GITTUTORIAL-2(7)          (08/17/2021)           GITTUTORIAL-2(7)

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

     SYNOPSIS
          git *

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

          The goal of this tutorial is to introduce two fundamental
          pieces of Gitcqs 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
          Letcqs 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 objectcqs 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 objectcqs 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

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          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. Doncqt 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 filecqs 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

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          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 wecqre 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

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          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 woncqt discuss here; refer to
          git-tag(1) for details.

          So now we know how Git uses the object database to represent
          a projectcqs history:

          +o   "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 theycqre connected into the project history.

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

          +o   "blob" objects contain file data without any other
              structure.

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

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

          Note, by the way, that lots of commands take a tree as an

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          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
          The primary tool wecqve been using to create commits is
          git-commit -a, which creates a commit including every change
          youcqve 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,
          wecqll see that there are more flexible ways creating
          commits.

          Continuing with our test-project, letcqs modify file.txt
          again:

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

          but this time instead of immediately making the commit,
          letcqs take an intermediate step, and ask for diffs along the
          way to keep track of whatcqs 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 doesncqt 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

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          So git diff is comparing against something other than the
          head. The thing that itcqs 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, wecqll 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

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          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, itcqs 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)

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                      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 arencqt
          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 didncqt 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?
          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 m[blue]Git Usercqs Manualm[][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
          m[blue]howtosm[][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
          gittutorial(7), gitcvs-migration(7), gitcore-tutorial(7),
          gitglossary(7), git-help(1), giteveryday(7), m[blue]The Git
          Usercqs Manualm[][1]

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     GIT
          Part of the git(1) suite

     NOTES
           1. Git Usercqs Manual
              file:///usr/share/doc/git/html/user-manual.html

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

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