    "\n",
    "Name your repository `git_tutorial` and then click “Create Repository”.\n",
    "\n",
    "Note: Since this repository will be connected to a local repository, it needs to be empty. Leave “Initialize this repository with a README” unchecked, and keep “None” as options for both “Add .gitignore” and “Add a license.” See the “GitHub License and README files” exercise below for a full explanation of why the repository needs to be empty.\n",
    "\n",
    "&nbsp;\n",
    "\n",
    "<img src=\"static/images/github-create-repo-02.png\">\n",
    "\n",
    "&nbsp;\n",
    "\n",
    "As soon as the repository is created, GitHub displays a page with a URL and some information on how to configure your local repository:\n",
    "\n",
    "\n",
    "<img src=\"static/images/github-create-repo-03.png\">\n",
    "\n",
    "This effectively does the following on GitHub’s servers:\n",
    "\n",
    "```bash\n",
    "mkdir planets\n",
    "cd planets\n",
    "git init\n",
    "```\n",
    "\n",
    "If you remember back to the earlier lesson where we added and committed our earlier work on mars.txt, we had a diagram of the local repository which looked like this:\n",
    "\n",
    "The Local Repository with Git Staging Area\n",
    "\n",
    "<img src=\"static/images/git-staging-area.svg\">\n",
    "\n",
    "\n",
    "Now that we have two repositories, we need a diagram like this:\n",
    "\n",
    "<img src=\"static/images/git-freshly-made-github-repo.svg\">\n",
    "\n",
    "Note that our local repository still contains our earlier work on `keyval.py`, but the remote repository on GitHub appears empty as it doesn’t contain any files yet.\n",
    "\n",
    "The next step is to connect the two repositories. We do this by making the GitHub repository a [`remote`](https://swcarpentry.github.io/reference.html#remote) for the local repository. The home page of the repository on GitHub includes the string we need to identify it:\n",
    "\n",
    "<img src=\"static/images/github-find-repo-string.png\">\n",
    "\n",
    "\n",
    "Copy that URL from the browser, go into the local `git_tutorial` repository, and run this command:\n",
    "\n"
   ]
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  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [],
   "source": [
    "cd SRC"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [],
   "source": [
    "# git remote add origin https://github.com/epinux/git_tutorial.git"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Make sure to use the URL for your repository rather than epinux’s: the only difference should be your username instead of `epinux`.\n",
    "\n",
    "`origin` is a local name used to refer to the remote repository. It could be called anything, but `origin` is a convention that is often used by default in git and GitHub, so it’s helpful to stick with this unless there’s a reason not to.\n",
    "\n",
    "We can check that the command has worked by running `git remote -v:"
   ]
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     "text": [
      "origin\thttps://github.com/epinux/git_tutorial.git (fetch)\n",
      "origin\thttps://github.com/epinux/git_tutorial.git (push)\n"
     ]
    }
   ],
   "source": [
    "git remote -v"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "We’ll discuss remotes in more detail in the next episode, while talking about how they might be used for collaboration.\n",
    "\n",
    "Once the remote is set up, this command will push the changes from our local repository to the repository on GitHub:"
   ]
  },
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   "cell_type": "code",
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   "metadata": {},
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   "source": [
    "# git push origin master"
   ]
  },
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   "source": [
    "\n",
    "```\n",
    "Enumerating objects: 16, done.\n",
    "Counting objects: 100% (16/16), done.\n",
    "Delta compression using up to 8 threads.\n",
    "Compressing objects: 100% (11/11), done.\n",
    "Writing objects: 100% (16/16), 1.45 KiB | 372.00 KiB/s, done.\n",
    "Total 16 (delta 2), reused 0 (delta 0)\n",
    "remote: Resolving deltas: 100% (2/2), done.\n",
    "To https://github.com/epinux/git_tutorial.git\n",
    " * [new branch]      master -> master\n",
    "```\n",
    "\n",
    "**NOTE:**  To run the above command from the jupyter environment we need to open a new terminal, as we need to issue an username and password!\n"
   ]
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   "metadata": {},
   "source": [
    "<blockquote class=\"callout\">\n",
    "  <h2 id=\"proxy\">Proxy</h2>\n",
    "\n",
    "If the network you are connected to uses a proxy, there is a chance that your last command failed with “Could not resolve hostname” as the error message. To solve this issue, you need to tell Git about the proxy:\n",
    "    \n",
    "```bash\n",
    "git config --global http.proxy http://user:password@proxy.url\n",
    "git config --global https.proxy http://user:password@proxy.url\n",
    "```\n",
    "\n",
    "&nbsp;\n",
    "    \n",
    "When you connect to another network that doesn’t use a proxy, you will need to tell Git to disable the proxy using:\n",
    "    \n",
    "```bash\n",
    "git config --global --unset http.proxy\n",
    "git config --global --unset https.proxy\n",
    "```\n",
    "</blockquote>"
   ]
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   "metadata": {},
   "source": [
    "Our local and remote repositories are now in this state:\n",
    "\n",
    "<img src=\"../static/images/github-repo-after-first-push.svg\">\n",
    "\n"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Note:**\n",
    "\n",
    "* The `-u` Flag**\n",
    "\n",
    "You may see a `-u` option used with `git push` in some documentation. This option is synonymous with the `--set-upstream-to` option for the `git branch` command, and is used to associate the current branch with a remote branch so that the git pull command can be used without any arguments. To do this, simply use `git push -u origin master` once the remote has been set up."
   ]
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   "source": [
    "We can pull changes from the remote repository to the local one as well:"
   ]
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     "text": [
      "From https://github.com/epinux/git_tutorial\n",
      " * branch            master     -> FETCH_HEAD\n",
      "Already up to date.\n"
     ]
    }
   ],
   "source": [
    "git pull origin master"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Pulling has no effect in this case because the two repositories are already synchronized. If someone else had pushed some changes to the repository on GitHub, though, this command would download them to our local repository."
   ]
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   "metadata": {},
   "source": [
    "**Explore the GitHub GUI**\n",
    "\n",
    "Browse to your `git_tutorial` repository on GitHub. Under the Code tab, find and click on the text that says “XX commits” (where “XX” is some number). Hover over, and click on, the three buttons to the right of each commit. What information can you gather/explore from these buttons? \n",
    "\n",
    "* How would you get that same information in the shell?\n",
    "\n",
    "The left-most button (with the picture of a clipboard) copies the full identifier of the commit to the clipboard. In the shell, `git log` will show you the full commit identifier for each commit.\n",
    "\n",
    "When you click on the middle button, you’ll see all of the changes that were made in that particular commit. Green shaded lines indicate additions and red ones removals. In the shell we can do the same thing with `git diff`. In particular, `git diff ID1..ID2` where ID1 and ID2 are commit identifiers (e.g. `git diff a3bf1e5..041e637`) will show the differences between those two commits.\n",
    "\n",
    "The right-most button lets you view all of the files in the repository at the time of that commit. To do this in the shell, we’d need to checkout the repository at that particular time. We can do this with `git checkout ID` where ID is the identifier of the commit we want to look at. If we do this, we need to remember to put the repository back to the right state afterwards!"
   ]
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   "source": [
    "**GitHub Timestamp**\n",
    "\n",
    "GitHub displays timestamps in a human readable relative format (i.e. “22 hours ago” or “three weeks ago”). However, if you hover over the timestamp, you can see the exact time at which the last change to the file occurred."
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Push vs. Commit**\n",
    "\n",
    "How is “git push” different from “git commit”?\n",
    "\n",
    "When we push changes, we’re interacting with a remote repository to update it with the changes we’ve made locally (often this corresponds to sharing the changes we’ve made with others). Commit only updates your local repository."
   ]
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   "source": [
    "**Extra:**\n",
    "\n",
    "In this section we learned about creating a remote repository on GitHub, but when you initialized your GitHub repo, you didn’t add a README.md or a license file. If you had, what do you think would have happened when you tried to link your local and remote repositories?\n",
    "\n",
    "In this case, we’d see a **merge conflict** due to unrelated histories. When GitHub creates a `README.md` file, it performs a commit in the remote repository. When you try to pull the remote repository to your local repository, Git detects that they have histories that do not share a common origin and refuses to merge.\n",
    "\n",
    "```bash\n",
    "git pull origin master\n",
    "\n",
    "warning: no common commits\n",
    "remote: Enumerating objects: 3, done.\n",
    "remote: Counting objects: 100% (3/3), done.\n",
    "remote: Total 3 (delta 0), reused 0 (delta 0), pack-reused 0\n",
    "Unpacking objects: 100% (3/3), done.\n",
    "From https://github.com/epinux/git_tutorial\n",
    " * branch            master     -> FETCH_HEAD\n",
    " * [new branch]      master     -> origin/master\n",
    "fatal: refusing to merge unrelated histories\n",
    "```\n",
    "\n",
    "You can force git to merge the two repositories with the option `--allow-unrelated-histories`. Be careful when you use this option and carefully examine the contents of local and remote repositories before merging.\n",
    "\n",
    "```bash\n",
    "git pull --allow-unrelated-histories origin master\n",
    "\n",
    "From https://github.com/epinux/git_tutorial\n",
    " * branch            master     -> FETCH_HEAD\n",
    "Merge made by the 'recursive' strategy.\n",
    "README.md | 1 +\n",
    "1 file changed, 1 insertion(+)\n",
    "create mode 100644 README.md\n",
    "```\n"
   ]
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   "metadata": {},
   "source": [
    "<blockquote class=\"keypoints\">\n",
    "  <h2>Key Points</h2>\n",
    "    \n",
    "* A local Git repository can be connected to one or more remote repositories.\n",
    "* Use the HTTPS protocol to connect to remote repositories until you have learned how to set up SSH.\n",
    "* `git push` copies changes from a local repository to a remote repository.\n",
    "* `git pull` copies changes from a remote repository to a local repository.\n",
    "</blockquote>"
   ]
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   "cell_type": "markdown",
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   "source": [
    "<div id=\"body\">\n",
    "    <center>\n",
    "    <a href=\"05 Ignore Things.ipynb\">  <font size=\"4\"> &lt; </font></a>\n",
    "    <a href=\"index.ipynb\">  <font size=\"4\"> Version Control with Git </font> </a>\n",
    "    <a href=\"07 Collaborating.ipynb\">  <font size=\"4\"> &gt; </font></a>\n",
    "    </center>\n",
    "</div>"
   ]
  }
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