305 lines
12 KiB
Plaintext
305 lines
12 KiB
Plaintext
git for CVS users
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=================
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So you're a CVS user. That's ok, it's a treatable condition. The job of
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this document is to put you on the road to recovery, by helping you
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convert an existing cvs repository to git, and by showing you how to use a
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git repository in a cvs-like fashion.
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Some basic familiarity with git is required. This
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link:tutorial.html[tutorial introduction to git] should be sufficient.
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First, note some ways that git differs from CVS:
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* Commits are atomic and project-wide, not per-file as in CVS.
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* Offline work is supported: you can make multiple commits locally,
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then submit them when you're ready.
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* Branching is fast and easy.
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* Every working tree contains a repository with a full copy of the
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project history, and no repository is inherently more important than
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any other. However, you can emulate the CVS model by designating a
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single shared repository which people can synchronize with; see below
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for details.
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Importing a CVS archive
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-----------------------
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First, install version 2.1 or higher of cvsps from
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link:http://www.cobite.com/cvsps/[http://www.cobite.com/cvsps/] and make
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sure it is in your path. The magic command line is then
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-------------------------------------------
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$ git cvsimport -v -d <cvsroot> -C <destination> <module>
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-------------------------------------------
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This puts a git archive of the named CVS module in the directory
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<destination>, which will be created if necessary. The -v option makes
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the conversion script very chatty.
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The import checks out from CVS every revision of every file. Reportedly
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cvsimport can average some twenty revisions per second, so for a
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medium-sized project this should not take more than a couple of minutes.
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Larger projects or remote repositories may take longer.
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The main trunk is stored in the git branch named `origin`, and additional
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CVS branches are stored in git branches with the same names. The most
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recent version of the main trunk is also left checked out on the `master`
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branch, so you can start adding your own changes right away.
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The import is incremental, so if you call it again next month it will
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fetch any CVS updates that have been made in the meantime. For this to
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work, you must not modify the imported branches; instead, create new
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branches for your own changes, and merge in the imported branches as
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necessary.
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Development Models
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------------------
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CVS users are accustomed to giving a group of developers commit access to
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a common repository. In the next section we'll explain how to do this
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with git. However, the distributed nature of git allows other development
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models, and you may want to first consider whether one of them might be a
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better fit for your project.
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For example, you can choose a single person to maintain the project's
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primary public repository. Other developers then clone this repository
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and each work in their own clone. When they have a series of changes that
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they're happy with, they ask the maintainer to pull from the branch
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containing the changes. The maintainer reviews their changes and pulls
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them into the primary repository, which other developers pull from as
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necessary to stay coordinated. The Linux kernel and other projects use
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variants of this model.
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With a small group, developers may just pull changes from each other's
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repositories without the need for a central maintainer.
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Emulating the CVS Development Model
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-----------------------------------
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Start with an ordinary git working directory containing the project, and
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remove the checked-out files, keeping just the bare .git directory:
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------------------------------------------------
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$ mv project/.git /pub/repo.git
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$ rm -r project/
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------------------------------------------------
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Next, give every team member read/write access to this repository. One
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easy way to do this is to give all the team members ssh access to the
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machine where the repository is hosted. If you don't want to give them a
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full shell on the machine, there is a restricted shell which only allows
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users to do git pushes and pulls; see gitlink:git-shell[1].
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Put all the committers should in the same group, and make the repository
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writable by that group:
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------------------------------------------------
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$ chgrp -R $group repo.git
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$ find repo.git -mindepth 1 -type d |xargs chmod ug+rwx,g+s
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$ GIT_DIR=repo.git git repo-config core.sharedrepository true
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------------------------------------------------
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Make sure committers have a umask of at most 027, so that the directories
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they create are writable and searchable by other group members.
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Suppose this repository is now set up in /pub/repo.git on the host
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foo.com. Then as an individual commiter you can clone the shared
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repository:
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------------------------------------------------
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$ git clone foo.com:/pub/repo.git/ my-project
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$ cd my-project
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------------------------------------------------
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and hack away. The equivalent of `cvs update` is
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------------------------------------------------
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$ git pull origin
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------------------------------------------------
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which merges in any work that others might have done since the clone
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operation.
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[NOTE]
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================================
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The first `git clone` places the following in the
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`my-project/.git/remotes/origin` file, and that's why the previous step
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and the next step both work.
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------------
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URL: foo.com:/pub/project.git/ my-project
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Pull: master:origin
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------------
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================================
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You can update the shared repository with your changes using:
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------------------------------------------------
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$ git push origin master
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------------------------------------------------
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If someone else has updated the repository more recently, `git push`, like
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`cvs commit`, will complain, in which case you must pull any changes
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before attempting the push again.
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In the `git push` command above we specify the name of the remote branch
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to update (`master`). If we leave that out, `git push` tries to update
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any branches in the remote repository that have the same name as a branch
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in the local repository. So the last `push` can be done with either of:
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------------
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$ git push origin
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$ git push repo.shared.xz:/pub/scm/project.git/
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------------
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as long as the shared repository does not have any branches
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other than `master`.
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[NOTE]
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============
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Because of this behaviour, if the shared repository and the developer's
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repository both have branches named `origin`, then a push like the above
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attempts to update the `origin` branch in the shared repository from the
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developer's `origin` branch. The results may be unexpected, so it's
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usually best to remove any branch named `origin` from the shared
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repository.
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============
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Advanced Shared Repository Management
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-------------------------------------
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Git allows you to specify scripts called "hooks" to be run at certain
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points. You can use these, for example, to send all commits to the shared
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repository to a mailing list. See link:hooks.txt[Hooks used by git].
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You can enforce finer grained permissions using update hooks. See
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link:howto/update-hook-example.txt[Controlling access to branches using
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update hooks].
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CVS annotate
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------------
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So, something has gone wrong, and you don't know whom to blame, and
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you're an ex-CVS user and used to do "cvs annotate" to see who caused
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the breakage. You're looking for the "git annotate", and it's just
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claiming not to find such a script. You're annoyed.
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Yes, that's right. Core git doesn't do "annotate", although it's
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technically possible, and there are at least two specialized scripts out
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there that can be used to get equivalent information (see the git
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mailing list archives for details).
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git has a couple of alternatives, though, that you may find sufficient
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or even superior depending on your use. One is called "git-whatchanged"
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(for obvious reasons) and the other one is called "pickaxe" ("a tool for
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the software archaeologist").
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The "git-whatchanged" script is a truly trivial script that can give you
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a good overview of what has changed in a file or a directory (or an
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arbitrary list of files or directories). The "pickaxe" support is an
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additional layer that can be used to further specify exactly what you're
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looking for, if you already know the specific area that changed.
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Let's step back a bit and think about the reason why you would
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want to do "cvs annotate a-file.c" to begin with.
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You would use "cvs annotate" on a file when you have trouble
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with a function (or even a single "if" statement in a function)
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that happens to be defined in the file, which does not do what
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you want it to do. And you would want to find out why it was
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written that way, because you are about to modify it to suit
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your needs, and at the same time you do not want to break its
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current callers. For that, you are trying to find out why the
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original author did things that way in the original context.
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Many times, it may be enough to see the commit log messages of
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commits that touch the file in question, possibly along with the
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patches themselves, like this:
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$ git-whatchanged -p a-file.c
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This will show log messages and patches for each commit that
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touches a-file.
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This, however, may not be very useful when this file has many
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modifications that are not related to the piece of code you are
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interested in. You would see many log messages and patches that
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do not have anything to do with the piece of code you are
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interested in. As an example, assuming that you have this piece
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of code that you are interested in in the HEAD version:
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if (frotz) {
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nitfol();
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}
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you would use git-rev-list and git-diff-tree like this:
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$ git-rev-list HEAD |
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git-diff-tree --stdin -v -p -S'if (frotz) {
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nitfol();
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}'
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We have already talked about the "\--stdin" form of git-diff-tree
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command that reads the list of commits and compares each commit
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with its parents (otherwise you should go back and read the tutorial).
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The git-whatchanged command internally runs
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the equivalent of the above command, and can be used like this:
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$ git-whatchanged -p -S'if (frotz) {
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nitfol();
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}'
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When the -S option is used, git-diff-tree command outputs
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differences between two commits only if one tree has the
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specified string in a file and the corresponding file in the
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other tree does not. The above example looks for a commit that
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has the "if" statement in it in a file, but its parent commit
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does not have it in the same shape in the corresponding file (or
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the other way around, where the parent has it and the commit
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does not), and the differences between them are shown, along
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with the commit message (thanks to the -v flag). It does not
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show anything for commits that do not touch this "if" statement.
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Also, in the original context, the same statement might have
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appeared at first in a different file and later the file was
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renamed to "a-file.c". CVS annotate would not help you to go
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back across such a rename, but git would still help you in such
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a situation. For that, you can give the -C flag to
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git-diff-tree, like this:
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$ git-whatchanged -p -C -S'if (frotz) {
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nitfol();
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}'
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When the -C flag is used, file renames and copies are followed.
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So if the "if" statement in question happens to be in "a-file.c"
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in the current HEAD commit, even if the file was originally
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called "o-file.c" and then renamed in an earlier commit, or if
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the file was created by copying an existing "o-file.c" in an
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earlier commit, you will not lose track. If the "if" statement
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did not change across such a rename or copy, then the commit that
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does rename or copy would not show in the output, and if the
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"if" statement was modified while the file was still called
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"o-file.c", it would find the commit that changed the statement
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when it was in "o-file.c".
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NOTE: The current version of "git-diff-tree -C" is not eager
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enough to find copies, and it will miss the fact that a-file.c
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was created by copying o-file.c unless o-file.c was somehow
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changed in the same commit.
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You can use the --pickaxe-all flag in addition to the -S flag.
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This causes the differences from all the files contained in
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those two commits, not just the differences between the files
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that contain this changed "if" statement:
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$ git-whatchanged -p -C -S'if (frotz) {
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nitfol();
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}' --pickaxe-all
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NOTE: This option is called "--pickaxe-all" because -S
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option is internally called "pickaxe", a tool for software
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archaeologists.
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