that various users push changes to; see gitlink:git-push[1] and
link:cvs-migration.html[git for CVS users].
-Keeping track of history
-------------------------
+Exploring history
+-----------------
-Git history is represented as a series of interrelated commits. The
-most recent commit in the currently checked-out branch can always be
-referred to as HEAD, and the "parent" of any commit can always be
-referred to by appending a caret, "^", to the end of the name of the
-commit. So, for example,
+Git history is represented as a series of interrelated commits. We
+have already seen that the git log command can list those commits.
+Note that first line of each git log entry also gives a name for the
+commit:
-------------------------------------
-git diff HEAD^ HEAD
+$ git log
+commit c82a22c39cbc32576f64f5c6b3f24b99ea8149c7
+Author: Junio C Hamano <junkio@cox.net>
+Date: Tue May 16 17:18:22 2006 -0700
+
+ merge-base: Clarify the comments on post processing.
-------------------------------------
-shows the difference between the most-recently checked-in state of
-the tree and the previous state, and
+We can give this name to git show to see the details about this
+commit.
-------------------------------------
-git diff HEAD^^ HEAD^
+$ git show c82a22c39cbc32576f64f5c6b3f24b99ea8149c7
-------------------------------------
-shows the difference between that previous state and the state two
-commits ago. Also, HEAD~5 can be used as a shorthand for HEAD{caret}{caret}{caret}{caret}{caret},
-and more generally HEAD~n can refer to the nth previous commit.
-Commits representing merges have more than one parent, and you can
-specify which parent to follow in that case; see
-gitlink:git-rev-parse[1].
+But there other ways to refer to commits. You can use any initial
+part of the name that is long enough to uniquely identify the commit:
-The name of a branch can also be used to refer to the most recent
-commit on that branch; so you can also say things like
+-------------------------------------
+$ git show c82a22c39c # the first few characters of the name are
+ # usually enough
+$ git show HEAD # the tip of the current branch
+$ git show experimental # the tip of the "experimental" branch
+-------------------------------------
+
+Every commit has at least one "parent" commit, which points to the
+previous state of the project:
-------------------------------------
-git diff HEAD experimental
+$ git show HEAD^ # to see the parent of HEAD
+$ git show HEAD^^ # to see the grandparent of HEAD
+$ git show HEAD~4 # to see the great-great grandparent of HEAD
-------------------------------------
-to see the difference between the most-recently committed tree in
-the current branch and the most-recently committed tree in the
-experimental branch.
+Note that merge commits may have more than one parent:
+
+-------------------------------------
+$ git show HEAD^1 # show the first parent of HEAD (same as HEAD^)
+$ git show HEAD^2 # show the second parent of HEAD
+-------------------------------------
-But you may find it more useful to see the list of commits made in
-the experimental branch but not in the current branch, and
+You can also give commits names of your own; after running
-------------------------------------
-git log HEAD..experimental
+$ git-tag v2.5 1b2e1d63ff
-------------------------------------
-will do that, just as
+you can refer to 1b2e1d63ff by the name "v2.5". If you intend to
+share this name with other people (for example, to identify a release
+version), you should create a "tag" object, and perhaps sign it; see
+gitlink:git-tag[1] for details.
+
+Any git command that needs to know a commit can take any of these
+names. For example:
-------------------------------------
-git log experimental..HEAD
+$ git diff v2.5 HEAD # compare the current HEAD to v2.5
+$ git branch stable v2.5 # start a new branch named "stable" based
+ # at v2.5
+$ git reset --hard HEAD^ # reset your current branch and working
+ # directory its state at HEAD^
-------------------------------------
-will show the list of commits made on the HEAD but not included in
-experimental.
+Be careful with that last command: in addition to losing any changes
+in the working directory, it will also remove all later commits from
+this branch. If this branch is the only branch containing those
+commits, they will be lost. (Also, don't use "git reset" on a
+publicly-visible branch that other developers pull from, as git will
+be confused by history that disappears in this way.)
-You can also give commits convenient names of your own: after running
+The git grep command can search for strings in any version of your
+project, so
-------------------------------------
-$ git-tag v2.5 HEAD^^
+$ git grep "hello" v2.5
-------------------------------------
-you can refer to HEAD^^ by the name "v2.5". If you intend to share
-this name with other people (for example, to identify a release
-version), you should create a "tag" object, and perhaps sign it; see
-gitlink:git-tag[1] for details.
+searches for all occurences of "hello" in v2.5.
-You can revisit the old state of a tree, and make further
-modifications if you wish, using git branch: the command
+If you leave out the commit name, git grep will search any of the
+files it manages in your current directory. So
-------------------------------------
-$ git branch stable-release v2.5
+$ git grep "hello"
-------------------------------------
-will create a new branch named "stable-release" starting from the
-commit which you tagged with the name v2.5.
+is a quick way to search just the files that are tracked by git.
-You can reset the state of any branch to an earlier commit at any
-time with
+Many git commands also take sets of commits, which can be specified
+in a number of ways. Here are some examples with git log:
-------------------------------------
-$ git reset --hard v2.5
+$ git log v2.5..v2.6 # commits between v2.5 and v2.6
+$ git log v2.5.. # commits since v2.5
+$ git log --since="2 weeks ago" # commits from the last 2 weeks
+$ git log v2.5.. Makefile # commits since v2.5 which modify
+ # Makefile
-------------------------------------
-This will remove all later commits from this branch and reset the
-working tree to the state it had when the given commit was made. If
-this branch is the only branch containing the later commits, those
-later changes will be lost. Don't use "git reset" on a
-publicly-visible branch that other developers pull from, as git will
-be confused by history that disappears in this way.
+You can also give git log a "range" of commits where the first is not
+necessarily an ancestor of the second; for example, if the tips of
+the branches "stable-release" and "master" diverged from a common
+commit some time ago, then
+
+-------------------------------------
+$ git log stable..experimental
+-------------------------------------
+
+will list commits made in the experimental branch but not in the
+stable branch, while
+
+-------------------------------------
+$ git log experimental..stable
+-------------------------------------
+
+will show the list of commits made on the stable branch but not
+the experimental branch.
+
+The "git log" command has a weakness: it must present commits in a
+list. When the history has lines of development that diverged and
+then merged back together, the order in which "git log" presents
+those commits is meaningless.
+
+Most projects with multiple contributors (such as the linux kernel,
+or git itself) have frequent merges, and gitk does a better job of
+visualizing their history. For example,
+
+-------------------------------------
+$ gitk --since="2 weeks ago" drivers/
+-------------------------------------
+
+allows you to browse any commits from the last 2 weeks of commits
+that modified files under the "drivers" directory.
+
+Finally, most commands that take filenames will optionally allow you
+to precede any filename by a commit, to specify a particular version
+fo the file:
+
+-------------------------------------
+$ git diff v2.5:Makefile HEAD:Makefile.in
+-------------------------------------
Next Steps
----------
Some good commands to explore next:
- * gitlink:git-diff[1]: This flexible command does much more than
- we've seen in the few examples above.
-
* gitlink:git-format-patch[1], gitlink:git-am[1]: These convert
series of git commits into emailed patches, and vice versa,
useful for projects such as the linux kernel which rely heavily
projects / git.git / commitdiff