I tried to incorporate all the suggestions. There are still a few things I have to research, and now I'm worried it's getting too long. Sigh.
Generally, it's wonderful when the whole is greater than the sum of the parts. But trying to explain that is difficult, because you have to explain all the parts before you can explain how they work together to deliver a feature.
Oh, well. Perhaps I can rearrange this to talk about remote branches *after* local merging?
Thanks to everyone who commented.
* Branching and merging in git
In CVS, branches are difficult and awkward to use, and generally considered an advanced technique. Many people use CVS for a long time without departing from the trunk.
Git is very different. Branching and merging are central to effective use of git, and if you aren't comfortable with them, you won't be comfortable with git. In particular, they are required to share work with other people.
The only things that are a bit confusing are some of the names.
In particular, at least when beginning:
- You create new branches with "git checkout -b".
"git branch" should only be used to list and delete branches.
- You share work with "git fetch" and "git push". These are opposites.
- You merge with "git pull", not "git merge". "git pull" can also do a
"git fetch", but that's optional. What's not optional is the merge.
Also, a good habit it to never commit directly to your main "master" branch. Do all work in temporary "topic" branches, and then merge them into the master. Most experienced users don't bother to be quite this purist, but you should err on the side of using separate topic branches, so it's excellent practice.
* A brief digression on command names
All git commands can be invoked as "git-foo" and "git foo". This document
uses them interchangably. But you have to ask for the "git-foo" man page.
Git provides a few other ways to get the man page as well:
man git-foo
git help foo
git foo --help
Git doesn't have a specialized built-in help system; it just shows you the man pages.
One outstanding problem with git's man pages is that often the most detail is in the command page that was written first, not the user-friendly one that you should use. For example, there are a number of special cases of the "git diff" command that were written first, and the man pages for these commands (git-diff-index, git-diff-files, git-diff-tree, and git-diff-stages) are considerably more informative than the page for plain git-diff, even though that's the command that you should use 99% of the time.
Likewise, the man page for the incredibly useful "git show" command is a sterile wasteland; you have to refer to the git-rev-list and git-diff-tree man pages to find the many interesting options it supports.
"git-foo --help" often gives you the man page as well, but sometimes
gives a different help listing. (Try "git-rebase --help" and compare
it with "git rebase --help".)
If you care, here's why:
There are over 100 git-* commands. This has led to some complaints about the clutter in /usr/bin. So it's now possible to move the git-* commands into a separate directory, and you can either add that directory to your $PATH, or always use the "git" wrapper to find and invoke them.
This is kind of academic as, despite the complaints, I don't know anyone who has actually removed the git-* forms from the default $PATH.
Since the wrapper was developed, some simple commands have been made "builtin", so for example, "git diff" is done internally. There's a git-diff link to retain compatability.
* Git's representation of history
As you recall from Git 101 (or the git(7) man page), git's largest data strucure is the object database, holding exactly four kidnds of objects. Each of them has a globally unique 40-character hex name (a.k.a. object IDs) made by hashing its type and contents. Since this is an (effectively unforgeable) cryptographic hash, the name of an object identifies its contents uniquely. This is what is meant when people say that git's history is immutable; you can erase it and rewrite it, but any alterations will have a different name, and it will be obvious to anyone looking that something has been changed.
Blob objects are the simplest: they record file contents, and contain uninterpreted bytes.
Tree objects record directory contents; they contain file names, permissions, and the associated tree or blob object names.
Tag objects are shareable pointers to other objects; they're generally used to store a digital signature, and generally point to commits. (Although you can tag any object, including another tag.)
Finaly, there are commit objects. Every commit points to (contains the name of) an associated tree object which records the state of the source code at the time of the commit, and some descriptive data (time, author, committer, commit comment) about the commit.
Because every commit is associated with one tree, it's considered "tree-ish", and almost anywhere that git expect a tree object, you can supply a commit, and it'll understand. (Tags on commits are likewise considered "commit-ish", and can be used almost anywhere a commit or tree is required.)
Most importantly, each commit contains a list of "parent commits", older commits from which this one is derived. These pointers are what produce the history graph.
Typically only one commit (the initial commit) has zero parents. It's possible to have more than one such commit (if you merge two projects with separate histories), but that's unusual.
Many commits have exactly one parent. These are made by a normal commit after editing. From a branching and merging point of view, they're not too exciting.
And then there are commits which have multiple parents. Two is most common, but git allows many more. There's a limit of sixteen in the source code, and the most anyone's ever used in real life is 12, which was generally regarded as overdoing it. The famous "dodecapus" is commit 9fdb62af in the linux kernel repository.
Finally, there are references, stored in the .git/refs directory. These are the human-readable names associated with commits, and the "root set" from which all other commits should be reachable.
These references are generally divided into two types, although
there is no fundamental difference:
- Tags are references that are intended to be immutable.
A tag like "v1.2" is a historical record. Tag references may or may not
point to tag objects! If they do, this is called a "heavyweight tag";
the tag can hold a digital signature and can be shared between repositores.
"Lightweight tags" point to commits directly, and are not automatically
shared.
- Heads are references that are intended to be updated.
They are invariably "lightweight." "Head" is actually synonymous with
"branch", although one emphasizes the tip more, while the other directs
your attention to the entire path that got there.
Either way, they're just a 41-byte file that contains a 40-byte hex object ID, plus a newline. Tags are stored in .git/refs/tags, and heads are stored in .git/refs/heads. Creating a new branch is literally just picking a file name and writing the ID of an existing commit into it.
The git programs enforce the immutability of tags, but that's a safety feature, not something fundamental. You can rename a tag to the heads directory and go wild. (But the git-update-ref helper takes care of a few corner cases involving symlinks.)
The only limit on branches is clutter. A number of git commands have ways to operate on "all heads", and if you have too many, it can get annoying. If you're not using a branch, either delete it, or move it somewhere (like the tags directory) where it won't clutter up the list of "currently active heads".
(Note that CVS doesn't have this all-heads default, so people tend to use longer branch names and keep them around after they've been merged into the trunk. Old CVS repositories converted to git generally need an old-branch cleanup.)
Another thing that's worth mentioning is that head and tag names can contain slashes; i.e. you're allowed to make subdirectories in the .git/refs/heads and .git/refs/tags directories. See the man page for "git-check-ref-format" for full details of legal names.
* Naming revisions
CVS encourages you to tag like crazy, because the only other way to find a given revision is by date. Git makes it a lot easier, so most revisions don't need names.
You can find a full description in the git-rev-parse man page, but here's a summary.
First of all, every commit has a globally unique name, its 40-digit hex object ID. It's a bit long and awkward, but always works. This is useful for talking about a specific commit on a mailing list. You can abbreviate it to a unique prefix; most people find about 8 digits sufficient.
(Subversion is easier yet, because it assigns a sequential number to each commit. However, that isn't possible in a distributed system like git.)
Second, you can refer to a head or tag name. Git looks in the
following places, in order, for a reference:
1) .git/<name>
2) .git/refs/<name>
3) .git/refs/tags/<name>
4) .git/refs/heads/<name>
5) .git/refs/remotes/<name>
6) .git/refs/remotes/<name>/HEAD
You should avoid having e.g. a head and a tag with the same name, but if you do, you can specify one or the other with heads/foo and tags/foo.
Third, you can specify a commit relative to another. The simplest one is "the parent", specified by appending ^ to a name. E.g. HEAD^ or deadbeef^. If there are multiple parents, then ^ is the same as ^1, and the others are ^2, ^3, etc.
So the last few commits you've made are HEAD, HEAD^, HEAD^^, HEAD^^^, etc. After a while, counting carets becomes annoying, so you can abbreviate ^^^^ as ~4. Note that this only lets you specify the first parent. If you want to follow a side branch, you have to specify something like "master~305^2~22".
* Converting between names
Git has three helpers (programs designed mainly for use in shell scripts) to convert between global object IDs and human-readable names.
The first is git-rev-parse. This is a general git shell script helper, which validates the command line and converts object names to absolute object IDs. Its man page has a detailed description of the object name syntax.
The second is git-name-rev, which converts the other way around, giving a name for the specified commit in terms of the existing references. This searches the references for the closest descendant of the given commit. If called with the --tags option, it'll search only tags, showing you the next tag on the development path.
Third is git-describe. This is something like git-name-rev, but
backwards: it finds the closest reference that is an ancestor of the
specified commit. Its output is not acceptable git input, but takes
the form of either
v1.2 (tag name), or
v1.2-g12345678 (commit 12345678, whose nearest ancestor is v1.2)
git-describe's output is intended to be used as a software version number, something like the "rcsid" feature in RCS and CVS. By default, git-describe uses only heavyweight tags for its naming, so they are globally unique, but you can ask for it to be more liberal.
Git has chosen not to implement anything like the RCS/CVS "keyword substitution" feature, but you can invoke git-describe from a Makefile and incorporate its output into an executable to achieve a similar effect.
* Working with branches, the trivial cases.
By convention, the local "trunk" of git development is called "master". This is just the name of the branch it creates when you start an empty repository. You can delete it if you don't like the name.
If you create your repository by cloning someone else's repository, the remote "master" branch is copied to a local branch named "origin". You get your own "master" branch which is not tied to the remote repository.
There is always a current head, known as HEAD. (This is actually a symbolic link, .git/HEAD, to a file like refs/heads/master.) Git requires that this always point to the refs/heads directory.
Minor technical details:
1) HEAD used to be a Unix symlink, and can still be though of that
way, but for Microsoft support, this is now what's called a
"symbolic reference" or symref, and is a plain file containing
"ref: refs/heads/master". Git treats it just like a symlink.
There's a git-update-ref helper which writes these.
2) While HEAD must point to refs/heads, it's legal for it to
point to a file that doesn't exist. This is what happens
before the first commit in a brand new repository.
When you do "git commit", a new commit object is created with the old HEAD as a parent, and the new commit is written to the current head (pointed to by HEAD).
* The three uses of "git checkout"
Git checkout can do three separate things:
1) Change to a new head
git checkout [-f|-m] <branch>
This changes the HEAD symref to point to <branch>, and copies its
state to the index and the working directory.
If a file has unsaved changes in the working directory, this tries
to preserve them. This is a simple attempt, and requires that the
modified files(s) are not altered between the old and new HEADs.
In that case, the version in the working directory is left untouched.
A more aggressive option is -m, which will try to do a three-way
(intra-file) merge. This can fail, leaving unmerged files in the
index; dealing with this is described later.
An alternative is to use -f, which will overwrite any unsaved changes
in the working directory. This option can be used with no <branch>
specified (defaults to HEAD) to undo local edits.
2) Revert changes to a small number of files.
git checkout [<revision>] [--] <paths>
will copy the version of the <paths> from the index to the working
directory. If a <revision> is given, the index for those paths will
be updated from the given revision before copying from the index to
the working tree.
Unlike the version with no <paths> specified, this does NOT change
HEAD, even if <paths> is ".".
The "--" is only required if the first <path> could be
mistaken for a revision.
3) Create a branch.
git checkout [-f|-m] -b <branch> [revision]
will create, and switch to, a new branch with the given name.
This is equivalent to
git branch <branch> [<revision>]
git checkout [-f|-m] <branch>
If <revision> is omitted, it defaults to the current HEAD, in which
case no working directory files are altered.
This is the usual way that one creates a new branch to start work
on it, or checks out a revision that does not have an existing head
pointing to it.
Note that you can create a branch at any time up until the git-commit;
* Deleting branches
"git branch -d <head>" is safe. It deletes the given <head>, but first it checks that the commit is reachable from the current HEAD. That is, you merged the branch in to the current HEAD, or you never did any edits on that branch.
It's a good idea to create a "topic branch" when you're working on anything bigger than a one-liner, but it's also a good idea to delete them when you're done. The commits are still there in the history.
* Doing rude things to heads: git reset
If you need to overwrite the current HEAD for some reason, the tool to do it with is "git reset". There are three levels of reset:
git reset --soft <head>
This overwrites the current HEAD with the contents of <head>.
If you omit <head>, it defaults to HEAD, so this does nothing.
git reset [<head>]
git reset --mixed [<head>]
These overwrite the current HEAD, and copy it to the index,
undoing any git-update-index commands you may have executed.
If you omit <head>, it default to HEAD, so there is no change
to the current branch, but all index changes are undone.
git reset --hard [<head>]
This does everything mentioned above, and updates the
working directory. This throws away all of your in-progress
edits and gets you a clean copy. This is also commonly
used without an explicit <head>, in which case the current
HEAD is used.
* Using git-reset to fix mistakes
"Oh, no! I didn't mean to commit *that*! How do I undo it?"
If you just want to undo a commit, then you can use "git reset HEAD^" to return the current HEAD to the previous version. If you want to leave the commit in the index (this only applies to you if you are familiar with using the index; see below), then you can use "git reset --soft HEAD^".
And if you want to blow away every record of the changes you made, you can use "git reset --hard HEAD^"
If you just want a stupid trivial mistake and want to replace the most recent commit with a corrected one, "git commit --amend" is your friend. It makes a new commit with HEAD^ rather than HEAD as its ancestor.
It's often much easier to use --amend than fiddle around with git-reset.
* Fixing mistakes without git-reset
git-reset has the problem that it doesn't preserve hacking in progress in the working directory. It can leave the working directory alone (making everything a "hack in progress"), but it can't merge in-progress changes like git checkout.
So, suppose you've been trying something that should have been simple, and made three commits to your main branch before realizing that the problem is harder than you thought, and you want your work so far to be on a new branch of its own; committing them on the current HEAD (I'll call it "old") was a mistake.
You don't want to erase anything, just rename it. Make "new" a copy of the current "old" and move old back to HEAD^^^ (three commits ago).
While there are ways to do that using git-reset, but far better is to use "git branch -f":
git checkout -b new
Create (and switch to) the "new" branch.
git branch -f old HEAD^^^
Forcibly move "old" back three versions.
(You could also use old~3 or new^^^ or any synonymous name.)
You can use a similar trick to rename a branch. If it's the current HEAD, then:
git checkout -b newname
git branch -d oldname
and if it's not, then
git branch newname oldname
git branch -d oldname
An alternative in the latter case is to just use mv on the raw .git/refs/heads/oldname file.
* How do I check out an old version?
A very common beginning question is how to check out an old version. Say you need to compile an old release for test purposes. "git checkout v1.2" gives a funny error message. What's going on?
Well, "git checkout" makes the current HEAD point to the head that you specify. And, as previously mentioned, git requires that it point to something in the .git/refs/heads directory. So you can't do that.
If you're busy doing things in your working directory, and don't want to overwrite your work with an old version, then you can get a snapshot with the (old) git-tar-tree or (new) git-archive commands. These produce a tar file (git-archive can also produce a zip file) which is a snapshot of any version you like. You can then unpack this file in a different directory and build it.
However, if you haven't got any edits in progress, and want to check out the old version into your working directory, just create a temp branch!
git checkout -b temp v1.2
Will do what you want. This will also do what you want if you have a local edit (like the "#define DEBUG 1" mentioned above) that you want to preserve while working on the old version.
You'll see this in use if you ever use the (highly recommended) git-bisect tool. It creates a branch called "bisect" for the duration of the bisect.
(Yes, I have to confess, I sometimes wish that git would enforce the "HEAD must point to .git/refs/heads" rule when committing (checking in) rather than when checking out, but that's the way git has grown up.)
Note that if you want *exactly* an old version, with no local hacks, make sure there are none (with "git status") when doing this. It's more convenient if you do it before the checkout, but you'll get the same answer if you ask afterwards.
Now, what about the complex case: you have local hacks that you want to keep, but not have polluting the old version?
Well, one way of the other, you'll have to commit it. If you don't mind committing your changes to the current branch ("git commit -a"), do that.
If they're not ready to commit, you can commit them anyway, and back them out when you're done:
git commit -a -m "Temp commit"
git checkout -b temp v1.2
make ; make test ; whatever
git checkout master
git branch -d temp
git reset HEAD^
This leaves both the working directory and the master head in the states they were in at the beginning.
If you don't like committing to the master branch, you can make a new one. In this example, it's "work in progress", a.k.a. "wip":
git checkout -b wip
git commit -a -m "Temp commit"
git checkout -b temp v1.2
make ; make test ; whatever
git checkout wip
git branch -d temp
git reset master
git checkout master # Won't change working directory
git branch -d wip
* Examining history: git-log and git-rev-list
In another example of docs being better on the first command written, the all-purpose utility for examining history is "git log", but all of the examples of clever ways to use it are in the git-rev-list man page. And git-log also has most of git-diff's options.
Other utilities, notably the gitk and qgit GUIs, also use the git-rev-list command-line options, so it's well worth learning them.
git-rev-list gives you a filtered subset of the repository history. There are two basic ways that you can do the filtering:
1) By ancestry. You specify a set of commits to include all the
ancestors of, and another set to exclude all the ancestors of.
(For this purpose, a commit is considered an ancestor of itself.)
So if you want to see all commits between v1.1 and v1.2, you
can specify
git log ^v1.1 v1.2
or, with a more convenient syntax
git log v1.1..v1.2
However, there are times when you want to specify something more
complex. For example, if a big branch that had been in progress since
v1.0.7 was merged between v1.1 and v1.2, but you don't want to see it,
you could specify any of:
git log v1.2 ^v1.1 ^bigbranch
git log ^bigbranch v1.1..v1.2
git log ^v1.1 bigbranch..v1.2
They're all equivalent. Another special syntax that's sometimes
handy is
git log branch1...branch2
Note the three dots. This generates the symmetric difference between
the two; basically it's a diff between the commits that went into
each of them.
"git log" by default pipes its output through less(1), and generates
its output from newest to oldest on the fly, so there's no great
speed penalty to not specifying a starting place. It'll generate a
few screen fulls more than you look at, but not waste any more effort
than that.
2) By path name. This is a feature which appears to be unique to git.
If you give git-rev-list (or git-log, or gitk, or qgit) a list of
pathname prefixes, it will list only commits which touch those
paths. So "git log drivers/scsi include/scsi" will list only
commits which alters a file whose name begins with drivers/scsi
or include/scsi.
(If there's any possible ambiguity between a path name and a commit
name, git-rev-list will refuse to proceed. You can resolve it by
including "--" on the command line. Everything before that is a
commit name; everything after is a path.)
This filter is in addition to the ancestry filter. It's also rather
clever about omitting unnecessary detail. In particular, if there's
a side branch which does not touch the given paths, then the entire
branch, and the merge at the end, will be removed from the log.
You can additionally limit the commits to a certain number, or by date, author, committer, and so on.
By default, "git log" only shows the commit messages, so it's important to write good ones. Other tools compress commit messages down to the first line, so try to make that as informative as possible.
("git show" is the standard tool for examining a single commit, and it does show
* History diagrams
When talking about various situations involving multiple branches,
people often find it handy to draw pictures. Gitk draws nice pictures
vertically, but for e-mail, ASCII art drawn horizontally is often easier.
Commits are shown as "o", and the links between them with lines drawn with
- / and \. Time goes left to right, and heads may be labelled with names.
For example:
o--o--o <-- Branch A
/
o--o--o <-- master
\
o--o--o <-- Branch BIf someone needs to talk about a particular commit, the character "o" may be replaced with another letter or number.
* Trivial merges: fast-forward and already up-to-date.
There are two kinds of merge that are particularly simple, and you will encounter them in git a great deal. They are mirror images.
Suppose that you are working on branch A and merge in branch B, but no work has been done to branch B since the last time you merged, or since you spawned branch A from it. That is, the history looks like
o--o--o--o <-- B
\
o--o--o <-- A
or
o--o--o--o--o--o <-- B
\ \
o--o--o--o--o <-- AIf you then merge B into A, A is described as "already up to date". It is already a strict superset of B, and the merge does nothing.
In particular, git will not create a dummy commit to record the fact that a merge was done. It turns out that are a number of bad things that would happen if you did this, but for now, I'll just say that git doesn't do it.
Now, the opposite scenario is the "fast-forward" merge. Suppose you merge A into B. Again, A is a strict superset of B.
In this case, git will simply change the head B to point to the same commit as A and say that it did a "fast-forward" merge. Again, no commit object is created to reflect this fact.
The effect is to unclutter the git history. If I create a topic branch to work on a feature, do some hacking, and then merge the result back into the (untouched!) master, the history will look just like I did all the work on the master directly. If I then delete the topic branch (because I'm done using it), the repository state is truly indistinguishable.
While the topic branch existed, you could have done something to the master branch, in which case the final merge would have been non-trivial, but if that didn't happen, git produces a simple, easy-to-follow linear history.
Some people used to heavyweight branches find this confusing; they think a merge is a big deal and it should be memorialized, but there are actually excellent reasons for doing this.
The most important one is that a fit of merging back and forth will eventually end. Suppose that branches A and B are maintained by separate developers who like to track each other's work closely.
If the fast-forward case did create a commit, then merging A into B would produce
o--o--o--o---------o <-- B
\ /
o--o--o <-- A
then merging B into A would produce:
o--o--o--o---------o <-- B
\ / \
o--o--o---o <-- Aand further merges would produce more and more dummy commits, all without ever reaching a steady state, and without making it obvious that the two heads are actually identical.
Since history lasts forever, cluttering it up with unimportant stuff is a burden to all future users, and not a good idea. Allowing the merge of a branch to be seamless in the simple case encourages lightweight branches. If you _might_ need a separate branch, create it. If it turned out that you didn't, it won't make a difference.
* Exchanging work with other repositories
The basic tools for exchanging work with other repositories are "git fetch" and "git push". The fact that "git pull" is not the opposite of "git push" is often confusing to beginners (it's a superset of git fetch), but that's the terminology that has grown up.
The unit of sharing in git is the branch. If you've used branches in CVS, you'll be familiar with using "CVS update" to pull changes from your "current branch" in the repository into your working directory.
In Git, you don't pull into the working directory, but rather into a tracking branch. You set up a branch in your repository which will be a copy of the branch in the remote repository. For example, if you use "git clone", then the remote "master" branch is tracked by the local "origin" branch.
Then, when you do a "git fetch", git fetches all of the new commits and sets the origin head to point to the newly fetched head of the remote branch.
By default, git checks that this is a trivial fast-forward merge, that is not throwing away history. If it finds something like:
o--o--o--o--o--o <-- remote master
\
o <-- Local originIt will complain and abort the fetch. This is usually a warning that something has gone wrong - in particular, you forgot that this was supposed to be a tracking branch and committed some work to it - and it aborts before throwing your work away.
However, sometimes the remote git user will have a branch name that they delete and re-create frequently. There are plenty of reasons to do this. The most common is doing a "test merge" between various branches in progress. They're all unfinished, so the developer of branch A doesn't want to merge in all the new bugs in branch B, but a tester might want to create a merged version with both sets of bugs for testing.
The merged version is not intended to be a permanent part of history - it'll get deleted after the test - but it can still be useful to have a draft copy.
In this case, you can mark the source branch with a leading "+", to disable this sanity check. (See the git-fetch man page for details.)
Note that in this case, you should specifically avoid merging from such a branch into any non-test branches of your own. It is, as mentioned, not intended to be a permanent part of history, so it would be rude to make it part of your permanent history. (You still might want to test-merge it with your work in progress, of course.)
The fact that you should know to treat such branches specially is why git doesn't try to automatically cope with them.
* Alternate branch naming
The original git scheme mixes tracking branches with all the other heads. This requires that you remember which branches are tracking branches and which aren't. Hopefully, you remember what all your branches are for, but if you track a lot of remote repositories, you might not remember what every remote branch is for and what you called it locally.
An alternative convention has been developed (which may become the default in future), that places copies of the remote servers' heads under .git/refs/remotes/<server>/<branch>.
Then you can refer to
If you use "git-line --use-separate-remote", it will set
If you want to use the "separate remotes" tracking branch There's an alternate way, using the --use-separate-remote option to git-clone. This sets up a copy of the remote server's heads under .git/refs/remotes/origin/<name>. Then you can refer to "origin/<branch>" whenever you want.
Because the branch names are got under .git/refs/heads, the git tools will not let you commit to the branch.
* Remotes files
You can specify what to fetch on the git-fetch command line. That is, you can type
git fetch <url> src1:dst1 src2:dst2
to fetch the given remote heads src1 and src2 However, if you intend to monitor another repository on an ongoing basis, it's generally easier to set up a short-cut by placing the options in .git/remotes/<name>.
The syntax is explained in the git-fetch man page. When this is st up, "git fetch <name>" will retrieve all the branches listed in the .git/remotes/<name> file. The ability to fetch multiple branches at once (such as release, beta, and development) is an advantage of using a remotes file.
git-fetch with no argument uses the default file .git/remotes/origin. If you have a single primary "upstream" repository that you sync to, place it in the origin remotes file, and you can just type "git fetch" to get all the latest changes.
Note that branches to fetch are identified by "Pull: " lines in the remotes file. This is another example of the fetch/pull confusion. git-pull will be explained eventually.
If you want to follow this convention when manually adding branches to track, just supply .git/-relative branch names:
Pull: refs/heads/master:refs/remotes/origin/master
Pull: refs/heads/beta:refs/remotes/origin/beta
Pull: refs/heads/alpha:refs/remotes/origin/alpha
[TODO: Explain the git config file alternative. I need to write a section on the config file itself first.]
* Cloning
If you want to watch a project that's hosted on a git server, the easiest way is to use "git clone".
git-clone creates a new repository, sets up a remotes file to reack every branch in the remote repository, and fetches all those branches. By default, it maps them to local heads as follows:
- The remote "master" is tracked by the local "origin"
- The local "master" is made a copy of that.
- The remote "origin" is not tracked at all.
- All other heads are tracked by local heads of the same name.
If you use git-clone --use-separate-remote, then a different, simpler convention for remote heads will be used:
- Every remote <head> is tracked by .git/refs/remotes/origin/<head>.
Either way, the fetch information is all placed in the .git/remotes/origin file, which (as mentioned above) is the default used by git-fetch if no argument is supplied.
* Fetching without tracking (advanced)
Whenever a fetch is done, the fetched heads are also stored in .git/FETCH_HEAD. This is for use by later merging (coming up really soon, promise!). In fact, it's possible to fetch without writing to any local heads at all. If you just
git fetch <url> src1 src2
Then the fetch will be done, but the results will be written nowhere but .git/FETCH_HEAD. This is actually the earliest form of git_fetch impllemented; everything else is a later addition. It's not something you'd do on purpose much, except as part of a script that uses FETCH_HEAD, but it's worth mentioning it in case you type it and wonder what the heck happened.
* Remote tags
When you fetch to a tracking branch, git-fetch also fetches every heavyweight tag (one that involves an actual tag object) that points to a commit reachable from the branch head and installs a copy locally.
It will never overwrite a pre-existing tag of the same name. This means that if you publish a heavyweight tag, and then try to change it, people who fetched the old tag won't see the change!
This is perhaps an excessively liberal policy, but it has worked well in practice so far. Still, it does mean that you should think about who you fetch code from.
Since tags can be PGP-signed, one option would be to verify them before installing them locally.
In case it helps, the primitive to get a list of remote heads and tags is git-ls-remote. It's used by git-clone to get the list of heads to track, and can be used manually to see what's been added since then that you might be interested in tracking.
* Git network protocols
There a few basic ways to share a git repository over the 'net, which break down into "dumb" protocols that just copy files, and the "smart" git native protocol that can deliver just the objects that you need.
1) http protocol. This is sub-optimal, but may be all you have.
The challenge with this is that you have to pack your repository
into chunks that are neither too big (because you have to download
all of a chunk to download any of it) not too small (because the
http request overhead would kill you). So think carefully about
how often you run git-repack, and don't run git-repack -a.
http clients also need some extra index information to help them
find which pack files they need. git-update-server-info is the
command that generates these files, but it's run automatically
from git-repack, so it's not too important to know.
2) rsync protocol. This is basically an alternative to http, and
has the same strictures and limitations.
3) The git protocol. This is represented with a git:// URL, and talks
to a dedicated git daemon (see the git-daemin man page) on the
remote machine. It uses TP port 9418 by default. This is a smart
protocol that understands the git format and does sophisticated
wire compression.
It uses more CPU on the server, but less bandwidth. And it doesn't
require any special care when repacking. git-daemon is purposefully
written to provide read-only service.
4) The git protocol over ssh. This is a git+ssh:// URL; ssh:// is
accepted as a synonym. It has the same efficiency issues as
plain git. If you want to limit ssh users to just the git commands
necessary to share work, git provides a git-shell command that can
be used as a very limited login shell.
It's been reported that using http or rsync for the initial clone is faster than the smart git protocol, because the smarts are wasted when you just need to download everything. Then you can go into the remotes/origin file and edit the URL line to git:// for later smart access.
But unless you want to bother with that very minor optimization, just remember to use the git native protocol whenever possible.
* Exchanging work with other repositories, part II: git-push
It's simpler to set up git sharing on a pull basis. If your source code isn't secret, you can set up a public read-only server very easily (see the git-daemon man page for details), and have other fetch from that.
However, N developers all pulling from each other is an N^2 mess. Some centralization helps.
One way is to have a central coordinator (like Linus) who pulls from all of the developers, and who they in turn pull from.
The other is familiar to users of centralized VCSs: have a central repository that people can push to. This generally requires an ssh login on the server. You can use git-shell as the login shell if all you want to allow the account to do is git fetch and push. (You can use the hook scripts to enforce rules about who's allowed to do what to which branch.)
Git-push to the remote machine works exactly like git-fetch from the remote machine. The objects are moved over, and the branches pushed to are fast-forwarded. If fast-forward is impossible, you get an error.
So if you have multiple people committing to a branch on the server, you will not be allowed to push if someone has pushed more to that branch since last time you fetched it.
You have to fix the problem locally, and re-try the push when you've got a new head that includes the most recently pushed work as an ancestor.
This is exactly like "cvs commit" not working if your recent checkout wasn't the (current) tip of the branch, but git can upload more than one commit.
The simplest way to resolve the conflict is to merge the remote head with your local head. Alternatively, you can rebase your work to the new head. (See below for details.)
In either case, this is easiest if you have different local branches for fetching the remote repository and for pushing to it.
That is, you have one head that just tracks the master repository's main branch, and another that you add your work to, and push from. It makes resolving conflicts between the two much easier if you have a name for each of them.
Like git-fetch, you can specify everything on the command line:
git-push <url> src1:dst1 src2:dst2 src3:dst3
to upload the local branches src1, src2 and src3 to the corresponding remote branches, or you can create a .git/remotes/<name> file. The URL: and any Push: lines in the file are
(If you need different ssh options for different hosts you push to, set them up in a .ssh_config file. You can even have different options for the same host, as .ssh_config lets you specify "Host alias" and "HostName real.dom.aim".)
Another use for git-push, even for a solo developer, is sharing your work with the world. You can set up a public git server on a high-bandwidth machine (possibly rented from a hosting service) and then push to it to publish something. This also helps keep your private development branches private; git-daemon doesn't have a way to limit which branches it exposes to which people.
* Merging (finally!)
I went through everything else first because the most common merge case is local changes with remote changes. Not that you can't merge two branches of your own, but beginners will encounter the local/remote case first.
The primitive that does the merging is called (guess what?) git-merge. And there's nothing too terribly wrong with using it. However, it's usually easier to use the git-pull wrapper. This merges the changes from some other branch into the current HEAD and generates a commit message automatically.
git-merge lets you specify the commit message (rather than generating it automatically), but that's about it.
The basic git-pull syntax is
git-pull <repository> <branch>
The repository can be any URL that git supports. Including, particularly, a local file. So to do a simple local merge, you just type
git-pull . <branch>
So after doing some hacking on branch "foo", you would
git checkout master
git pull . foo
and ba-boom, all is done.
Now, you can also specify a remote repository to merge from, using a git://, http:// or git+ssh:// URL. This is what Linus does all day long, and why the git-pull tool is optimized to allow that. It uses git-fetch to fetch the remote branch without assigning it a branch name (as mentioned above, it gets the magic name FETCH_HEAD), and them merges it into the current HEAD directly.
There is absolutely nothing wrong with doing that, but beginners often find it confusing to have a single short command do quite so much. And if you are working closely with someone, it's often more convenient and less confusing to keep local tracking branches. Then you can
git fetch # Fetches 'origin'
git pull . origin
It's also possible to give just a single remotes file name to git-pull:
git pull origin
That does a git fetch, updating all of the listed branches as usual, then merges the _first_ listed branch into HEAD. It would be more cinsistent to merge all the branches, but that's almost never what you want.
By the way: don't blink, you might miss it! As I mentioned, pulling is a very big part of Linus's daily routine, and he's made sure it's fast. (Actually, you cant miss all the output it produces.)
Just to clarify, because people often get confused:
git-pull is a MERGING tool. It always does a merge, as well as an optional fetch. If you just want to LOOK at a remote branch, use git-fetch.
* Undoing a merge
If you discover that a merge was a mistake, it can be undone just like any other commit. The HEAD you merged to is the first parent, so just do
git reset --hard HEAD^
Since your local git repository is private, it's easy to un-commit; it's your own private infinite undo. It's only when you publish your branch that
This is why Linus likes a git-pull command that does so much in one shot; if he doesn't like what he pulls, it's easy to undo.
* How merging operates
Git uses the basic three-way merge. First, it applies it to file names, then to whole files, and then to lines within files.
To do a three-way merge, you need three versions of a file. The versions A and B you want to merge, and a common ancestor, commonly called O. That is, history proceeds something like:
o--o--A
/
o--o--O
\
o--BThe basic idea is "I want the file O, plus all the changes made from O to A, plus all the changes made from O to B." Since the cases where one of A or B is a direct ancestor of the other have already been disposed of, the three commits must be different.
For each file, there are a few cases that are trivial, and git gets these out of the way immediately:
- If A and B are identical, the merged result is obvious.
- If O and A are the same, then the result should be B.
- If O and B are the same, then the result should be A.
In the completely trivial case when O, A and B are the same, then all three rules apply, they all produce the same obvious result.
Git automatically finds the masrge base O as the most recent common ancestor of the heads A and B to be merged.
When doing a merge, git uses the above 2-out-of-3 merging rules three times:
First, the rules are applied to file renames. So if foo.c is renamed to bar.c on branch A, then branch B's foo.c will be merged with it to produce the result.
Second, they're applied to whole files. If two out of three entire files are identical, there's nothing more to do. Since the name of a blob object uniquely identifies its contents, this lets git do a considerable amount of merging without even looking at the file contents.
Finally, any files where all three versions are different are loaded into the index file, with the same file name but marked "stage 1", "stage 2", and "stage 3", and the classic line-based three-way merge is used to resolve the mess, This looks for isolated "hunks" of change and uses the same 2-out-of-3 rules to resolve each hunk separately.
Only if all three commits have differing hunks that overlap (or come so glose that git can't be sure) is git unable to automatically resolve the problem. This requires manual correction, as described below.
If the merge goes well, it is automatically comitted and the HEAD branch updated to point to the new commit.
* Merge subtleties (advanced)
As mentioned before, the merge base is the most recent common ancestor of A and B. The only problem is, that's not necessarily unique!
The classic confusing case is called a "criss-cross merge", and looks like this:
o--b-o-o--B
/ \ /
o--o--o X
\ / \
o--a-o-o--AThere are two most-recent common ancestors of A and B, marked a and b in the graph above. And they're not the same. You could use either one and get reasonable results, but how to choose?
The details are too advanced for this discussion, but the default "recursive" merge strategy that git uses solves the answer by merging a and b into a temporary commit and using *that* as the merge base.
Of course, a and b could have the same problem, so merging them could require another merge of still-older commits. This is why the algorithm is called "recursive." It's been tested with pathological conditions, but multiply nested criss-cross merges are very rare, so the recursion isn't a performance limit in practice.
(This can lead to occasional confusing messages about merge conflicts that aren't real. That's because git computes the full merge base, but if the conflicting file is actually identical in A and B, the messed-up version in O doesn't matter.)
* Alternate merge strategies (advanced)
In every version control system prior to git, the merging algorithm was buried deep in the bowels of the software, and very difficult to change. One of particularly nice things that git did was allow for easily replaceable "merge strategies". Indeed, you can try multiple merge strategies, and the fallback - print an error message and let the user sort it out - can be thought of as just another merge strategy.
Enabling this is why the index is so important to git. It provides a place to store an unfinished merge, so you can try various strategies (including hand-editing) to finish it.
There are two non-default strategies that have their uses in special circumstances.
* Octopus merge (advanced)
The first is the "octopus" stratgy. This is special because it can do a three- or more-way merge. See 5401f304 in the git repository for an example. (Run gitk, double-click on the "SHA1 ID" box to select what's already there, enter "5401f304" instead, and click "Goto".)
The octopus strategy is invoked automatically when you specify more than one branch at a time to merge in with "git pull". It can't handle complicated overlaps and file renames as well as the 2-way recursive strategy, but if you have a number of simple, independent changes that you want to merge together, an octopus merge is the obvious way to document the fact that they're truly independent.
The only downside to using an octopus to combine a number of simple changes is that any merge makes git-bisect's job harder. If you have a development history like
/-b-\
/ \
| /-c-\ |
|/ \|
--a---d---g--
|\ /|
| \-e-/ |
\ /
\-f-/And you know that a works but g doesn't, there's no way to do a binary search on b through f; they have to be searched linearly. This is no harder to bisect, and a lot nicer-looking than the equivalent with 2-way merges:
/-b
/ \
| /-c-g
|/ \
--a---d---h--i--j--
|\ / /
| \-e----/ /
\ /
\-f-----/But if they were just done one after the other, you'd have
--a--b--c--d--e--f--
Which may imply a non-existent dependency between the changes, but is a bit simpler. Still, the first octopus merge in git's own development (211232ba) is of this form. It's a matter of taste.
For the beginner, the important thing to know is that you never *need* an octopus.
* "Ours" merge (advanced)
The other merge strategy that is surprisingly useful is specified with the "-s ours" option to git-pull.
This strategy instructs git that the merged result should be the same as the current HEAD. Any other branches are recorded as parents, but their contents are ignored.
What the heck is the use of that? Well, it lets you record the fact that some work has been done in the history, and that it shouldn't be merged again. For example, say you write and share a popular patch set. People are always merging it in to their local source trees. But then you discover a much better way to achieve the goal of that patch set, and you want to publish the fact that the new patch supersedes the old one.
If you developed the new set starting from the old one, that would happen automatically. But another way to achieve the same goal is to merge the old branch it in using the "ours" strategy. Everyone else's git will notice that the patch is already included, and stop trying to merge it in.
* When merging goes wrong
This is the fun part. Git's default recursive-merge strategy is pretty clever, but sometimes changes truly do conflict and need manual fix-up.
When git is unable to complete a merge, it leaves the three different versions in the index and places a file with CVS-style conflict markers in the working directory.
As long as there is a "staged" file like this in the index, you will not be able to commit. You must resolve the conflict, and update the index with the resolved versions. You can do this one at a time with git-update-index, or at the end by giving the files as arguments to git-commit.
Doing them one at a time is probably safest; checking in a file which still has conflict markers makes a bit of a mess. Git will still use the automatically generated commit message when you finally commit. (It's stashed in .git/MERGE_MSG, if you care.)
Note that "git diff" knows how to be useful with a staged file. By default, it displays a multi-way diff. For example, suppose I take a (slightly buggy) hello.c:
--- hello.c --- #include <stdio.h>
int main(void)
{
printf("Hello, world!");
}
--- end ---Now, suppose that in branch A, I fix some bugs - add the missing newline and "return 0;". In branch B, I display my angst and change it to "Goodbye, cruel world!". When I try to merge A into B, obviously I'll get a conflict. The resultant file, with conflict markers, looks like:
--- hello.c --- #include <stdio.h>
int
main(void)
{
<<<<<<< HEAD/hello.c
printf("Goodbye, cruel world!");
=======
printf("Hello, world!\n");
return 0;
Junio C Hamano
