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As you can see, a commit shows who made the latest change, what they
did, and why.
-Every commit has a 40-hexdigit id, sometimes called the "object name"
-or the "SHA1 id", shown on the first line of the "git show" output.
-You can usually refer to a commit by a shorter name, such as a tag or a
-branch name, but this longer name can also be useful. Most
-importantly, it is a globally unique name for this commit: so if you
-tell somebody else the object name (for example in email), then you are
-guaranteed that name will refer to the same commit in their repository
-that it does in yours (assuming their repository has that commit at
-all).
+Every commit has a 40-hexdigit id, sometimes called the "object name" or the
+"SHA1 id", shown on the first line of the "git show" output. You can usually
+refer to a commit by a shorter name, such as a tag or a branch name, but this
+longer name can also be useful. Most importantly, it is a globally unique
+name for this commit: so if you tell somebody else the object name (for
+example in email), then you are guaranteed that name will refer to the same
+commit in their repository that it does in yours (assuming their repository
+has that commit at all). Since the object name is computed as a hash over the
+contents of the commit, you are guaranteed that the commit can never change
+without its name also changing.
+
+In fact, in <<git-internals>> we shall see that everything stored in git
+history, including file data and directory contents, is stored in an object
+with a name that is a hash of its contents.
Understanding history: commits, parents, and reachability
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
below. Commits are shown as "o", and the links between them with
lines drawn with - / and \. Time goes left to right:
+
+................................................
o--o--o <-- Branch A
/
o--o--o <-- master
\
o--o--o <-- Branch B
+................................................
If we need to talk about a particular commit, the character "o" may
be replaced with another letter or number.
$ cat .git/config
...
[remote "linux-nfs"]
- url = git://linux-nfs.org/~bfields/git.git
- fetch = +refs/heads/*:refs/remotes/linux-nfs-read/*
+ url = git://linux-nfs.org/pub/nfs-2.6.git
+ fetch = +refs/heads/*:refs/remotes/linux-nfs/*
...
-------------------------------------------------
branch--then you are warned; the output may look something like this:
-------------------------------------------------
-$ git pull . next
-Trying really trivial in-index merge...
-fatal: Merge requires file-level merging
-Nope.
-Merging HEAD with 77976da35a11db4580b80ae27e8d65caf5208086
-Merging:
-15e2162 world
-77976da goodbye
-found 1 common ancestor(s):
-d122ed4 initial
-Auto-merging file.txt
+$ git merge next
+ 100% (4/4) done
+Auto-merged file.txt
CONFLICT (content): Merge conflict in file.txt
Automatic merge failed; fix conflicts and then commit the result.
-------------------------------------------------
information you need to help resolve the merge.
Files with conflicts are marked specially in the index, so until you
-resolve the problem and update the index, git commit will fail:
+resolve the problem and update the index, gitlink:git-commit[1] will
+fail:
-------------------------------------------------
$ git commit
file.txt: needs merge
-------------------------------------------------
-Also, git status will list those files as "unmerged".
+Also, gitlink:git-status[1] will list those files as "unmerged", and the
+files with conflicts will have conflict markers added, like this:
+
+-------------------------------------------------
+<<<<<<< HEAD:file.txt
+Hello world
+=======
+Goodbye
+>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
+-------------------------------------------------
+
+All you need to do is edit the files to resolve the conflicts, and then
+
+-------------------------------------------------
+$ git add file.txt
+$ git commit
+-------------------------------------------------
+
+Note that the commit message will already be filled in for you with
+some information about the merge. Normally you can just use this
+default message unchanged, but you may add additional commentary of
+your own if desired.
+
+The above is all you need to know to resolve a simple merge. But git
+also provides more information to help resolve conflicts:
+
+Getting conflict-resolution help during a merge
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All of the changes that git was able to merge automatically are
already added to the index file, so gitlink:git-diff[1] shows only
-the conflicts. Also, it uses a somewhat unusual syntax:
+the conflicts. It uses an unusual syntax:
-------------------------------------------------
$ git diff
will be HEAD, the tip of the current branch; the other will be the
tip of the other branch, which is stored temporarily in MERGE_HEAD.
-The diff above shows the differences between the working-tree version
-of file.txt and two previous versions: one version from HEAD, and one
-from MERGE_HEAD. So instead of preceding each line by a single "+"
-or "-", it now uses two columns: the first column is used for
-differences between the first parent and the working directory copy,
-and the second for differences between the second parent and the
-working directory copy. Thus after resolving the conflict in the
-obvious way, the diff will look like:
+During the merge, the index holds three versions of each file. Each of
+these three "file stages" represents a different version of the file:
+
+-------------------------------------------------
+$ git show :1:file.txt # the file in a common ancestor of both branches
+$ git show :2:file.txt # the version from HEAD, but including any
+ # nonconflicting changes from MERGE_HEAD
+$ git show :3:file.txt # the version from MERGE_HEAD, but including any
+ # nonconflicting changes from HEAD.
+-------------------------------------------------
+
+Since the stage 2 and stage 3 versions have already been updated with
+nonconflicting changes, the only remaining differences between them are
+the important ones; thus gitlink:git-diff[1] can use the information in
+the index to show only those conflicts.
+
+The diff above shows the differences between the working-tree version of
+file.txt and the stage 2 and stage 3 versions. So instead of preceding
+each line by a single "+" or "-", it now uses two columns: the first
+column is used for differences between the first parent and the working
+directory copy, and the second for differences between the second parent
+and the working directory copy. (See the "COMBINED DIFF FORMAT" section
+of gitlink:git-diff-files[1] for a details of the format.)
+
+After resolving the conflict in the obvious way (but before updating the
+index), the diff will look like:
-------------------------------------------------
$ git diff
first parent, deleted "Goodbye" from the second parent, and added
"Goodbye world", which was previously absent from both.
-The gitlink:git-log[1] command also provides special help for merges:
+Some special diff options allow diffing the working directory against
+any of these stages:
+
+-------------------------------------------------
+$ git diff -1 file.txt # diff against stage 1
+$ git diff --base file.txt # same as the above
+$ git diff -2 file.txt # diff against stage 2
+$ git diff --ours file.txt # same as the above
+$ git diff -3 file.txt # diff against stage 3
+$ git diff --theirs file.txt # same as the above.
+-------------------------------------------------
+
+The gitlink:git-log[1] and gitk[1] commands also provide special help
+for merges:
-------------------------------------------------
$ git log --merge
+$ gitk --merge
-------------------------------------------------
-This will list all commits which exist only on HEAD or on MERGE_HEAD,
-and which touch an unmerged file.
+These will display all commits which exist only on HEAD or on
+MERGE_HEAD, and which touch an unmerged file.
-We can now add the resolved version to the index and commit:
+Each time you resolve the conflicts in a file and update the index:
-------------------------------------------------
$ git add file.txt
-$ git commit
-------------------------------------------------
-Note that the commit message will already be filled in for you with
-some information about the merge. Normally you can just use this
-default message unchanged, but you may add additional commentary of
-your own if desired.
+the different stages of that file will be "collapsed", after which
+git-diff will (by default) no longer show diffs for that file.
[[undoing-a-merge]]
undoing a merge
Or, if you've already commited the merge that you want to throw away,
-------------------------------------------------
-$ git reset --hard HEAD^
+$ git reset --hard ORIG_HEAD
-------------------------------------------------
However, this last command can be dangerous in some cases--never
conflicts manually, just as in the case of <<resolving-a-merge,
resolving a merge>>.
+[[fixing-a-mistake-by-editing-history]]
Fixing a mistake by editing history
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gitlink:git-show[1]:
-------------------------------------------------
-$ git show HEAD^ path/to/file
+$ git show HEAD^:path/to/file
-------------------------------------------------
which will display the given version of the file.
You have performed no merges into mywork, so it is just a simple linear
sequence of patches on top of "origin":
-
+................................................
o--o--o <-- origin
\
o--o--o <-- mywork
+................................................
Some more interesting work has been done in the upstream project, and
"origin" has advanced:
+................................................
o--o--O--o--o--o <-- origin
\
a--b--c <-- mywork
+................................................
At this point, you could use "pull" to merge your changes back in;
the result would create a new merge commit, like this:
-
+................................................
o--o--O--o--o--o <-- origin
\ \
a--b--c--m <-- mywork
+................................................
However, if you prefer to keep the history in mywork a simple series of
commits without any merges, you may instead choose to use
patches to the new mywork. The result will look like:
+................................................
o--o--O--o--o--o <-- origin
\
a'--b'--c' <-- mywork
+................................................
In the process, it may discover conflicts. In that case it will stop
and allow you to fix the conflicts; after fixing conflicts, use "git
$ git rebase --abort
-------------------------------------------------
+Modifying a single commit
+-------------------------
+
+We saw in <<fixing-a-mistake-by-editing-history>> that you can replace the
+most recent commit using
+
+-------------------------------------------------
+$ git commit --amend
+-------------------------------------------------
+
+which will replace the old commit by a new commit incorporating your
+changes, giving you a chance to edit the old commit message first.
+
+You can also use a combination of this and gitlink:git-rebase[1] to edit
+commits further back in your history. First, tag the problematic commit with
+
+-------------------------------------------------
+$ git tag bad mywork~5
+-------------------------------------------------
+
+(Either gitk or git-log may be useful for finding the commit.)
+
+Then check out a new branch at that commit, edit it, and rebase the rest of
+the series on top of it:
+
+-------------------------------------------------
+$ git checkout -b TMP bad
+$ # make changes here and update the index
+$ git commit --amend
+$ git rebase --onto TMP bad mywork
+-------------------------------------------------
+
+When you're done, you'll be left with mywork checked out, with the top patches
+on mywork reapplied on top of the modified commit you created in TMP. You can
+then clean up with
+
+-------------------------------------------------
+$ git branch -d TMP
+$ git tag -d bad
+-------------------------------------------------
+
+Note that the immutable nature of git history means that you haven't really
+"modified" existing commits; instead, you have replaced the old commits with
+new commits having new object names.
+
Reordering or selecting from a patch series
-------------------------------------------
with merging. Suppose somebody fetches your branch and merges it into
their branch, with a result something like this:
+................................................
o--o--O--o--o--o <-- origin
\ \
t--t--t--m <-- their branch:
+................................................
Then suppose you modify the last three commits:
+................................................
o--o--o <-- new head of origin
/
o--o--O--o--o--o <-- old head of origin
+................................................
If we examined all this history together in one repository, it will
look like:
+................................................
o--o--o <-- new head of origin
/
o--o--O--o--o--o <-- old head of origin
\ \
t--t--t--m <-- their branch:
+................................................
Git has no way of knowing that the new head is an updated version of
the old head; it treats this situation exactly the same as it would if
A fast forward looks something like this:
+................................................
o--o--o--o <-- old head of the branch
\
o--o--o <-- new head of the branch
+................................................
In some cases it is possible that the new head will *not* actually be
realized she made a serious mistake, and decided to backtrack,
resulting in a situation like:
+................................................
o--o--o--o--a--b <-- old head of the branch
\
o--o--o <-- new head of the branch
-
-
+................................................
In this case, "git fetch" will fail, and print out a warning.
options mentioned above.
+[[git-internals]]
Git internals
=============
Simplify beginning by suggesting disconnected head instead of
temporary branch creation?
-Explain how to refer to file stages in the "how to resolve a merge"
-section: diff -1, -2, -3, --ours, --theirs :1:/path notation. The
-"git ls-files --unmerged --stage" thing is sorta useful too,
-actually. And note gitk --merge.
-
Add more good examples. Entire sections of just cookbook examples
might be a good idea; maybe make an "advanced examples" section a
standard end-of-chapter section?