{
    "mode": "man",
    "parameter": "gittutorial-2",
    "section": "7",
    "url": "https://www.chedong.com/phpMan.php/man/gittutorial-2/7/json",
    "generated": "2026-06-03T01:22:19Z",
    "synopsis": "git *",
    "sections": {
        "NAME": {
            "content": "gittutorial-2 - A tutorial introduction to Git: part two\n",
            "subsections": []
        },
        "SYNOPSIS": {
            "content": "git *\n\n",
            "subsections": []
        },
        "DESCRIPTION": {
            "content": "You should work through gittutorial(7) before reading this tutorial.\n\nThe goal of this tutorial is to introduce two fundamental pieces of Git’s architecture—the\nobject database and the index file—and to provide the reader with everything necessary to\nunderstand the rest of the Git documentation.\n",
            "subsections": []
        },
        "THE GIT OBJECT DATABASE": {
            "content": "Let’s start a new project and create a small amount of history:\n\n$ mkdir test-project\n$ cd test-project\n$ git init\nInitialized empty Git repository in .git/\n$ echo 'hello world' > file.txt\n$ git add .\n$ git commit -a -m \"initial commit\"\n[master (root-commit) 54196cc] initial commit\n1 file changed, 1 insertion(+)\ncreate mode 100644 file.txt\n$ echo 'hello world!' >file.txt\n$ git commit -a -m \"add emphasis\"\n[master c4d59f3] add emphasis\n1 file changed, 1 insertion(+), 1 deletion(-)\n\n\nWhat are the 7 digits of hex that Git responded to the commit with?\n\nWe saw in part one of the tutorial that commits have names like this. It turns out that every\nobject in the Git history is stored under a 40-digit hex name. That name is the SHA-1 hash of\nthe object’s contents; among other things, this ensures that Git will never store the same\ndata twice (since identical data is given an identical SHA-1 name), and that the contents of\na Git object will never change (since that would change the object’s name as well). The 7\nchar hex strings here are simply the abbreviation of such 40 character long strings.\nAbbreviations can be used everywhere where the 40 character strings can be used, so long as\nthey are unambiguous.\n\nIt is expected that the content of the commit object you created while following the example\nabove generates a different SHA-1 hash than the one shown above because the commit object\nrecords the time when it was created and the name of the person performing the commit.\n\nWe can ask Git about this particular object with the cat-file command. Don’t copy the 40 hex\ndigits from this example but use those from your own version. Note that you can shorten it to\nonly a few characters to save yourself typing all 40 hex digits:\n\n$ git cat-file -t 54196cc2\ncommit\n$ git cat-file commit 54196cc2\ntree 92b8b694ffb1675e5975148e1121810081dbdffe\nauthor J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500\ncommitter J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500\n\ninitial commit\n\n\nA tree can refer to one or more \"blob\" objects, each corresponding to a file. In addition, a\ntree can also refer to other tree objects, thus creating a directory hierarchy. You can\nexamine the contents of any tree using ls-tree (remember that a long enough initial portion\nof the SHA-1 will also work):\n\n$ git ls-tree 92b8b694\n100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad    file.txt\n\n\nThus we see that this tree has one file in it. The SHA-1 hash is a reference to that file’s\ndata:\n\n$ git cat-file -t 3b18e512\nblob\n\n\nA \"blob\" is just file data, which we can also examine with cat-file:\n\n$ git cat-file blob 3b18e512\nhello world\n\n\nNote that this is the old file data; so the object that Git named in its response to the\ninitial tree was a tree with a snapshot of the directory state that was recorded by the first\ncommit.\n\nAll of these objects are stored under their SHA-1 names inside the Git directory:\n\n$ find .git/objects/\n.git/objects/\n.git/objects/pack\n.git/objects/info\n.git/objects/3b\n.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad\n.git/objects/92\n.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe\n.git/objects/54\n.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7\n.git/objects/a0\n.git/objects/a0/423896973644771497bdc03eb99d5281615b51\n.git/objects/d0\n.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59\n.git/objects/c4\n.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241\n\n\nand the contents of these files is just the compressed data plus a header identifying their\nlength and their type. The type is either a blob, a tree, a commit, or a tag.\n\nThe simplest commit to find is the HEAD commit, which we can find from .git/HEAD:\n\n$ cat .git/HEAD\nref: refs/heads/master\n\n\nAs you can see, this tells us which branch we’re currently on, and it tells us this by naming\na file under the .git directory, which itself contains a SHA-1 name referring to a commit\nobject, which we can examine with cat-file:\n\n$ cat .git/refs/heads/master\nc4d59f390b9cfd4318117afde11d601c1085f241\n$ git cat-file -t c4d59f39\ncommit\n$ git cat-file commit c4d59f39\ntree d0492b368b66bdabf2ac1fd8c92b39d3db916e59\nparent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7\nauthor J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500\ncommitter J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500\n\nadd emphasis\n\n\nThe \"tree\" object here refers to the new state of the tree:\n\n$ git ls-tree d0492b36\n100644 blob a0423896973644771497bdc03eb99d5281615b51    file.txt\n$ git cat-file blob a0423896\nhello world!\n\n\nand the \"parent\" object refers to the previous commit:\n\n$ git cat-file commit 54196cc2\ntree 92b8b694ffb1675e5975148e1121810081dbdffe\nauthor J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500\ncommitter J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500\n\ninitial commit\n\n\nThe tree object is the tree we examined first, and this commit is unusual in that it lacks\nany parent.\n\nMost commits have only one parent, but it is also common for a commit to have multiple\nparents. In that case the commit represents a merge, with the parent references pointing to\nthe heads of the merged branches.\n\nBesides blobs, trees, and commits, the only remaining type of object is a \"tag\", which we\nwon’t discuss here; refer to git-tag(1) for details.\n\nSo now we know how Git uses the object database to represent a project’s history:\n\n•   \"commit\" objects refer to \"tree\" objects representing the snapshot of a directory tree at\na particular point in the history, and refer to \"parent\" commits to show how they’re\nconnected into the project history.\n\n•   \"tree\" objects represent the state of a single directory, associating directory names to\n\"blob\" objects containing file data and \"tree\" objects containing subdirectory\ninformation.\n\n•   \"blob\" objects contain file data without any other structure.\n\n•   References to commit objects at the head of each branch are stored in files under\n.git/refs/heads/.\n\n•   The name of the current branch is stored in .git/HEAD.\n\nNote, by the way, that lots of commands take a tree as an argument. But as we can see above,\na tree can be referred to in many different ways—by the SHA-1 name for that tree, by the name\nof a commit that refers to the tree, by the name of a branch whose head refers to that tree,\netc.--and most such commands can accept any of these names.\n\nIn command synopses, the word \"tree-ish\" is sometimes used to designate such an argument.\n",
            "subsections": []
        },
        "THE INDEX FILE": {
            "content": "The primary tool we’ve been using to create commits is git-commit -a, which creates a commit\nincluding every change you’ve made to your working tree. But what if you want to commit\nchanges only to certain files? Or only certain changes to certain files?\n\nIf we look at the way commits are created under the cover, we’ll see that there are more\nflexible ways creating commits.\n\nContinuing with our test-project, let’s modify file.txt again:\n\n$ echo \"hello world, again\" >>file.txt\n\n\nbut this time instead of immediately making the commit, let’s take an intermediate step, and\nask for diffs along the way to keep track of what’s happening:\n\n$ git diff\n--- a/file.txt\n+++ b/file.txt\n@@ -1 +1,2 @@\nhello world!\n+hello world, again\n$ git add file.txt\n$ git diff\n\n\nThe last diff is empty, but no new commits have been made, and the head still doesn’t contain\nthe new line:\n\n$ git diff HEAD\ndiff --git a/file.txt b/file.txt\nindex a042389..513feba 100644\n--- a/file.txt\n+++ b/file.txt\n@@ -1 +1,2 @@\nhello world!\n+hello world, again\n\n\nSo git diff is comparing against something other than the head. The thing that it’s comparing\nagainst is actually the index file, which is stored in .git/index in a binary format, but\nwhose contents we can examine with ls-files:\n\n$ git ls-files --stage\n100644 513feba2e53ebbd2532419ded848ba19de88ba00 0       file.txt\n$ git cat-file -t 513feba2\nblob\n$ git cat-file blob 513feba2\nhello world!\nhello world, again\n\n\nSo what our git add did was store a new blob and then put a reference to it in the index\nfile. If we modify the file again, we’ll see that the new modifications are reflected in the\ngit diff output:\n\n$ echo 'again?' >>file.txt\n$ git diff\nindex 513feba..ba3da7b 100644\n--- a/file.txt\n+++ b/file.txt\n@@ -1,2 +1,3 @@\nhello world!\nhello world, again\n+again?\n\n\nWith the right arguments, git diff can also show us the difference between the working\ndirectory and the last commit, or between the index and the last commit:\n\n$ git diff HEAD\ndiff --git a/file.txt b/file.txt\nindex a042389..ba3da7b 100644\n--- a/file.txt\n+++ b/file.txt\n@@ -1 +1,3 @@\nhello world!\n+hello world, again\n+again?\n$ git diff --cached\ndiff --git a/file.txt b/file.txt\nindex a042389..513feba 100644\n--- a/file.txt\n+++ b/file.txt\n@@ -1 +1,2 @@\nhello world!\n+hello world, again\n\n\nAt any time, we can create a new commit using git commit (without the \"-a\" option), and\nverify that the state committed only includes the changes stored in the index file, not the\nadditional change that is still only in our working tree:\n\n$ git commit -m \"repeat\"\n$ git diff HEAD\ndiff --git a/file.txt b/file.txt\nindex 513feba..ba3da7b 100644\n--- a/file.txt\n+++ b/file.txt\n@@ -1,2 +1,3 @@\nhello world!\nhello world, again\n+again?\n\n\nSo by default git commit uses the index to create the commit, not the working tree; the \"-a\"\noption to commit tells it to first update the index with all changes in the working tree.\n\nFinally, it’s worth looking at the effect of git add on the index file:\n\n$ echo \"goodbye, world\" >closing.txt\n$ git add closing.txt\n\n\nThe effect of the git add was to add one entry to the index file:\n\n$ git ls-files --stage\n100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0       closing.txt\n100644 513feba2e53ebbd2532419ded848ba19de88ba00 0       file.txt\n\n\nAnd, as you can see with cat-file, this new entry refers to the current contents of the file:\n\n$ git cat-file blob 8b9743b2\ngoodbye, world\n\n\nThe \"status\" command is a useful way to get a quick summary of the situation:\n\n$ git status\nOn branch master\nChanges to be committed:\n(use \"git restore --staged <file>...\" to unstage)\n\nnew file:   closing.txt\n\nChanges not staged for commit:\n(use \"git add <file>...\" to update what will be committed)\n(use \"git restore <file>...\" to discard changes in working directory)\n\nmodified:   file.txt\n\n\nSince the current state of closing.txt is cached in the index file, it is listed as \"Changes\nto be committed\". Since file.txt has changes in the working directory that aren’t reflected\nin the index, it is marked \"changed but not updated\". At this point, running \"git commit\"\nwould create a commit that added closing.txt (with its new contents), but that didn’t modify\nfile.txt.\n\nAlso, note that a bare git diff shows the changes to file.txt, but not the addition of\nclosing.txt, because the version of closing.txt in the index file is identical to the one in\nthe working directory.\n\nIn addition to being the staging area for new commits, the index file is also populated from\nthe object database when checking out a branch, and is used to hold the trees involved in a\nmerge operation. See gitcore-tutorial(7) and the relevant man pages for details.\n",
            "subsections": [
                {
                    "name": "WHAT NEXT?",
                    "content": "At this point you should know everything necessary to read the man pages for any of the git\ncommands; one good place to start would be with the commands mentioned in giteveryday(7). You\nshould be able to find any unknown jargon in gitglossary(7).\n\nThe Git User’’s Manual[1] provides a more comprehensive introduction to Git.\n\ngitcvs-migration(7) explains how to import a CVS repository into Git, and shows how to use\nGit in a CVS-like way.\n\nFor some interesting examples of Git use, see the howtos[2].\n\nFor Git developers, gitcore-tutorial(7) goes into detail on the lower-level Git mechanisms\ninvolved in, for example, creating a new commit.\n"
                }
            ]
        },
        "SEE ALSO": {
            "content": "gittutorial(7), gitcvs-migration(7), gitcore-tutorial(7), gitglossary(7), git-help(1),\ngiteveryday(7), The Git User’’s Manual[1]\n",
            "subsections": []
        },
        "GIT": {
            "content": "Part of the git(1) suite\n",
            "subsections": []
        },
        "NOTES": {
            "content": "1. Git User’s Manual\nfile:///usr/share/doc/git/html/user-manual.html\n\n2. howtos\nfile:///usr/share/doc/git/html/howto-index.html\n\n\n\nGit 2.34.1                                   02/26/2026                             GITTUTORIAL-2(7)",
            "subsections": []
        }
    },
    "summary": "gittutorial-2 - A tutorial introduction to Git: part two",
    "flags": [],
    "examples": [],
    "see_also": [
        {
            "name": "gittutorial",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/gittutorial/7/json"
        },
        {
            "name": "gitcvs-migration",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/gitcvs-migration/7/json"
        },
        {
            "name": "gitcore-tutorial",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/gitcore-tutorial/7/json"
        },
        {
            "name": "gitglossary",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/gitglossary/7/json"
        },
        {
            "name": "git-help",
            "section": "1",
            "url": "https://www.chedong.com/phpMan.php/man/git-help/1/json"
        },
        {
            "name": "giteveryday",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/giteveryday/7/json"
        }
    ]
}