Documentation/gittutorial.adoc

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 folder 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 folder with the command `git init`.
   This creates the `.git` folder 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 2.23.0.

[[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 move 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 folder, 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!

`git status` is very important
------------------------------

`git status` shows you the current state of your Git repository.
It can give a huge amount of different information that takes
practice to interpret, so this tutorial has several examples of
how to interpret the output of `git status` in different situations.

It's often useful to run `git status` before and after running a Git
command to see how it changed things.

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 (though see the warning in
the next section!).

Warning about terminal text editors
-----------------------------------

If you run `git commit` without any arguments, Git will open a
text editor in the terminal for you to use to edit your commit message.
This lets you write a longer commit message with more details.
By default, Git uses the `vi` text editor, which is extremely difficult
to use if you're not used to it. On Windows, the Git installer asks you
to set your text editor during the install to avoid this problem.

Some signs that you're trapped in `vi`: suddenly typing doesn't work
anymore, and there are tilde `~` characters on the bottom left of the
screen.

If Git opens `vi` and you'd rather use a different editor, you can
handle it like this:

1. Quit by pressing the `Escape` key, then typing `:q!` and then `Enter`
2. Run `git config --global core.editor YOUR_PREFERRED_EDITOR`
   to configure which editor to use. For example, `nano` is available
   on a lot of systems and has basic instructions on the screen for how
   to use it. Here is a list of
   https://git-scm.com/book/en/v2/Appendix-C:-Git-Commands-Setup-and-Config#ch_core_editor[how to configure some common editors].
3. Run `git commit` again

If you don't want to set up a text editor yet, you can also just use
`git commit -m"your message"` like we did earlier in this tutorial.

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 `README.md`, 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 (Mac/Linux only): 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.

On Windows, you can skip this step and the next one because Git for
Windows comes with
https://github.com/git-ecosystem/git-credential-manager[Git Credential Manager]
to handle logging in.

If you don't already have an SSH key, you'll need to create one.
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 (Mac/Linux only): Tell the Git host your SSH public key
---------------------------------------------------------------

First, find your SSH key like this:

-------------------------------------------
$ ls ~/.ssh/*.pub
-------------------------------------------

That will output something like this. The exact filename depends on the type
of key. The important thing is that you're looking for a filename that
ends in `.pub` ("pub" stands for "public"):

-------------------------------------------
/home/alice/.ssh/id_ed25519.pub
-------------------------------------------

Copy the contents of that file to your clipboard. One way to do that is
to run this command (replacing the filename with the right filename),
and then copy the output.

-------------------------------------------
cat /home/alice/.ssh/id_ed25519.pub
-------------------------------------------

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.

If you followed the Mac/Linux instructions to set up an SSH key, look
for an "SSH" address like this:

-------------------------------------------
git@git.example.com:username/repo.git
-------------------------------------------

If you're on Windows, look for an "HTTPS" address like this:

-------------------------------------------
https://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://` and you need
an SSH address 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 address from Step 13):

-------------------------------------------
$ 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
-------------------------------------------

On Windows, this should pop up a dialog from Git Credential Manager
asking you to log in to the Git host in your browser.

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])
* To get a list of common Git commands, run `git help` (linkgit:git-help[1])

GIT
---
Part of the linkgit:git[1] suite