gittutorial-2(7) - A tutorial introduction to Git: part two



  • GITTUTORIAL-2(7)			   Git Manual				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 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
           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 <[email protected]> 1143414668 -0500
    	   committer J. Bruce Fields <[email protected]> 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 <[email protected]> 1143418702 -0500
    	   committer J. Bruce Fields <[email protected]> 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 <[email protected]> 1143414668 -0500
    	   committer J. Bruce Fields <[email protected]> 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
           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 reset HEAD <file>..." to unstage)
    
    		   new file:   closing.txt
    
    	   Changes not staged for commit:
    	     (use "git add <file>..." to update what will be committed)
    	     (use "git checkout -- <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?
           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
           gittutorial(7), gitcvs-migration(7), gitcore-tutorial(7), gitglossary(7), git-help(1),
           giteveryday(7), The Git User’s Manual[1]
    
    GIT
           Part of the git(1) suite
    
    NOTES
    	1. Git User’s Manual
    	   file:///usr/share/doc/git/html/user-manual.html
    
    	2. howtos
    	   file:///usr/share/doc/git/html/howto-index.html
    
    Git 2.17.1				   11/26/2018				GITTUTORIAL-2(7)
    

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