Now you and Sally are working on parallel branches of the project: you're working on a private branch, and Sally is working on the trunk, or main line of development.
For projects that have a large number of contributors, it's common for most people to have working copies of the trunk. Whenever someone needs to make a long-running change that is likely to disrupt the trunk, a standard procedure is to create a private branch and commit changes there until all the work is complete.
So, the good news is that you and Sally aren't interfering with each other. The bad news is that it's very easy to drift too far apart. Remember that one of the problems with the “crawl in a hole” strategy is that by the time you're finished with your branch, it may be near-impossible to merge your changes back into the trunk without a huge number of conflicts.
Instead, you and Sally might continue to share changes as you work. It's up to you to decide which changes are worth sharing; Subversion gives you the ability to selectively “copy” changes between branches. And when you're completely finished with your branch, your entire set of branch changes can be copied back into the trunk. In Subversion terminology, the general act of replicating changes from one branch to another is called merging, and it is performed using various invocations of the svn merge subcommand.
In the examples that follow, we're assuming that both your Subversion client and server are running Subversion 1.7 (or later). If either client or server is older than version 1.5, things are more complicated: the system won't track changes automatically, forcing you to use painful manual methods to achieve similar results. That is, you'll always need to use the detailed merge syntax to specify specific ranges of revisions to replicate (see the section called “Merge Syntax: Full Disclosure” later in this chapter), and take special care to keep track of what's already been merged and what hasn't. For this reason, we strongly recommend that you make sure your client and server are at least at version 1.5.
Before we proceed further, we should warn you that there's a lot of discussion of “changes” in the pages ahead. A lot of people experienced with version control systems use the terms “change” and “changeset” interchangeably, and we should clarify what Subversion understands as a changeset.
Everyone seems to have a slightly different definition of changeset, or at least a different expectation of what it means for a version control system to have one. For our purposes, let's say that a changeset is just a collection of changes with a unique name. The changes might include textual edits to file contents, modifications to tree structure, or tweaks to metadata. In more common speak, a changeset is just a patch with a name you can refer to.
In Subversion, a global revision
N names a tree in the
repository: it's the way the repository looked after the
Nth commit. It's also the name of
an implicit changeset: if you compare
N-1, you can derive the exact
patch that was committed. For this reason, it's easy to think
N as not just a tree,
but a changeset as well. If you use an issue tracker to
manage bugs, you can use the revision numbers to refer to
particular patches that fix bugs—for example,
“this issue was fixed by r9238.” Somebody
can then run
svn log -r 9238 to read about
the exact changeset that fixed the bug, and run
svn diff -c 9238 to see the patch itself.
And (as you'll see shortly)
Subversion's svn merge command is able to use
revision numbers. You can merge specific changesets from one
branch to another by naming them in the merge
to svn merge would merge changeset r9238
into your working copy.
Continuing with our running example, let's suppose that a
week has passed since you started working on your private
branch. Your new feature isn't finished yet, but at the same
time you know that other people on your team continue to make
important changes in the
/trunk. It's in your best
interest to replicate those changes to your own branch, just
to make sure they mesh well with your changes. This is done
by performing a sync merge—a
merge operation designed to bring your branch up to date with
any changes made to its ancestral parent branch since your
branch was created.
Frequently keeping your branch in sync with the main development line helps prevent “surprise” conflicts when the time comes for you to fold your changes back into the trunk.
Subversion is aware of the history of your branch and knows when it split away from the trunk. To perform a sync merge, first make sure your working copy of the branch is “clean”—that it has no local modifications reported by svn status. Then simply run:
$ pwd /home/user/my-calc-branch $ svn merge ^/calc/trunk --- Merging r345 through r356 into '.': U button.c U integer.c --- Recording mergeinfo for merge of r345 through r356 into '.': U . $
This basic syntax—
Subversion to merge all changes which have not been previously
merged from the URL to the current working directory (which is
typically the root of your working copy). Notice that we're
using the caret (
syntax to avoid having to type out the
/trunk URL. Also note
the “Recording mergeinfo for merge…”
notification. This tells you that the merge is updating
svn:mergeinfo property. We'll discuss
both this property and these notifications later in this
the section called “Mergeinfo and Previews”.
After running the prior example, your branch working copy now contains new local modifications, and these edits are duplications of all of the changes that have happened on the trunk since you first created your branch:
$ svn status M . M button.c M integer.c $
At this point, the wise thing to do is look at the changes
carefully with svn diff, and then build and
test your branch. Notice that the current working directory
.”) has also been
modified; svn diff will show that
svn:mergeinfo property has been either
created or modified. This is important merge-related metadata
that you should not touch, since it is
needed by future svn merge commands.
(We'll learn more about this metadata later in the
After performing the merge, you might also need to resolve
some conflicts—just as you do with svn
update—or possibly make some small edits to get
things working properly. (Remember, just because there are
no syntactic conflicts doesn't mean there
aren't any semantic conflicts!) If you
encounter serious problems, you can always abort the local
changes by running
svn revert . -R (which
will undo all local modifications) and starting a
long “what's going on?” discussion with your
collaborators. If things look good, however, you can
submit these changes into the repository:
$ svn commit -m "Merged latest trunk changes to my-calc-branch." Sending . Sending button.c Sending integer.c Transmitting file data .. Committed revision 357. $
At this point, your private branch is now “in sync” with the trunk, so you can rest easier knowing that as you continue to work in isolation, you're not drifting too far away from what everyone else is doing.
Suppose that another week has passed. You've committed more changes to your branch, and your comrades have continued to improve the trunk as well. Once again, you want to replicate the latest trunk changes to your branch and bring yourself in sync. Just run the same merge command again!
$ svn merge ^/calc/trunk svn: E195020: Cannot merge into mixed-revision working copy [357:378]; try up\ dating first $
Well that was unexpected! After making changes to your
branch over the past week you now find yourself with a working
copy that contains a mixture of revisions (see
the section called “Mixed-revision working copies”). With the
release of Subversion 1.7 the svn merge
subcommand disables merges into mixed-revision working copies
by default. Without going into too much detail, this is
because of limitations in the way merges are tracked by the
svn:mergeinfo property (see
the section called “Mergeinfo and Previews” for
details). These limitations mean that merges into
mixed-revision working copies can result in unexpected text
and tree conflicts. We don't want any needless conflicts, so
we update the working copy and then reattempt the
$ svn up Updating '.': At revision 380. $ svn merge ^/calc/trunk --- Merging r357 through r380 into '.': U integer.c U Makefile A README --- Recording mergeinfo for merge of r357 through r380 into '.': U . $
Subversion knows which trunk changes you previously replicated to your branch, so it carefully replicates only those changes you don't yet have. And once again, you build, test, and svn commit the local modifications to your branch.
Prior to Subversion 1.7, merges unconditionally updated all of the subtree mergeinfo under the target to describe the merge. For users with a lot of subtree mergeinfo this meant that relatively “simple” merges (e.g. one which applied a diff to only a single file) resulted in changes to every subtree with mergeinfo, even those that were not parents of the effected path(s). This caused some level of confusion and frustration. Subversion 1.7 addresses this problem by only updating the mergeinfo on subtrees which are parents of the paths modified by the merge (i.e. paths changed, added, or deleted by application of the difference, see the section called “Merge Syntax: Full Disclosure”). The one exception to this behavior regards the actual merge target; the merge target's mergeinfo is always updated to describe the merge, even if the applied difference made no changes.
What happens when you finally finish your work, though? Your new feature is done, and you're ready to merge your branch changes back to the trunk (so your team can enjoy the bounty of your labor). The process is simple. First, bring your branch into sync with the trunk again, just as you've been doing all along:
$ svn merge ^/calc/trunk --- Merging r381 through r385 into '.': U button.c U README --- Recording mergeinfo for merge of r381 through r385 into '.': U . $ # build, test, ... $ svn commit -m "Final merge of trunk changes to my-calc-branch." Sending . Sending button.c Sending README Transmitting file data .. Committed revision 390.
Now, use svn merge with the
--reintegrate option to replicate your branch
changes back into the trunk. You'll need a working copy
/trunk. You can get one by doing
an svn checkout, dredging up an old trunk
working copy from somewhere on your disk, or
using svn switch (see
the section called “Traversing Branches”). Your trunk
working copy cannot have any local edits or contain a mixture
of revisions (see
the section called “Mixed-revision working copies”). While
these are typically best practices for merging anyway, they
are required when using the
Once you have a clean working copy of the trunk, you're ready to merge your branch back into it:
$ pwd /home/user/calc-trunk $ svn update # (make sure the working copy is up to date) Updating '.': At revision 390. $ svn merge --reintegrate ^/calc/branches/my-calc-branch --- Merging differences between repository URLs into '.': U button.c U integer.c U Makefile --- Recording mergeinfo for merge between repository URLs into '.': U . $ # build, test, verify, ... $ svn commit -m "Merge my-calc-branch back into trunk!" Sending . Sending button.c Sending integer.c Sending Makefile Transmitting file data .. Committed revision 391.
Congratulations, your branch-specific changes have now
been merged back into the main line of development. Notice
our use of the
--reintegrate option this time
around. The option is critical for reintegrating changes from
a branch back into its original line of
development—don't forget it! It's needed because this
sort of “merge back” is a different sort of work
than what you've done up until now. Previously, we were
asking svn merge to grab the “next
set” of changes from one line of development (the
trunk) and duplicate them to another (your branch). This is
fairly straightforward, and each time Subversion knows how to
pick up where it left off. In our prior examples, you can see
that first it merges the ranges 345:356 from trunk to branch;
later on, it continues by merging the next contiguously
available range, 356:380. When doing the final sync, it
merges the range 380:385.
When merging your branch back to the trunk, however, the
underlying mathematics are quite different. Your feature
branch is now a mishmash of both duplicated trunk changes and
private branch changes, so there's no simple contiguous range
of revisions to copy over. By specifying
--reintegrate option, you're asking
Subversion to carefully replicate only
those changes unique to your branch. (And in fact, it does
this by comparing the latest trunk tree with the latest branch
tree: the resulting difference is exactly your branch
Keep in mind that the
option is quite specialized in contrast to the more general
nature of most Subversion subcommand options. It supports the
use case described above, but has little applicability outside
of that. Because of this narrow focus, in addition to
requiring an up-to-date working copy with no mixed-revisions,
it will not function in combination with most of the other
svn merge options. You'll get an error if you
use any non-global options but these:
Now that your private branch is merged to trunk, you may wish to remove it from the repository:
$ svn delete ^/calc/branches/my-calc-branch \ -m "Remove my-calc-branch, reintegrated with trunk in r391." Committed revision 392.
But wait! Isn't the history of that branch valuable?
What if somebody wants to audit the evolution of your feature
someday and look at all of your branch changes? No need to
worry. Remember that even though your branch is no longer
visible in the
/branches directory, its
existence is still an immutable part of the repository's
history. A simple svn log command on
/branches URL will show the entire
history of your branch. Your branch can even be resurrected
at some point, should you desire (see
the section called “Resurrecting Deleted Items”).
--reintegrate merge is done from
branch to trunk, the branch is no longer usable for further
work. It's not able to correctly absorb new trunk changes,
nor can it be properly reintegrated to trunk again. For this
reason, if you want to keep working on your feature branch, we
recommend destroying it and then re-creating it from the
$ svn delete http://svn.example.com/repos/calc/branches/my-calc-branch \ -m "Remove my-calc-branch, reintegrated with trunk in r391." Committed revision 392. $ svn copy http://svn.example.com/repos/calc/trunk \ http://svn.example.com/repos/calc/branches/my-calc-branch -m "Recreate my-calc-branch from trunk@HEAD." Committed revision 393.
There is another way of making the branch usable again after reintegration, without deleting the branch. See the section called “Keeping a Reintegrated Branch Alive”.
The basic mechanism Subversion uses to track
changesets—that is, which changes have been merged to
which branches—is by recording data in versioned
properties. Specifically, merge data is tracked in
svn:mergeinfo property attached to
files and directories. (If you're not familiar with
Subversion properties, see the section called “Properties”.)
You can examine the property, just like any other:
$ cd my-calc-branch $ svn propget svn:mergeinfo . /trunk:341-390 $
While it is possible to
svn:mergeinfo just as you might
any other versioned property, we strongly discourage doing
so unless you really know what you're
The amount of
svn:mergeinfo on a single
path can get quite large, as can the output of a
svn propget --recursive or
svn proplist --recursive when dealing with
large amounts of subtree mergeinfo, see
Subtree Merges and Subtree Mergeinfo
. The formatted output produced by the
--verbose option with either of these
subcommands is often very helpful in these cases.
svn:mergeinfo property is
automatically maintained by Subversion whenever you
run svn merge. Its value indicates which
changes made to a given path have been replicated into the
directory in question. In our previous example, the path
which is the source of the merged changes is
/trunk and the directory which has
received the changes is
Earlier versions of Subversion maintained the
svn:mergeinfo property silently. You could
still detect the changes, after a merge completed, with the
svn diff or svn status
subcommands, but the merge itself gave no indication when it
svn:mergeinfo property. This is no
longer true in Subversion 1.7, which has several new notifications
to alert you when a merge updates the
svn:mergeinfo property. These notifications
all begin with “--- Recording mergeinfo for”
and appear at the end of the merge. Unlike other merge
notifications, these don't describe the application of a
difference to a working copy
(see the section called “Merge Syntax: Full Disclosure”),
but instead describe "housekeeping" changes made to keep
track of what was merged.
Subversion also provides a subcommand, svn mergeinfo, which is helpful in seeing not only which changesets a directory has absorbed, but also which changesets it's still eligible to receive. This gives a sort of preview of which changes a subsequent svn merge operation would replicate to your branch.
$ cd my-calc-branch # Which changes have already been merged from trunk to branch? $ svn mergeinfo ^/calc/trunk r341 r342 r343 … r388 r389 r390 # Which changes are still eligible to merge from trunk to branch? $ svn mergeinfo ^/calc/trunk --show-revs eligible r391 r392 r393 r394 r395 $
The svn mergeinfo command requires
a “source” URL (where the changes come
from), and takes an optional “target” URL (where
the changes merge to). If no target URL is given,
it assumes that the current working directory is the
target. In the prior example, because we're querying our
branch working copy, the command assumes we're interested in
receiving changes to
from the specified trunk URL.
With the release of Subversion 1.7, the
svn mergeinfo subcommand can also account for
subtree mergeinfo and non-inheritable mergeinfo. It accounts for
subtree mergeinfo by use of the
--depth options, while non-inheritable mergeinfo
is considered by default.
Let's say we have a branch with both subtree and non-inheritable mergeinfo:
$ svn propget svn:mergeinfo --recursive -v # Non-inheritable mergeinfo Properties on '.': svn:mergeinfo /trunk:651-652,758* # Subtree mergeinfo Properties on 'doc/INSTALL': svn:mergeinfo /trunk/doc/INSTALL:651-652,958,1060
From the above mergeinfo we see that r758 has only been
merged into the root of the branch, but not any of the root's
children. We also see that both r958 and r1060 have been
merged only to the
When we use svn mergeinfo with the
--recursive option to see what has been merged
^/trunk to this branch, we see two
revisions are flagged with the
$ svn mergeinfo --show-revs=merged ^/trunk . --recursive 651 652 758* 958* 1060
* indicates revisions that are only
partially merged to the target in question
(the meaning is the same if we are checking for eligible
revisions). What this means in this example is that if we tried
to merge r758 or r958 from
^/trunk then more
changes would result. Likewise, because r1060 is
not flagged with a
we know that it only affects
and that trying to merge it would have no result.
Another way to get a more precise preview of a merge
operation is to use the
$ svn merge ^/calc/trunk --dry-run --- Merging r391 through r395 into 'branch': U integer.c $ svn status # nothing printed, working copy is still unchanged.
--dry-run option doesn't actually
apply any local changes to the working copy. It shows only
status codes that would be printed in a
real merge. It's useful for getting a “high-level”
preview of the potential merge, for those times
when running svn diff gives too much
After performing a merge operation, but before
committing the results of the merge, you can
to see only the changes to the immediate target of your
merge. If your merge target was a directory, only property
differences are displayed. This is a handy way to see
the changes to the
svn:mergeinfo property recorded by the
merge operation, which will remind you about what you've
Of course, the best way to preview a merge operation is to
just do it! Remember, running svn merge
isn't an inherently risky thing (unless you've made local
modifications to your working copy—but we already
stressed that you shouldn't merge into such an
environment). If you don't like the results of the merge,
svn revert . -R to revert
the changes from your working copy and retry the command with
different options. The merge isn't final until you
actually svn commit the results.
An extremely common use for svn merge
is to roll back a change that has already been committed.
Suppose you're working away happily on a working copy of
/calc/trunk, and you discover that the
change made way back in revision 303, which changed
integer.c, is completely wrong. It never
should have been committed. You can use svn
merge to “undo” the change in your
working copy, and then commit the local modification to the
repository. All you need to do is to specify a
reverse difference. (You can do this by
--revision 303:302, or by an
$ svn merge -c -303 ^/calc/trunk --- Reverse-merging r303 into 'integer.c': U integer.c --- Recording mergeinfo for reverse merge of r303 into 'integer.c': U A-branch $ svn status M . M integer.c $ svn diff … # verify that the change is removed … $ svn commit -m "Undoing change committed in r303." Sending integer.c Transmitting file data . Committed revision 350.
As we mentioned earlier, one way to think about a
repository revision is as a specific changeset. By using the
-r option, you can ask svn
merge to apply a changeset, or a whole range of
changesets, to your working copy. In our case of undoing a
change, we're asking svn merge to apply
changeset r303 to our working copy
Keep in mind that rolling back a change like this is just
like any other svn merge operation, so you
should use svn status and svn
diff to confirm that your work is in the state you
want it to be in, and then use svn commit
to send the final version to the repository. After
committing, this particular changeset is no longer reflected
Again, you may be thinking: well, that really didn't undo
the commit, did it? The change still exists in revision 303.
If somebody checks out a version of the
calc project between revisions 303 and
349, she'll still see the bad change, right?
Yes, that's true. When we talk about
“removing” a change, we're really talking about
removing it from the
HEAD revision. The
original change still exists in the repository's history. For
most situations, this is good enough. Most people are only
interested in tracking the
HEAD of a
project anyway. There are special cases, however, where you
really might want to destroy all evidence of the commit.
(Perhaps somebody accidentally committed a confidential
document.) This isn't so easy, it turns out, because
Subversion was deliberately designed to never lose
information. Revisions are immutable trees that build upon
one another. Removing a revision from history would cause a
domino effect, creating chaos in all subsequent revisions and
possibly invalidating all working copies.
The great thing about version control systems is that
information is never lost. Even when you delete a file or
directory, it may be gone from the
revision, but the object still exists in earlier revisions.
One of the most common questions new users ask is, “How
do I get my old file or directory back?”
The first step is to define exactly which item you're trying to resurrect. Here's a useful metaphor: you can think of every object in the repository as existing in a sort of two-dimensional coordinate system. The first coordinate is a particular revision tree, and the second coordinate is a path within that tree. So every version of your file or directory is defined by a specific coordinate pair. (Remember the “peg revision” syntax—foo.c@224—mentioned back in the section called “Peg and Operative Revisions”.)
First, you might need to use svn log to
discover the exact coordinate pair you wish to resurrect. A
good strategy is to run
svn log --verbose
in a directory that used to contain your deleted item. The
-v) option shows
a list of all changed items in each revision; all you need to
do is find the revision in which you deleted the file or
directory. You can do this visually, or by using another tool
to examine the log output (via grep, or
perhaps via an incremental search in an editor).
$ cd parent-dir $ svn log -v … ------------------------------------------------------------------------ r808 | joe | 2003-12-26 14:29:40 -0600 (Fri, 26 Dec 2003) | 3 lines Changed paths: D /calc/trunk/real.c M /calc/trunk/integer.c Added fast fourier transform functions to integer.c. Removed real.c because code now in double.c. …
In the example, we're assuming that you're looking for a
real.c. By looking through
the logs of a parent directory, you've spotted that this file
was deleted in revision 808. Therefore, the last version of
the file to exist was in the revision right before that.
Conclusion: you want to resurrect the path
/calc/trunk/real.c from revision
That was the hard part—the research. Now that you know what you want to restore, you have two different choices.
One option is to use svn merge to apply
revision 808 “in reverse.” (We already
discussed how to undo changes in
the section called “Undoing Changes”.) This
would have the effect of re-adding
as a local modification. The file would be scheduled for
addition, and after a commit, the file would again exist
In this particular example, however, this is probably not
the best strategy. Reverse-applying revision 808 would not
real.c for addition, but
the log message indicates that it would also undo certain
integer.c, which you don't
want. Certainly, you could reverse-merge revision 808 and
then svn revert the local modifications to
integer.c, but this technique doesn't
scale well. What if 90 files were changed in revision
A second, more targeted strategy is not to use svn merge at all, but rather to use the svn copy command. Simply copy the exact revision and path “coordinate pair” from the repository to your working copy:
$ svn copy ^/calc/trunk/real.c@807 ./real.c $ svn status A + real.c $ svn commit -m "Resurrected real.c from revision 807, /calc/trunk/real.c." Adding real.c Transmitting file data . Committed revision 1390.
The plus sign in the status output indicates that the item
isn't merely scheduled for addition, but scheduled for
addition “with history.” Subversion remembers
where it was copied from. In the future, running svn
log on this file will traverse back through the
file's resurrection and through all the history it had prior
to revision 807. In other words, this new
real.c isn't really new; it's a direct
descendant of the original, deleted file. This is usually
considered a good and useful thing. If, however, you wanted
to resurrect the file without
maintaining a historical link to the old file, this technique
works just as well:
$ svn cat ^/calc/trunk/real.c@807 > ./real.c $ svn add real.c A real.c $ svn commit -m "Re-created real.c from revision 807." Adding real.c Transmitting file data . Committed revision 1390.
Although our example shows us resurrecting a file, note that these same techniques work just as well for resurrecting deleted directories. Also note that a resurrection doesn't have to happen in your working copy—it can happen entirely in the repository:
$ svn copy ^/calc/trunk/real.c@807 ^/calc/trunk/ \ -m "Resurrect real.c from revision 807." Committed revision 1390. $ svn update Updating '.': A real.c Updated to revision 1390.
 This was introduced in svn 1.6.
 The svn
--allow-mixed-revisions allows you to
override this prohibition, but you should only do so if you
understand the ramifications and have a good reason for
 With Subversion 1.7 you don't absolutely have to do all your sync merges to the root of your branch as we do in this example. If your branch is effectively synced via a series of subtree merges then the reintegrate will work, but ask yourself, if the branch is effectively synced, then why are you doing subtree merges? Doing so is almost always needlessly complex.
 Reintegrate merges are allowed if the target is a shallow checkout (see the section called “Sparse Directories”) but any paths affected by the diff which are “missing” due to the sparse working copy will be skipped, probably not what you intended!
 This is often termed an “inoperative” merge. Though in this example the merge of r1060 would do something: It would update the mergeinfo on the root of the branch, but it would be inoperative in the sense that no diff would be applied.