Git For SBCL Hackers ==================== Git is structured as a ton of programs called git-foo, accessible either directly, or as "git foo". Commands "git help" and "git help foo" provide the documentation -- which is there also as the manual pages. Make sure you have Git 1.5.something. When running on tty the commands pipe their output through less automatically, so you don't need to mentally add "| less" anywhere. Let's get started. First off, we need a gitified SBCL repository to clone. At the time of writing there are two Git mirrors for the CVS history: git://sbcl.boinkor.net/sbcl.git and git://repo.or.cz/sbcl.git but the latter is actually a mirror of the first one, so it doesn't matter which one you use, really. The command git clone git://sbcl.boinkor.net/sbcl.git sbcl-git clones the SBCL Git mirror into the directory sbcl-git (there's a naming convention in play here, but ignore that for now.) The clone contains full history, and is an independent repository on its own right. Doing the clone takes as long as downloading 25Mb takes on your line, but after that things are very fast. To update your clone from the original source at a later date, use git-pull. Go ahead and try out now, to see that it works, and see how snazzy a no-op pull is. git pull The directory .git inside the clone contains Git's view of the repository -- no other Git files or directories are in the tree. Just for kicks: cd sbcl-git rm -rf * # leaves .git alone time git reset --hard # this takes < 2 seconds, restores everything So, two commands so far: git-clone One-time operation to clone a repository. git-reset Used to restore state: using the --hard flag resets the entire tree to the state of the last commit but doesn't delete new files which may be in the tree. (It doesn't actually "restore state", or rather does it as a side, effect, but you can learn the details on your own.) Let's hack a bit. git branch hack-a-bit # creates a new branch called "hack-a-bit" git branch # shows all branches and tells which one is active git checkout hack-a-bit # switches to the "hack-a-bit" branch git branch # shows that we're now on "hack-a-bit" Once you get your bearings you can do the create-branch-and-checkout in one step, which you will be doing a lot, since branches are really nice in Git! The magic is: "git checkout -b hack-a-bit-more" -- but leave that for another time. Let's do something. Edit BUGS, and maybe give SBCL some love. git diff # shows you the changes in your tree git status # shows you the state of files in the tree git-status will tell you that some files have been modified, but it has "no changes added to commit". If you tried git-commit right now, nothing would happen. What's going on? Git has a notion of a separate "staging area", from which commits are made. You can add content to the it by using git-add: git add BUGS git status git diff Now git-status shows changes as part of a pending commit, but git-diff is silent! By default git-diff shows the differences between the working tree and the staging area (which starts out identical to HEAD after a checkout). Edit BUGS again. Now you have three versions of it (ignoring all the historical versions for a second) to compare: git diff # between tree and staging area git diff HEAD # between tree and last commit git diff --cached # between staging area and last commit If we were to do a git-commit now, it would commit the version in the staging area, not the one in the tree. We like our latest version, so do git add BUGS again. Now the latest version is in the staging area, and version that used to be there is gone. You don't need to worry about the staging area. Either you will find out that it's pretty neat (consider it a temporary commit), or you can mostly ignore it. Do git commit now, and we'll move on in a second. Just to call spade a spade, the staging area is really called "index". Now, our changes are in the repository, and git-status should show a clean tree. (By the way, can you imagine how long all the diffs and statuses you've done during this tutorial would have taken with CVS?) To get the differences between your current branch, and the master branch, do git diff master To view the last commit on master, do git diff master^..master To view the commit logs, do git log You'll see long hex-string at the head of each commit. This is the commit id, which is a SHA1 hash based on the content of the tree, so you can share these with other hackers, and they can use the same commit id to poke at their own repositories. Locally any unique prefix of the hash is enough to identify a commit, so you don't need to use the full hashes. This is where the fun starts. Pick an interesting looking commit a while back, copy the commit id, and do git diff <> Git will show you all the changes between the commit you selected and current version as a single diff. Similarly, git diff -w <> <> can be used to compare two arbitrary versions. The -w switch tells Git to ignore whitespace changes -- you can usually leave it out, but it's nice when diffing across the great whitespacification patches. Onwards: just so that we have a bit more history on the branch, edit BUGS again, and git-commit again. You can use git commit -a to automatically add all the changed files to the staging area to speed things up. Repeat. Now git-log should show a few local changes. Let's see how merging works. To create a new branch based on master and switch to it do git checkout -b merge-experiment master Merge the changes we did on the hack-a-bit branch. git merge hack-a-bit Bing-bada-bing! Done. Have a look at git-log. You see that we have full branch history merged. If there had been conflicts, the merge would not have been automatic, but you would have been called to resolve conflicts, etc. This is very nice for merging short-lived local branches, but not always so good for merging things back onto master, or into "mainline" history since you get all the commits that happened on the branch: "first cut at x" "first cut at y" "oops, x was wrong" "implemented z" "aargh, x was still wrong" ... When merging a branch like this, with oopses in the history, it is far better if we are able to merge it as a few logical patches -- the resulting history will be easier to understand. First option is to use --squash to squash all the commits into a single one. git checkout -b merge-experiment-2 master git merge --squash hack-a-bit This has the side-effect of not committing the changes immediately. This is in effect similar to the usual way of merging back changes from a CVS branch: if we're told that the patch came from a branch, we can then go and look at the branch history, but we don't see the evolution of the branch in the mainline history. If the changeset is small, --squash is exactly what we want, but for long-lived branches that are best merged in a few steps we can use git merge --squash <> to merge the changes upto a certain commit. Repeat a few times, and you have the whole branch merged. Other Git commands provide more advanced options. See eg. git-cherry-pick, which you will end up using sooner or later. Now, let's assume we have a private Git repository in ~/sbcl-git, and we want to publish it to the world. The easiest way is to fire up a browser, and go to http://repo.or.cz/w/sbcl.git There, click on the "fork" link, and set up your own SBCL repository, called sbcl/yourname. Select the "push" mode, and write something along the lines of "My private SBCL tree. Pulls from main sbcl.git repository, and trickles changes back to upstream CVS manually -- from where they end up in sbcl.git. Turtles, you see." in the comment box. Then you will be directed to set up an account, which you will then have to add as a "pusher" to your SBCL fork. Finally, add the following snippet (adjusting for your own name) in ~/sbcl-git/.git/config [remote "public"] url = git+ssh://repo.or.cz/srv/git/sbcl/yourname.git After that is done, you're ready to publish your tree. git push --all public Now anyone can get at your repository at git://repo.or.cz/sbcl/yourname.git and you can publish your own hacks on it via git-push. git push public <> Since we're not (yet?) officially using Git, we want to get our changes back into the CVS. git-cvsexport is the perfect tool for this. First, to make things a bit easier, we add a command of our own to Git, by adding the following to ~/sbcl-git/.git/config: [alias] sbcl-export = cvsexportcommit -w /home/yourname/sbcl-cvs -v This assumes that you have a developer CVS checkout in ~/sbcl-cvs. To commit the the changes you have wrought on branch foo-hacks (compared to master): # These three steps could be replaced by any other sequence that # ends with all the changes you want to export to CVS at the top # of the current branch (so that the final commit introduces all # the changes.) # # In practice for small changes you may eg. just want to # # git checkout -b wip-fix-bug-xxx master # ...fix bug on the branch... # git commit -a # git checkout -b foo-hacks-to-cvs master git merge --squash foo-hacks git commit -a # This exports our stuff to the CVS tree. HEAD can be any <>, # so if you have a large number of commits on a branch that you want to # commit to CVS one by one, you do that as well. git sbcl-export HEAD cd ../sbcl-cvs Review, fix problems, etc. Edit version.lisp-expr, and add the version number to .msg (which contains the commit message from Git.) cvs commit -F .msg git-cvsexportcommit is fairly conservative by default, and will fail if the patch doens't apply cleanly. If that happens, you can fix the issues manually: .cvsexportcommit.diff -- holds the patch .msg -- holds the commit message Finally, delete the foo-hacks-to-cvs branch after you've committed code to CVS. Of course, instead of using git-cvsexportcommit you can also manually make and apply patches, etc. For hairier cases it may even be easier in the end: git format-patch -p master..foo-hacks Generates a patch for each commit between master and the HEAD of foo-hacks, naming them 0001-first-line-of-commit-message.patch 0002-and-so-and-so-forth.patch ... Due to, among other things, the cvs->git synchronization lag it is easy to get conflicts on version.lisp-expr so generally speaking you never want to edit version-lisp.expr in Git, and only edit it (and add the version to the commit message) just when you are about to commit to CVS. It is, however, nice to have an idea how a given image came to be, which you can take care of by putting the following code in branch-version.lisp-expr: #.(flet ((git (command &rest args) (with-output-to-string (s) ;; Adjust for your git installation location... (sb-ext:run-program (merge-pathnames "bin/git" (user-homedir-pathname)) (cons command args) :output s)))) (format nil "~A.~A~@[-dirty~]" (with-input-from-string (s (git "branch")) (loop for line = (read-line s) when (eql #\* (char line 0)) return (subseq line 2))) (count #\newline (git "rev-list" "HEAD...master")) (plusp (length (git "diff" "HEAD"))))) which leads to your Git tree build to have version names like 1.0.20.28.master.0-dirty 1.0.20.28 on branch master, uncommitted changes 1.0.20.19.wip-foo.4 1.0.20.19 on branch wip-foo, 4 commits between current HEAD and master, no uncommitted changes. To get latest changes from the CVS Git mirror you originally cloned from, do git pull on the branch you want to update on your private repository. Note that if you edit version.lisp-expr in the CVS tree and not before then the cvs commit command that git-cvsexportcommit prints does not list version.lisp-expr, so don't copy paste it. This completes our whirlwind tour. I'm not saying this makes you proficient in using Git, but at least you should be up and walking. Reading http://eagain.net/articles/git-for-computer-scientists/ http://www.kernel.org/pub/software/scm/git/docs/everyday.html http://www.kernel.org/pub/software/scm/git/docs/user-manual.html and various Git manual pages is a good idea. Two commands I can in particular recommend getting familiar with are git-rebase and git-cherry-pick. git-gui, git-citool, and gitk provide graphical interfaces for working with Git -- particularly gitk is an invaluable tool for visualizing history.