gittutorial-2(7) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | THE GIT OBJECT DATABASE | THE INDEX FILE | WHAT NEXT? | SEE ALSO | GIT | NOTES | COLOPHON

GITTUTORIAL-2(7)               Git Manual               GITTUTORIAL-2(7)

NAME         top

       gittutorial-2 - A tutorial introduction to Git: part two

SYNOPSIS         top

       git *

DESCRIPTION         top

       You should work through gittutorial(7) before reading this
       tutorial.

       The goal of this tutorial is to introduce two fundamental pieces
       of Git’s architecture—the object database and the index file—and
       to provide the reader with everything necessary to understand the
       rest of the Git documentation.

THE GIT OBJECT DATABASE         top

       Let’s start a new project and create a small amount of history:

           $ mkdir test-project
           $ cd test-project
           $ git init
           Initialized empty Git repository in .git/
           $ echo 'hello world' > file.txt
           $ git add .
           $ git commit -a -m "initial commit"
           [master (root-commit) 54196cc] initial commit
            1 file changed, 1 insertion(+)
            create mode 100644 file.txt
           $ echo 'hello world!' >file.txt
           $ git commit -a -m "add emphasis"
           [master c4d59f3] add emphasis
            1 file changed, 1 insertion(+), 1 deletion(-)

       What are the 7 digits of hex that Git responded to the commit
       with?

       We saw in part one of the tutorial that commits have names like
       this. It turns out that every object in the Git history is stored
       under a 40-digit hex name. That name is the SHA-1 hash of the
       object’s contents; among other things, this ensures that Git will
       never store the same data twice (since identical data is given an
       identical SHA-1 name), and that the contents of a Git object will
       never change (since that would change the object’s name as well).
       The 7 char hex strings here are simply the abbreviation of such
       40 character long strings. Abbreviations can be used everywhere
       where the 40 character strings can be used, so long as they are
       unambiguous.

       It is expected that the content of the commit object you created
       while following the example above generates a different SHA-1
       hash than the one shown above because the commit object records
       the time when it was created and the name of the person
       performing the commit.

       We can ask Git about this particular object with the cat-file
       command. Don’t copy the 40 hex digits from this example but use
       those from your own version. Note that you can shorten it to only
       a few characters to save yourself typing all 40 hex digits:

           $ git cat-file -t 54196cc2
           commit
           $ git cat-file commit 54196cc2
           tree 92b8b694ffb1675e5975148e1121810081dbdffe
           author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
           committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500

           initial commit

       A tree can refer to one or more "blob" objects, each
       corresponding to a file. In addition, a tree can also refer to
       other tree objects, thus creating a directory hierarchy. You can
       examine the contents of any tree using ls-tree (remember that a
       long enough initial portion of the SHA-1 will also work):

           $ git ls-tree 92b8b694
           100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad    file.txt

       Thus we see that this tree has one file in it. The SHA-1 hash is
       a reference to that file’s data:

           $ git cat-file -t 3b18e512
           blob

       A "blob" is just file data, which we can also examine with
       cat-file:

           $ git cat-file blob 3b18e512
           hello world

       Note that this is the old file data; so the object that Git named
       in its response to the initial tree was a tree with a snapshot of
       the directory state that was recorded by the first commit.

       All of these objects are stored under their SHA-1 names inside
       the Git directory:

           $ find .git/objects/
           .git/objects/
           .git/objects/pack
           .git/objects/info
           .git/objects/3b
           .git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
           .git/objects/92
           .git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
           .git/objects/54
           .git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
           .git/objects/a0
           .git/objects/a0/423896973644771497bdc03eb99d5281615b51
           .git/objects/d0
           .git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
           .git/objects/c4
           .git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241

       and the contents of these files is just the compressed data plus
       a header identifying their length and their type. The type is
       either a blob, a tree, a commit, or a tag.

       The simplest commit to find is the HEAD commit, which we can find
       from .git/HEAD:

           $ cat .git/HEAD
           ref: refs/heads/master

       As you can see, this tells us which branch we’re currently on,
       and it tells us this by naming a file under the .git directory,
       which itself contains a SHA-1 name referring to a commit object,
       which we can examine with cat-file:

           $ cat .git/refs/heads/master
           c4d59f390b9cfd4318117afde11d601c1085f241
           $ git cat-file -t c4d59f39
           commit
           $ git cat-file commit c4d59f39
           tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
           parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
           author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
           committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500

           add emphasis

       The "tree" object here refers to the new state of the tree:

           $ git ls-tree d0492b36
           100644 blob a0423896973644771497bdc03eb99d5281615b51    file.txt
           $ git cat-file blob a0423896
           hello world!

       and the "parent" object refers to the previous commit:

           $ git cat-file commit 54196cc2
           tree 92b8b694ffb1675e5975148e1121810081dbdffe
           author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
           committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500

           initial commit

       The tree object is the tree we examined first, and this commit is
       unusual in that it lacks any parent.

       Most commits have only one parent, but it is also common for a
       commit to have multiple parents. In that case the commit
       represents a merge, with the parent references pointing to the
       heads of the merged branches.

       Besides blobs, trees, and commits, the only remaining type of
       object is a "tag", which we won’t discuss here; refer to
       git-tag(1) for details.

       So now we know how Git uses the object database to represent a
       project’s history:

       •   "commit" objects refer to "tree" objects representing the
           snapshot of a directory tree at a particular point in the
           history, and refer to "parent" commits to show how they’re
           connected into the project history.

       •   "tree" objects represent the state of a single directory,
           associating directory names to "blob" objects containing file
           data and "tree" objects containing subdirectory information.

       •   "blob" objects contain file data without any other structure.

       •   References to commit objects at the head of each branch are
           stored in files under .git/refs/heads/.

       •   The name of the current branch is stored in .git/HEAD.

       Note, by the way, that lots of commands take a tree as an
       argument. But as we can see above, a tree can be referred to in
       many different ways—by the SHA-1 name for that tree, by the name
       of a commit that refers to the tree, by the name of a branch
       whose head refers to that tree, etc.--and most such commands can
       accept any of these names.

       In command synopses, the word "tree-ish" is sometimes used to
       designate such an argument.

THE INDEX FILE         top

       The primary tool we’ve been using to create commits is git-commit
       -a, which creates a commit including every change you’ve made to
       your working tree. But what if you want to commit changes only to
       certain files? Or only certain changes to certain files?

       If we look at the way commits are created under the cover, we’ll
       see that there are more flexible ways creating commits.

       Continuing with our test-project, let’s modify file.txt again:

           $ echo "hello world, again" >>file.txt

       but this time instead of immediately making the commit, let’s
       take an intermediate step, and ask for diffs along the way to
       keep track of what’s happening:

           $ git diff
           --- a/file.txt
           +++ b/file.txt
           @@ -1 +1,2 @@
            hello world!
           +hello world, again
           $ git add file.txt
           $ git diff

       The last diff is empty, but no new commits have been made, and
       the head still doesn’t contain the new line:

           $ git diff HEAD
           diff --git a/file.txt b/file.txt
           index a042389..513feba 100644
           --- a/file.txt
           +++ b/file.txt
           @@ -1 +1,2 @@
            hello world!
           +hello world, again

       So git diff is comparing against something other than the head.
       The thing that it’s comparing against is actually the index file,
       which is stored in .git/index in a binary format, but whose
       contents we can examine with ls-files:

           $ git ls-files --stage
           100644 513feba2e53ebbd2532419ded848ba19de88ba00 0       file.txt
           $ git cat-file -t 513feba2
           blob
           $ git cat-file blob 513feba2
           hello world!
           hello world, again

       So what our git add did was store a new blob and then put a
       reference to it in the index file. If we modify the file again,
       we’ll see that the new modifications are reflected in the git
       diff output:

           $ echo 'again?' >>file.txt
           $ git diff
           index 513feba..ba3da7b 100644
           --- a/file.txt
           +++ b/file.txt
           @@ -1,2 +1,3 @@
            hello world!
            hello world, again
           +again?

       With the right arguments, git diff can also show us the
       difference between the working directory and the last commit, or
       between the index and the last commit:

           $ git diff HEAD
           diff --git a/file.txt b/file.txt
           index a042389..ba3da7b 100644
           --- a/file.txt
           +++ b/file.txt
           @@ -1 +1,3 @@
            hello world!
           +hello world, again
           +again?
           $ git diff --cached
           diff --git a/file.txt b/file.txt
           index a042389..513feba 100644
           --- a/file.txt
           +++ b/file.txt
           @@ -1 +1,2 @@
            hello world!
           +hello world, again

       At any time, we can create a new commit using git commit (without
       the "-a" option), and verify that the state committed only
       includes the changes stored in the index file, not the additional
       change that is still only in our working tree:

           $ git commit -m "repeat"
           $ git diff HEAD
           diff --git a/file.txt b/file.txt
           index 513feba..ba3da7b 100644
           --- a/file.txt
           +++ b/file.txt
           @@ -1,2 +1,3 @@
            hello world!
            hello world, again
           +again?

       So by default git commit uses the index to create the commit, not
       the working tree; the "-a" option to commit tells it to first
       update the index with all changes in the working tree.

       Finally, it’s worth looking at the effect of git add on the index
       file:

           $ echo "goodbye, world" >closing.txt
           $ git add closing.txt

       The effect of the git add was to add one entry to the index file:

           $ git ls-files --stage
           100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0       closing.txt
           100644 513feba2e53ebbd2532419ded848ba19de88ba00 0       file.txt

       And, as you can see with cat-file, this new entry refers to the
       current contents of the file:

           $ git cat-file blob 8b9743b2
           goodbye, world

       The "status" command is a useful way to get a quick summary of
       the situation:

           $ git status
           On branch master
           Changes to be committed:
             (use "git restore --staged <file>..." to unstage)

                   new file:   closing.txt

           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:   file.txt

       Since the current state of closing.txt is cached in the index
       file, it is listed as "Changes to be committed". Since file.txt
       has changes in the working directory that aren’t reflected in the
       index, it is marked "changed but not updated". At this point,
       running "git commit" would create a commit that added closing.txt
       (with its new contents), but that didn’t modify file.txt.

       Also, note that a bare git diff shows the changes to file.txt,
       but not the addition of closing.txt, because the version of
       closing.txt in the index file is identical to the one in the
       working directory.

       In addition to being the staging area for new commits, the index
       file is also populated from the object database when checking out
       a branch, and is used to hold the trees involved in a merge
       operation. See gitcore-tutorial(7) and the relevant man pages for
       details.

WHAT NEXT?         top

       At this point you should know everything necessary to read the
       man pages for any of the git commands; one good place to start
       would be with the commands mentioned in giteveryday(7). You
       should be able to find any unknown jargon in gitglossary(7).

       The Git User’s Manual[1] provides a more comprehensive
       introduction to Git.

       gitcvs-migration(7) explains how to import a CVS repository into
       Git, and shows how to use Git in a CVS-like way.

       For some interesting examples of Git use, see the howtos[2].

       For Git developers, gitcore-tutorial(7) goes into detail on the
       lower-level Git mechanisms involved in, for example, creating a
       new commit.

SEE ALSO         top

       gittutorial(7), gitcvs-migration(7), gitcore-tutorial(7),
       gitglossary(7), git-help(1), giteveryday(7), The Git User’s
       Manual[1]

GIT         top

       Part of the git(1) suite

NOTES         top

        1. Git User’s Manual
           file:///usr/local/share/doc/git/user-manual.html

        2. howtos
           file:///usr/local/share/doc/git/howto-index.html

COLOPHON         top

       This page is part of the git (Git distributed version control
       system) project.  Information about the project can be found at
       ⟨http://git-scm.com/⟩.  If you have a bug report for this manual
       page, see ⟨http://git-scm.com/community⟩.  This page was obtained
       from the project's upstream Git repository
       ⟨https://github.com/git/git.git⟩ on 2021-04-01.  (At that time,
       the date of the most recent commit that was found in the
       repository was 2021-03-30.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

Git 2.31.1.163.ga65ce7         04/01/2021               GITTUTORIAL-2(7)

Pages that refer to this page: git(1)gitcore-tutorial(7)gitcvs-migration(7)gitglossary(7)gittutorial(7)