{
    "mode": "info",
    "parameter": "gittutorial-2",
    "section": "",
    "url": "https://www.chedong.com/phpMan.php/info/gittutorial-2/json",
    "generated": "2026-07-14T09:24:48Z",
    "synopsis": "git *",
    "sections": {
        "NAME": {
            "content": "gittutorial-2 - A tutorial introduction to Git: part two\n",
            "subsections": []
        },
        "SYNOPSIS": {
            "content": "git *\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\nGit's architecture--the object database and the index file--and to\nprovide the reader with everything necessary to understand the rest of\nthe 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\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.\nIt turns out that every object in the Git history is stored under a\n40-digit hex name. That name is the SHA-1 hash of the object's\ncontents; among other things, this ensures that Git will never store\nthe same data twice (since identical data is given an identical SHA-1\nname), and that the contents of a Git object will never change (since\nthat would change the object's name as well). The 7 char hex strings\nhere are simply the abbreviation of such 40 character long strings.\nAbbreviations can be used everywhere where the 40 character strings can\nbe used, so long as they are unambiguous.\n\nIt is expected that the content of the commit object you created while\nfollowing the example above generates a different SHA-1 hash than the\none shown above because the commit object records the time when it was\ncreated and the name of the person performing the commit.\n\nWe can ask Git about this particular object with the cat-file command.\nDon't copy the 40 hex digits from this example but use those from your\nown version. Note that you can shorten it to only a few characters to\nsave 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\nA tree can refer to one or more \"blob\" objects, each corresponding to a\nfile. In addition, a tree can also refer to other tree objects, thus\ncreating a directory hierarchy. You can examine the contents of any\ntree using ls-tree (remember that a long enough initial portion of the\nSHA-1 will also work):\n\n$ git ls-tree 92b8b694\n100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad    file.txt\n\nThus we see that this tree has one file in it. The SHA-1 hash is a\nreference to that file's data:\n\n$ git cat-file -t 3b18e512\nblob\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\nNote that this is the old file data; so the object that Git named in\nits response to the initial tree was a tree with a snapshot of the\ndirectory state that was recorded by the first commit.\n\nAll of these objects are stored under their SHA-1 names inside the Git\ndirectory:\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\nand the contents of these files is just the compressed data plus a\nheader identifying their length and their type. The type is either a\nblob, a tree, a commit, or a tag.\n\nThe simplest commit to find is the HEAD commit, which we can find from\n.git/HEAD:\n\n$ cat .git/HEAD\nref: refs/heads/master\n\nAs you can see, this tells us which branch we're currently on, and it\ntells us this by naming a file under the .git directory, which itself\ncontains a SHA-1 name referring to a commit object, which we can\nexamine 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\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\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\nThe tree object is the tree we examined first, and this commit is\nunusual in that it lacks any parent.\n\nMost commits have only one parent, but it is also common for a commit\nto have multiple parents. In that case the commit represents a merge,\nwith the parent references pointing to the heads of the merged\nbranches.\n\nBesides blobs, trees, and commits, the only remaining type of object is\na \"tag\", which we won't discuss here; refer to git-tag(1) for details.\n\nSo now we know how Git uses the object database to represent a\nproject's history:\n\no   \"commit\" objects refer to \"tree\" objects representing the snapshot\nof a directory tree at a particular point in the history, and refer\nto \"parent\" commits to show how they're connected into the project\nhistory.\n\no   \"tree\" objects represent the state of a single directory,\nassociating directory names to \"blob\" objects containing file data\nand \"tree\" objects containing subdirectory information.\n\no   \"blob\" objects contain file data without any other structure.\n\no   References to commit objects at the head of each branch are stored\nin files under .git/refs/heads/.\n\no   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\nas we can see above, a tree can be referred to in many different ways--\nby the SHA-1 name for that tree, by the name of a commit that refers to\nthe 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\nsuch an argument.\n",
            "subsections": []
        },
        "THE INDEX FILE": {
            "content": "The primary tool we've been using to create commits is git-commit -a,\nwhich creates a commit including every change you've made to your\nworking tree. But what if you want to commit changes only to certain\nfiles? Or only certain changes to certain files?\n\nIf we look at the way commits are created under the cover, we'll see\nthat there are more flexible ways creating commits.\n\nContinuing with our test-project, let's modify file.txt again:\n\n$ echo \"hello world, again\" >>file.txt\n\nbut this time instead of immediately making the commit, let's take an\nintermediate step, and ask for diffs along the way to keep track of\nwhat'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\nThe last diff is empty, but no new commits have been made, and the head\nstill doesn't contain the 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\nSo git diff is comparing against something other than the head. The\nthing that it's comparing against is actually the index file, which is\nstored in .git/index in a binary format, but whose contents we can\nexamine 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\nSo what our git add did was store a new blob and then put a reference\nto it in the index file. If we modify the file again, we'll see that\nthe new modifications are reflected in the git 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\nWith the right arguments, git diff can also show us the difference\nbetween the working directory and the last commit, or between the index\nand 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\nAt any time, we can create a new commit using git commit (without the\n\"-a\" option), and verify that the state committed only includes the\nchanges stored in the index file, not the additional change that is\nstill 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\nSo by default git commit uses the index to create the commit, not the\nworking tree; the \"-a\" option to commit tells it to first update the\nindex 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\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\nAnd, as you can see with cat-file, this new entry refers to the current\ncontents of the file:\n\n$ git cat-file blob 8b9743b2\ngoodbye, world\n\nThe \"status\" command is a useful way to get a quick summary of the\nsituation:\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\nSince the current state of closing.txt is cached in the index file, it\nis listed as \"Changes to be committed\". Since file.txt has changes in\nthe working directory that aren't reflected in the index, it is marked\n\"changed but not updated\". At this point, running \"git commit\" would\ncreate a commit that added closing.txt (with its new contents), but\nthat didn't modify file.txt.\n\nAlso, note that a bare git diff shows the changes to file.txt, but not\nthe addition of closing.txt, because the version of closing.txt in the\nindex file is identical to the one in the working directory.\n\nIn addition to being the staging area for new commits, the index file\nis also populated from the object database when checking out a branch,\nand is used to hold the trees involved in a merge operation. See\ngitcore-tutorial(7) and the relevant man pages for details.\n\nWHAT NEXT?\nAt this point you should know everything necessary to read the man\npages for any of the git commands; one good place to start would be\nwith the commands mentioned in giteveryday(7). You should be able to\nfind any unknown jargon in gitglossary(7).\n\nThe Git User's Manual[1] provides a more comprehensive introduction to\nGit.\n\ngitcvs-migration(7) explains how to import a CVS repository into Git,\nand shows how to use Git 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\nlower-level Git mechanisms involved in, for example, creating a new\ncommit.\n",
            "subsections": []
        },
        "SEE ALSO": {
            "content": "gittutorial(7), gitcvs-migration(7), gitcore-tutorial(7),\ngitglossary(7), git-help(1), giteveryday(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\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"
        }
    ]
}