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404 lines
13 KiB
404 lines
13 KiB
A tutorial introduction to git |
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This tutorial explains how to import a new project into git, make |
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changes to it, and share changes with other developers. |
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First, note that you can get documentation for a command such as "git |
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diff" with: |
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$ man git-diff |
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Importing a new project |
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----------------------- |
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Assume you have a tarball project.tar.gz with your initial work. You |
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can place it under git revision control as follows. |
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$ tar xzf project.tar.gz |
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$ cd project |
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$ git init-db |
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Git will reply |
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defaulting to local storage area |
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You've now initialized the working directory--you may notice a new |
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directory created, named ".git". Tell git that you want it to track |
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every file under the current directory with |
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$ git add . |
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Finally, |
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$ git commit -a |
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will prompt you for a commit message, then record the current state |
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of all the files to the repository. |
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Try modifying some files, then run |
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$ git diff |
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to review your changes. When you're done, |
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$ git commit -a |
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will again prompt your for a message describing the change, and then |
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record the new versions of the modified files. |
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A note on commit messages: Though not required, it's a good idea to |
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begin the commit message with a single short (less than 50 character) |
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line summarizing the change, followed by a blank line and then a more |
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thorough description. Tools that turn commits into email, for |
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example, use the first line on the Subject line and the rest of the |
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commit in the body. |
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To add a new file, first create the file, then |
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$ git add path/to/new/file |
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then commit as usual. No special command is required when removing a |
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file; just remove it, then commit. |
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At any point you can view the history of your changes using |
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$ git whatchanged |
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If you also want to see complete diffs at each step, use |
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$ git whatchanged -p |
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Managing branches |
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----------------- |
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A single git repository can maintain multiple branches of |
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development. To create a new branch named "experimental", use |
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$ git branch experimental |
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If you now run |
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$ git branch |
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you'll get a list of all existing branches: |
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experimental |
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* master |
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The "experimental" branch is the one you just created, and the |
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"master" branch is a default branch that was created for you |
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automatically. The asterisk marks the branch you are currently on; |
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type |
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$ git checkout experimental |
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to switch to the experimental branch. Now edit a file, commit the |
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change, and switch back to the master branch: |
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(edit file) |
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$ git commit -a |
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$ git checkout master |
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Check that the change you made is no longer visible, since it was |
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made on the experimental branch and you're back on the master branch. |
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You can make a different change on the master branch: |
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(edit file) |
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$ git commit -a |
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at this point the two branches have diverged, with different changes |
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made in each. To merge the changes made in the two branches, run |
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$ git pull . experimental |
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If the changes don't conflict, you're done. If there are conflicts, |
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markers will be left in the problematic files showing the conflict; |
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$ git diff |
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will show this. Once you've edited the files to resolve the |
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conflicts, |
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$ git commit -a |
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will commit the result of the merge. Finally, |
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$ gitk |
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will show a nice graphical representation of the resulting history. |
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If you develop on a branch crazy-idea, then regret it, you can always |
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delete the branch with |
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$ git branch -D crazy-idea |
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Branches are cheap and easy, so this is a good way to try something |
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out. |
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Using git for collaboration |
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Suppose that Alice has started a new project with a git repository in |
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/home/alice/project, and that Bob, who has a home directory on the |
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same machine, wants to contribute. |
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Bob begins with: |
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$ git clone /home/alice/project myrepo |
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This creates a new directory "myrepo" containing a clone of Alice's |
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repository. The clone is on an equal footing with the original |
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project, posessing its own copy of the original project's history. |
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Bob then makes some changes and commits them: |
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(edit files) |
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$ git commit -a |
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(repeat as necessary) |
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When he's ready, he tells Alice to pull changes from the repository |
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at /home/bob/myrepo. She does this with: |
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$ cd /home/alice/project |
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$ git pull /home/bob/myrepo |
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This actually pulls changes from the branch in Bob's repository named |
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"master". Alice could request a different branch by adding the name |
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of the branch to the end of the git pull command line. |
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This merges Bob's changes into her repository; "git whatchanged" will |
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now show the new commits. If Alice has made her own changes in the |
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meantime, then Bob's changes will be merged in, and she will need to |
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manually fix any conflicts. |
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A more cautious Alice might wish to examine Bob's changes before |
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pulling them. She can do this by creating a temporary branch just |
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for the purpose of studying Bob's changes: |
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$ git fetch /home/bob/myrepo master:bob-incoming |
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which fetches the changes from Bob's master branch into a new branch |
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named bob-incoming. (Unlike git pull, git fetch just fetches a copy |
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of Bob's line of development without doing any merging). Then |
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$ git whatchanged -p master..bob-incoming |
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shows a list of all the changes that Bob made since he branched from |
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Alice's master branch. |
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After examing those changes, and possibly fixing things, Alice can |
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pull the changes into her master branch: |
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$ git checkout master |
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$ git pull . bob-incoming |
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The last command is a pull from the "bob-incoming" branch in Alice's |
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own repository. |
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Later, Bob can update his repo with Alice's latest changes using |
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$ git pull |
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Note that he doesn't need to give the path to Alice's repository; |
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when Bob cloned Alice's repository, git stored the location of her |
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repository in the file .git/remotes/origin, and that location is used |
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as the default for pulls. |
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Bob may also notice a branch in his repository that he didn't create: |
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$ git branch |
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* master |
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origin |
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The "origin" branch, which was created automatically by "git clone", |
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is a pristine copy of Alice's master branch; Bob should never commit |
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to it. |
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If Bob later decides to work from a different host, he can still |
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perform clones and pulls using the ssh protocol: |
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$ git clone alice.org:/home/alice/project myrepo |
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Alternatively, git has a native protocol, or can use rsync or http; |
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see gitlink:git-pull[1] for details. |
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Git can also be used in a CVS-like mode, with a central repository |
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that various users push changes to; see gitlink:git-push[1] and |
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link:cvs-migration.html[git for CVS users]. |
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Keeping track of history |
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Git history is represented as a series of interrelated commits. The |
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most recent commit in the currently checked-out branch can always be |
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referred to as HEAD, and the "parent" of any commit can always be |
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referred to by appending a caret, "^", to the end of the name of the |
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commit. So, for example, |
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git diff HEAD^ HEAD |
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shows the difference between the most-recently checked-in state of |
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the tree and the previous state, and |
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git diff HEAD^^ HEAD^ |
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shows the difference between that previous state and the state two |
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commits ago. Also, HEAD~5 can be used as a shorthand for HEAD^^^^^, |
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and more generally HEAD~n can refer to the nth previous commit. |
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Commits representing merges have more than one parent, and you can |
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specify which parent to follow in that case; see |
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gitlink:git-rev-parse[1]. |
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The name of a branch can also be used to refer to the most recent |
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commit on that branch; so you can also say things like |
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git diff HEAD experimental |
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to see the difference between the most-recently committed tree in |
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the current branch and the most-recently committed tree in the |
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experimental branch. |
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But you may find it more useful to see the list of commits made in |
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the experimental branch but not in the current branch, and |
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git whatchanged HEAD..experimental |
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will do that, just as |
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git whatchanged experimental..HEAD |
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will show the list of commits made on the HEAD but not included in |
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experimental. |
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You can also give commits convenient names of your own: after running |
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$ git-tag v2.5 HEAD^^ |
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you can refer to HEAD^^ by the name "v2.5". If you intend to share |
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this name with other people (for example, to identify a release |
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version), you should create a "tag" object, and perhaps sign it; see |
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gitlink:git-tag[1] for details. |
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You can revisit the old state of a tree, and make further |
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modifications if you wish, using git branch: the command |
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$ git branch stable-release v2.5 |
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will create a new branch named "stable-release" starting from the |
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commit which you tagged with the name v2.5. |
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You can reset the state of any branch to an earlier commit at any |
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time with |
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$ git reset --hard v2.5 |
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This will remove all later commits from this branch and reset the |
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working tree to the state it had when the given commit was made. If |
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this branch is the only branch containing the later commits, those |
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later changes will be lost. Don't use "git reset" on a |
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publicly-visible branch that other developers pull from, as git will |
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be confused by history that disappears in this way. |
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Next Steps |
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---------- |
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Some good commands to explore next: |
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* gitlink:git-diff[1]: This flexible command does much more than |
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we've seen in the few examples above. |
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* gitlink:git-format-patch[1], gitlink:git-am[1]: These convert |
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series of git commits into emailed patches, and vice versa, |
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useful for projects such as the linux kernel which rely heavily |
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on emailed patches. |
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* gitlink:git-bisect[1]: When there is a regression in your |
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project, one way to track down the bug is by searching through |
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the history to find the exact commit that's to blame. Git bisect |
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can help you perform a binary search for that commit. It is |
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smart enough to perform a close-to-optimal search even in the |
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case of complex non-linear history with lots of merged branches. |
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Other good starting points include link:everyday.html[Everday GIT |
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with 20 Commands Or So] and link:cvs-migration.html[git for CVS |
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users]. Also, link:core-tutorial.html[A short git tutorial] gives an |
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introduction to lower-level git commands for advanced users and |
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developers.
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