[PATCH v2 2/2] doc: add new Git tutorial for beginners

["Julia Evans via GitGitGadget" <gitgitgadget@gmail.com>]

From: Julia Evans <julia@jvns.ca>

Introduce Git in two parts:

Steps 1-7: Create an empty repo & make 2 commits (`git init`,
`git add`, `git commit`, `git status`, `git diff`)
Steps 8-12: Push the repo to a remote host like GitHub or GitLab
(`git remote add`, `git push`)

Some design choices:

- Introduce a much smaller set of commands than the old tutorial, to
  keep the tutorial quick (doable in an hour). Some users found it to be
  very fast and some wanted to stop early (after step 7) so this seems
  to be a good balance.
- Have users start with an empty repository instead of an existing
  project, because many of the testers did not have an existing project
  to test, or were scared of using an existing project.
- Add a troubleshooting section, since in the testing some users got
  extremely stuck (for 4+ hours) on minor issues like needing to say
  "yes" when SSHing to a new host.
- Don't explain branches, because explaining branches and collaboration
  is complicated and deserves its own tutorial. Instead just tell users
  we're not going to explain them in this document.
- Tell users to set their default branch to `main`, because many Git
  hosts use it as their default branch and it really simplifies things.
- Keep it as vendor neutral as possible, while acknowledging the reality
  that Git hosting services like GitHub, GitLab, etc are extremely
  popular. Do this by creating the repo locally and then optionally
  pushing it later.
- Use SSH for authentication, explaining how to set up SSH in the
  easiest way possible (with disclaimers to check your security team's
  policy if applicable since the "easiest way" may not be the best), and
  giving some instructions for how to translate an HTTPS URL to an SSH
  URL.

At least 20 people have successfully completed it, and feedback was
incorporated from problems the testers ran into.

Co-authored-by: Marie Claire LeBlanc Flanagan <hello@marieflanagan.com>
Signed-off-by: Marie Claire LeBlanc Flanagan <hello@marieflanagan.com>
Signed-off-by: Julia Evans <julia@jvns.ca>
---
 Documentation/gittutorial.adoc | 739 +++++++++++++++++++++++++++++++++
 1 file changed, 739 insertions(+)
 create mode 100644 Documentation/gittutorial.adoc

diff --git a/Documentation/gittutorial.adoc b/Documentation/gittutorial.adoc
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+gittutorial(7)
+==============
+
+NAME
+----
+gittutorial - Introduction to Git for beginners
+
+DESCRIPTION
+-----------
+
+Git is a program that lets you collaborate with others on code, back up
+your code, roll back to an earlier version of your project, track how
+your project has changed over time, and much more.
+
+This tutorial explains how to create a new Git project, make
+changes to it, and upload the project to the Internet. It usually takes
+less than an hour if you have a little previous experience with Git, and
+1-2 hours if not.
+
+We'll show you how to use the commands `git commit`, `git add`,
+`git diff`, `git status`, and `git push`.
+
+What we'll be doing
+-------------------
+
+Git lets you take snapshots of your code, like a checkpoint in a video
+game. These snapshots are called "commits". Which files to save in a
+commit is up to you.
+
+When Git stores a commit, it stores a copy of every file in the commit
+in a database in a directory called `.git`. This is called a "Git repository".
+
+In this tutorial we're going to show you how to:
+
+1. Create a Git repository in a directory with the command `git init`.
+   This creates the `.git` directory where Git stores all your commits.
+2. Tell Git which files you want to put in a commit
+3. Make a commit
+4. Change a file and make a new commit with the new version
+5. Make experimental changes and throw them away
+
+We'll also explain how to use Git to put your code on the Internet.
+
+Command line vs graphical interfaces
+------------------------------------
+
+Git is a command line program, but there are many graphical interfaces
+for Git built by the community. Your text editor might even have a
+built-in Git integration.
+
+In this tutorial, we'll give instructions for using Git from the
+command line. We can't give you instructions for how to do it in your
+graphical interface because there are so many of them.
+
+You can always switch back and forth between using the command line and
+a graphical interface at any time.
+
+Step 1: Make sure Git is installed
+----------------------------------
+
+To do this tutorial, you'll need:
+
+1. Git to be installed. You might already have it installed,
+   and if not there are directions at https://git-scm.com/install/
+2. A terminal to use. On Windows, you can use Git Bash which comes
+   when you install Git.
+
+You can check if Git is installed by running this in your terminal:
+
+------------------------------------------------
+$ git --version
+------------------------------------------------
+
+It should output something like this:
+
+------------------------------------------------
+git version 2.54.0
+------------------------------------------------
+
+The exact version isn't too important: this tutorial is designed to work
+with any version of Git after 2.0.
+
+[[step_2]]
+Step 2: Introduce yourself to Git
+---------------------------------
+
+The first time you use Git on a computer, it's a good idea to tell Git
+your name and public email address. If you don't, you'll get a lot of
+warnings from Git. To do this, run:
+
+------------------------------------------------
+$ git config --global user.name "Namey McName"
+$ git config --global user.email you@example.com
+------------------------------------------------
+
+The name and email is so that other people you're collaborating with
+can know who made the changes. You can set them to anything you want.
+
+To check if the name and email have been set, you can run these
+commands. They should display your name and email:
+
+------------------------------------------------
+$ git config user.name
+$ git config user.email
+------------------------------------------------
+
+Next, configure Git to set the default branch name to `main` (instead
+of `master`) by running this command. We can't explain what a branch is
+yet, but Git is going to
+https://sfconservancy.org/news/2020/jun/23/gitbranchname/[make `main` the default soon],
+and setting this avoids some extremely annoying problems in Step 14.
+
+------------------------------------------------
+$ git config --global init.defaultBranch main
+------------------------------------------------
+
+If there's no output, then it succeeded.
+
+[NOTE]
+It's also possible to set a different email address for every project,
+but we're not covering that here.
+
+Step 3: Create an empty folder and 1 file
+------------------------------------------
+
+In your terminal, create an empty folder and change directories into it.
+Later on you could follow the same steps in this tutorial to take an
+existing project and start managing it with Git, but we're going to use
+an empty project so that everyone following this tutorial has the exact
+same files.
+
+For example, run these commands to make a folder called `myproject`
+
+------------------------------------------------
+$ mkdir myproject
+$ cd myproject
+------------------------------------------------
+
+Next, using your text editor, create a file in this directory, so we
+have something to work with.
+
+Make a file called README.md, with these contents:
+
+------------------------------------------------
+We're doing a Git tutorial!
+We need 2 lines in this file so here's a second one
+------------------------------------------------
+
+Save `README.md` and keep it open in your text editor so you can
+easily edit it later. The exact contents aren't too important, but
+we'll use those contents in the example output later.
+
+You can check that the file is in the right place by running `ls`,
+like this:
+
+------------------------------------------------
+$ ls
+------------------------------------------------
+
+The output should look like this:
+
+------------------------------------------------
+README.md
+------------------------------------------------
+
+Step 4: Create a Git repository
+-------------------------------
+
+A Git repository lets you save snapshots of your code. Every time you make a
+snapshot, Git stores a copy of the contents of every file in the snapshot.
+
+This might feel unbelievable, but Git uses compression so you can easily
+store tens of thousands of snapshots without using too much disk space.
+
+You can create a Git repository for any folder in your computer by
+running the command `git init`.
+
+Run `git init` in the folder you just created, like this:
+
+------------------------------------------------
+$ git init
+------------------------------------------------
+
+Git will reply:
+
+------------------------------------------------
+Initialized empty Git repository in .git/
+------------------------------------------------
+
+You might notice a new hidden folder was created, named `.git`.
+You don't need to use this folder directly in any way, but it's where
+your commits will be stored.
+
+Step 5: Make your first commit
+------------------------------
+
+Next, we're going to take a snapshot of all the files in the directory
+and tell Git to store them. Git calls these snapshots "commits".
+Making a commit in Git is a two step process: first you need to run `git
+add` and then `git commit`.
+
+The reason it's a two step process is that sometimes you might not
+actually want Git to store a commit with _all_ the files in a folder.
+For example, if you're compiling your program to a binary, you might
+want Git to put the program's source code in your commit but not
+compiled binary. You can also use a `.gitignore` file to help with this.
+
+For now, we're going to store a commit with every file in your folder,
+as well as in every subfolder.
+
+The first step of making a commit is to tell Git which files you
+want to include in the commit using `git add`. To tell Git
+"include all files", run this command:
+
+------------------------------------------------
+$ git add .
+------------------------------------------------
+
+To check if it succeeded, you can run `git status`.
+The output should look like this:
+
+
+------------------------------------------------
+$ git status
+On branch main
+
+No commits yet
+
+Changes to be committed:
+  (use "git rm --cached <file>..." to unstage)
+        new file:   README.md
+------------------------------------------------
+
+Next you can create the commit using the `git commit` command, like
+this:
+
+------------------------------------------------
+$ git commit -m "Initial commit"
+------------------------------------------------
+
+The `-m` flag stands for "message", and it lets you set a reminder
+for your future self of what you were doing. The message
+("Initial commit" in this example) can say anything you want.
+
+The output should look like this:
+
+------------------------------------------------
+[main (root-commit) 4a7e367] Initial commit
+ 1 file changed, 2 insertions(+)
+ create mode 100644 README.md
+------------------------------------------------
+
+You've now stored the first version of your project in Git!
+
+Step 6: Make a change
+---------------------
+
+Now, let's learn how to make a change to a file and review the change you made.
+In this example, we'll update the file `README.md`, but you can edit a
+different file.
+
+First open `README.md` in your favourite text editor, make a tiny or
+silly change, and save it. In our example, we'll edit the first line to
+add more exclamation marks:
+
+------------------------------------------------
+We're doing a Git tutorial!!!!
+------------------------------------------------
+
+
+Next, in the terminal, run the command `git status` to get a summary of
+what you changed since the last Git commit.
+
+------------------------------------------------
+$ git status
+------------------------------------------------
+
+The output will look something like this:
+
+------------------------------------------------
+On branch main
+
+Changes not staged for commit:
+  (use "git add <file>..." to update what will be committed)
+  (use "git restore <file>..." to discard changes in working directory)
+        modified:   README.md
+------------------------------------------------
+
+This output says that we've changed `README.md` since the last time
+we committed.
+
+Next, if you want to see the details of how you've changed `README.md`,
+you can run `git diff`.
+
+------------------------------------------------
+$ git diff
+------------------------------------------------
+
+The output will look something like this:
+
+------------------------------------------------
+diff --git a/README.md b/README.md
+index ffa8b85..a49e4b0 100644
+--- a/README.md
++++ b/README.md
+@@ -1,2 +1,2 @@
+-We're doing a Git tutorial!
++We're doing a Git tutorial!!!!
+ We need 2 lines in this file so here's a second one
+------------------------------------------------
+
+This output says that we deleted one line (`We're doing a Git
+tutorial!`) and added one line (`We're doing a Git tutorial!!!!`). By
+default Git will show the lines you removed in red, and the lines you
+added in green. Removed lines start with a minus `-` and added lines
+start with a plus `+`.
+
+Step 7a: Make a second commit
+-----------------------------
+
+Now that we've changed `README.md`, let's make another commit.
+We're going to follow the same steps as before when we made our first
+commit.
+
+First, tell Git that we want to include the new version of `README.md`
+in the next commit:
+
+------------------------------------------------
+$ git add README.md
+------------------------------------------------
+
+and then run `git commit`, like this:
+
+------------------------------------------------
+$ git commit -m "Update README"
+------------------------------------------------
+
+The output should look like this:
+
+------------------------------------------------
+[main 32e0171] Update README
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+------------------------------------------------
+
+This says that the new commit is the same as the previous commit,
+except that we changed 1 line of 1 file.
+
+Step 7b: Make a second commit (the short way)
+---------------------------------------------
+
+Instead of running the two commands in Step 7a above, you
+could also run this command as a shortcut. It will:
+
+1. Get a list of every file in the most recent commit
+2. Commit the current version of all of those files
+
+------------------------------------------------
+$ git commit -am "Update README"
+------------------------------------------------
+
+The `-a` stands for "all".
+
+Understanding `git status` output: "untracked files"
+----------------------------------------------------
+
+Now let's talk about a term you'll see in the `git status` output:
+"untracked files".
+
+First create a new file called `example.txt`. It can be empty.
+
+Then run the command `git status`. You should see this output:
+
+----------
+On branch main
+
+Untracked files:
+  (use "git add <file>..." to include in what will be committed)
+        example.txt
+
+nothing added to commit but untracked files present (use "git add" to track)
+----------
+
+This has a list of "untracked files". An untracked file is a file which
+was not part of the latest commit. By default Git will ignore these
+files.
+
+If you want to commit an untracked file, you can add it like this:
+
+----------
+$ git add example.txt
+----------
+
+Then you can run `git commit` to commit it.
+
+Step 8: Make a change and throw it away
+---------------------------------------
+
+One of the most useful things about Git is that it lets you safely
+experiment: you never need to be scared to change your code because you
+can always go back to the old version.
+
+Before starting to experiment, run `git status` to check the current
+state of your Git repository:
+
+------------------------------------------------
+$ git status
+On branch main
+
+nothing to commit, working tree clean
+------------------------------------------------
+
+This "working tree clean" message means that there haven't been any
+changes since your last commit.
+
+Now make a change to your code, and run `git status` again. Like last
+time, you should see a message like this:
+
+------------------------------------------------
+$ git status
+On branch main
+
+Changes not staged for commit:
+  (use "git add <file>..." to update what will be committed)
+  (use "git restore <file>..." to discard changes in working directory)
+        modified:   README.md
+------------------------------------------------
+
+This tells us that `README.md` has been changed since the last commit.
+But we don't actually want this change, so let's undo it! We can restore
+`README.md` back to how it was at the most recent commit using the `git
+restore` command.
+
+**WARNING**: `git restore` can't be reversed! Any time you run it it's
+important to be absolutely sure that you're okay with throwing away your
+changes since the last commit.
+
+------------------------------------------------
+$ git restore README.md
+------------------------------------------------
+
+Now open `README.md` again. You should see that your changes have been undone.
+
+You can stop here!
+------------------
+
+The commands we've learned so far (`git init`, `git add`, `git commit`,
+`git diff`, `git status`, and `git restore`) are enough to get a lot out
+of Git on their own.
+
+With these commands, you can keep copies of past versions of your code
+on your computer and safely experiment with big changes to your code.
+
+When using Git this way, all of your code just lives on your computer.
+But if you want to put your code on the Internet so that other people
+can use it or back it up, then you'll need to learn about one more
+command: `git push`.
+
+The rest of this tutorial is about how to upload your code to a Git
+repository on the Internet. The high level process is:
+
+1. Create an account on the Git host
+2. Configure Git so that it can login to the Git host to make changes
+3. Run `git push origin main` to upload your code
+
+Navigating the Git host's UI can be tricky, and it's hard for us to
+give you exact directions because the UIs change a lot.
+There's a rich tradition of people learning Git from their "Git friend"
+who already knows Git, and if you have one of those it might be useful
+to ask them if you get stuck. There are many ways to do things in Git,
+so they might suggest a slightly different way.
+There's also a troubleshooting guide at the end with the most common
+issues folks have run into while doing this tutorial.
+
+Step 9: Create an account on a Git host
+---------------------------------------
+
+To put your code on the Internet, you need it to be hosted somewhere.
+The easiest way to do this is to sign up with a Git host. There's a list
+of Git hosts (many of them offer free accounts) at
+https://git-scm.com/tools/hosting. GitHub and GitLab are two popular hosts.
+
+If you're comfortable running a server, there are other ways to host
+a Git repository on the internet, like connecting through SSH or open
+source Git forge software to your server.
+
+Step 10: Create an SSH key (if needed)
+-------------------------------------
+
+For Git to upload changes to another Git repository, it needs a way to
+login to that repository. One of the most popular ways to do this is
+with an SSH key.
+
+If you don't already have an SSH key, you'll need to create one. On Mac
+or Linux, you can check if you already have an SSH key by running:
+
+-------------------------------------------
+$ ls ~/.ssh/*.pub
+-------------------------------------------
+
+If you don't already have an SSH key, run:
+
+-------------------------------------------
+$ ssh-keygen
+-------------------------------------------
+
+`ssh-keygen` will ask you for some details.
+The easiest way to do this is to just press Enter at every prompt until
+it's done.
+
+NOTE: All of the advice about how to use SSH in this tutorial is aimed
+at getting it to work as quickly as possible. Setting up SSH in a secure
+way is very far outside the scope of this tutorial. If you have a
+security team in your organization, they might have very different
+opinions about the appropriate way to configure authentication with Git.
+
+Step 11: Tell the Git host your SSH public key
+----------------------------------------------
+
+First, find your SSH key like this (on Mac or Linux):
+
+-------------------------------------------
+$ ls ~/.ssh/*.pub
+-------------------------------------------
+
+That will output something like
+
+-------------------------------------------
+/home/alice/.ssh/id_ed25519.pub
+-------------------------------------------
+
+Copy the contents of that file to your clipboard. It's important that
+the filename ends in `.pub` ("pub" stands for "public").
+
+Then login to your account on the Git host you chose and paste the SSH
+key into the appropriate place. Depending on the Git host, it's likely
+under "SSH keys" in your settings. If it asks you what type of SSH key,
+look for something like "Authentication Key".
+
+Once you've done this, you're done! Git will automatically use your SSH
+key to try to connect.
+
+[[step_12]]
+Step 12: Create an empty Git repository on the Git host
+-------------------------------------------------------
+
+Go to the Git host's website and create a new Git repository. If there's
+an option to "create a README.md", "add a license", or something
+similar, make sure to say no to all of them.
+
+**WARNING**: If you create a public repository and push your repository
+to it, then your name and email address, as well as the contents of
+any files you committed and every previous version of those files, will
+be on the public internet. Some Git hosts have the option to create
+a private repository instead, to keep your information private.
+
+Step 13: Find the address of the repository
+-------------------------------------------
+
+Now, find the address of the repository on your Git host. It should look
+something like this:
+
+-------------------------------------------
+git@git.example.com:username/repo.git
+-------------------------------------------
+
+(where `example.com`, `username` and `repo` will be replaced with the
+actual values).
+
+If you can only find an address starting with `https://`, then
+you can often translate it to the SSH format by replacing `https://`
+with `git@` and replacing the `/` after the domain name with an `:`.
+Here's how:
+
+-------------------------------------------
+https://git.example.com/username/repo
+^^^^^^^^               ^
+replace with "git@"    replace with ":"
+-------------------------------------------
+
+[[step_14]]
+Step 14: push your changes
+--------------------------
+
+Finally, we're ready to send the information in your local Git repository
+to the one on the internet!
+
+First, tell Git about the other repository by running `git remote add`
+in your terminal
+(replace `git@example.com:username/repo.git` with the actual address):
+
+-------------------------------------------
+$ git remote add origin git@example.com:username/repo.git
+-------------------------------------------
+
+This tells Git to save the URL `git@example.com:username/repo.git` in
+your Git repository's configuration under the name `origin`, so that you
+can use it later. If you make a mistake while doing this, you can run
+`git remote remove origin` and try again.
+
+Then tell Git to send the information, with `git push`
+(if this doesn't work, read the "Troubleshooting `git push`" section for advice!):
+
+-------------------------------------------
+$ git push --set-upstream origin main
+-------------------------------------------
+
+Let's break down this `git push` command:
+
+* `origin` refers to the Git repository that we just added with
+  `git remote add` (`git@example.com:username/repo.git`).
+  You can use any name (not just `origin`), but using `origin` often
+  makes things easier because it's the default for `git push` and `git pull`.
+* `main` is the name of your current Git branch.
+  We haven't covered branches yet, but `main` is the default branch name.
+* `--set-upstream` tells Git to make this the default for `git push` for
+  the `main` branch. After you set the upstream, `git status` will
+  also tell you if you have commits that haven't been pushed.
+
+The output should look like this:
+
+-------------------------------------------
+Enumerating objects: 7, done.
+Counting objects: 100% (7/7), done.
+Delta compression using up to 8 threads
+Compressing objects: 100% (6/6), done.
+Writing objects: 100% (7/7), 662 bytes | 662.00 KiB/s, done.
+Total 7 (delta 0), reused 0 (delta 0), pack-reused 0 (from 0)
+To example.com:username/repo
+ * [new branch]      main -> main
+branch 'main' set up to track 'origin/main'.
+-------------------------------------------
+
+Any time you want to set up a new repository, you can repeat steps
+12-14, with one exception: you only need to pass `--set-upstream` the first time
+you push. Afterwards you can just run the command:
+
+-------
+$ git push origin main
+-------
+
+Troubleshooting `git push`
+--------------------------
+
+There are 3 main errors you might run into when running `git push origin main`.
+Getting errors when running `git push` is actually a big part of using
+Git, so congratulations! If you get to this part of the tutorial, you
+get a little bonus experience in debugging.
+
+Here's a guide to why they're happening and how to fix them.
+
+**Problem 1: You pushed to `main`, but your branch actually isn't called `main`**
+
+Here's the error:
+
+-------
+$ git push origin main
+error: src refspec main does not match any
+-------
+
+This might happen if you didn't set `init.defaultBranch` to `main` in
+<<step_2,Step 2>>.
+
+If you want to check the name of your current branch, you can run `git
+status`. For example, in this output, the current branch is "master".
+
+-------------------------------------------
+$ git status
+On branch master
+
+...
+-------------------------------------------
+
+To push your branch, you have a few options:
+
+* rename your branch to `main`, by running `git branch -m main`
+  and then run `git push origin main`
+* run `git push origin master`
+  (where `master` is the name of your actual current branch)
+
+**Problem 2: SSH error**
+
+Here's the error:
+
+-----
+$ git push origin main
+The authenticity of host 'github.com (140.82.116.3)' can't be established.
+ED25519 key fingerprint is: SHA256:+DiY3wvvV6TuJJhbpZisF/zLDA0zPMSvHdkr4UvCOqU
+This key is not known by any other names.
+Are you sure you want to continue connecting (yes/no/[fingerprint])?
+-----
+
+Git uses SSH to connect during `git push`, and this message happens when
+SSH connects to a new site that you haven't connected to before.
+
+The easiest way to handle this is to type "yes" and press Enter.
+
+**Problem 3: the repository already has commits in it**
+
+Here's the error:
+
+---------------------
+$ git push origin main
+! [rejected]        main -> main (fetch first)
+error: failed to push some refs to 'example.com:username/repo.git'
+hint: Updates were rejected because the remote contains work that you do not
+hint: have locally.
+---------------------
+
+If you see an error like this, it means that when you created the
+repository on your Git host, it added a commit to try to help you out.
+That's the "work that you do not have locally" in the error message.
+
+You should be able to fix this by redoing <<step_12,Step 12>>.
+Then <<step_14,Step 14>> should work.
+
+MORE USEFUL GIT COMMANDS
+------------------------
+
+We're not going to cover collaboration at all in this tutorial, but here are a
+few more commands that might be useful when using Git on your own:
+
+* To download a Git repository to use on your computer you can run
+  `git clone https://example.com/some/repo` (linkgit:git-clone[1]).
+* To tell Git to "un-add" a file that you added by accident, run
+  `git rm --cached FILENAME` (linkgit:git-rm[1])
+* To get a list of past commits, run `git log` (linkgit:git-log[1])
+* Every commit has an ID that looks something like this:
+  `497588f53141b531a59ed888c7d2d9b8f8a6742d`.
+  If you see a commit with `git log` that you want more details about,
+  you can run `git show COMMIT_ID` to see what changes that made since
+  the previous commit (linkgit:git-show[1])
+
+SEE ALSO
+--------
+linkgit:git-help[1],
+
+GIT
+---
+Part of the linkgit:git[1] suite
-- 
gitgitgadget