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 number 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 tree N with tree N-1, you can derive the exact patch that was committed. For this reason, it's easy to think of revision 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 arguments: passing -c 9238 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 project's /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 an automatic 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. An “automatic” merge is simply one in which you provide the bare minimum of information required for a merge (i.e. a single merge source and a working copy target) and let Subversion determine which changes need merging—no changesets are passed to svn merge via the -r or -c options in an automatic merge.

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 r341 through r351 into '.':
U    doc/INSTALL
U    src/real.c
U    src/button.c
U    Makefile
--- Recording mergeinfo for merge of r341 through r351 into '.':
 U   .
 $

This basic syntax—svn merge URL—tells 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^[33] to avoid having to type out the entire /trunk URL. Also note the “Recording mergeinfo for merge…” notification. This tells you that the merge is updating the svn:mergeinfo property. We'll discuss both this property and these notifications later in this chapter, in the section called “Mergeinfo and Previews”.

$ svn log -v -r341
------------------------------------------------------------------------
r341 | user | 2013-02-15 07:41:25 -0500 (Fri, 15 Feb 2013) | 1 line
Changed paths:
   A /calc/branches/my-calc-branch (from /calc/trunk:340)

Creating a private branch of /calc/trunk.
------------------------------------------------------------------------

$ svn log -q -rHEAD http://svn.example.com/repos/calc/trunk
------------------------------------------------------------------------
r351 | sally | 2013-02-16 08:04:22 -0500 (Sat, 16 Feb 2013)
------------------------------------------------------------------------

$ svn merge http://svn.example.com/repos/calc/trunk -r340:351
U    doc/INSTALL
U    src/real.c
U    src/button.c
U    Makefile

$ svn ci -m "Sync the my-calc-branch with ^/calc/trunk through r351."
…

$ svn log -q -rHEAD http://svn.example.com/repos/calc/trunk
------------------------------------------------------------------------
r959 | sally | 2013-03-5 7:30:21 -0500 (Tue, 05 Mar 2013)
------------------------------------------------------------------------

$ svn merge http://svn.example.com/repos/calc/trunk -r351:959
…

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       Makefile
M       doc/INSTALL
M       src/button.c
M       src/real.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 shows that its svn:mergeinfo property has been created.

$ svn diff --depth empty .
Index: .
===================================================================
--- .   (revision 351)
+++ .   (working copy)

Property changes on: .
___________________________________________________________________
Added: svn:mergeinfo
   Merged /calc/trunk:r341-351

This new property 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 chapter.)

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 "Sync latest trunk changes to my-calc-branch."
Sending        .
Sending        Makefile
Sending        doc/INSTALL
Sending        src/button.c
Sending        src/real.c
Transmitting file data ....
Committed revision 352.

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.

$ cd my-calc-branch

$ svn diff -r 341:351 ^/calc/trunk > my-patch-file

$ svn patch my-patch-file
U         doc/INSTALL
U         src/real.c
U         src/button.c
U         Makefile

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 [352:357]; 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 Subversion 1.7 and later, 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.^[34] We don't want any needless conflicts, so we update the working copy and then reattempt the merge.

$ svn up
Updating '.':
At revision 361.

$ svn merge ^/calc/trunk
--- Merging r352 through r361 into '.':
U    src/real.c
U    src/main.c
--- Recording mergeinfo for merge of r352 through r361 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.

In most of the examples in this chapter the merge target is the root directory of a branch (see the section called “What's a Branch?”). While this is a best practice, you may occasionally need to merge directly to some child of the branch root. This type of merge is called a subtree merge and the mergeinfo recorded to describe it is called subtree mergeinfo. There is nothing special about subtree merges or subtree mergeinfo. In fact there is really only one important point to keep in mind about these concepts: the complete record of merges to a branch may not be contained solely in the mergeinfo on the branch root. You may have to consider subtree mergeinfo to get a full accounting. Fortunately Subversion does this for you and rarely will you need to concern yourself with it. A brief example will help explain:

# We need to merge r958 from trunk to branches/proj-X/doc/INSTALL,
# but that revision also affects main.c, which we don't want to merge:
$ svn log --verbose --quiet -r 958 ^/
------------------------------------------------------------------------
r958 | bruce | 2011-10-20 13:28:11 -0400 (Thu, 20 Oct 2011)
Changed paths:
   M /trunk/doc/INSTALL
   M /trunk/src/main.c
------------------------------------------------------------------------

# No problem, we'll do a subtree merge targeting the INSTALL file
# directly, but first take a note of what mergeinfo exists on the
# root of the branch:
$ cd branches/proj-X

$ svn propget svn:mergeinfo --recursive
Properties on '.':
  svn:mergeinfo
    /trunk:651-652

# Now we perform the subtree merge, note that merge source
# and target both point to INSTALL:
$ svn merge ^/trunk/doc/INSTALL doc/INSTALL -c 958
--- Merging r958 into 'doc/INSTALL':
U    doc/INSTALL
--- Recording mergeinfo for merge of r958 into 'doc/INSTALL':
 G   doc/INSTALL

# Once the merge is complete there is now subtree mergeinfo on INSTALL:
$ svn propget svn:mergeinfo --recursive
Properties on '.':
  svn:mergeinfo
    /trunk:651-652
Properties on 'doc/INSTALL':
  svn:mergeinfo
    /trunk/doc/INSTALL:651-652,958

# What if we then decide we do want all of r958? Easy, all we need do is
# repeat the merge of that revision, but this time to the root of the
# branch, Subversion notices the subtree mergeinfo on INSTALL and doesn't
# try to merge any changes to it, only the changes to main.c are merged:
$ svn merge ^/subversion/trunk . -c 958
--- Merging r958 into '.':
U    src/main.c
--- Recording mergeinfo for merge of r958 into '.':
 U   .
--- Eliding mergeinfo from 'doc/INSTALL':
 U   doc/INSTALL

You might be wondering why INSTALL in the above example has mergeinfo for r651-652, when we only merged r958. This is due to mergeinfo inheritance, which we'll cover in the sidebar Mergeinfo Inheritance. Also note that the subtree mergeinfo on doc/INSTALL was removed, or “elided”. This is called mergeinfo elision and it occurs whenever Subversion detects redundant subtree mergeinfo.

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^[35]:

$ svn up # (make sure the working copy is up to date)
Updating '.':
At revision 378.

$ svn merge ^/calc/trunk
--- Merging r362 through r378 into '.':
U    src/main.c
--- Recording mergeinfo for merge of r362 through r378 into '.':
 U   .

$ # build, test, ...

$ svn commit -m "Final merge of trunk changes to my-calc-branch."
Sending        .
Sending        src/main.c
Transmitting file data .
Committed revision 379.

Now, use svn merge subcommand to automatically replicate your branch changes back into the trunk. This type of merge is called an “automatic reintegrate” merge. You'll need a working copy of /calc/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, switched paths, 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 for automatic reintegrate merges.

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
Updating '.':
At revision 379.

$ svn merge ^/calc/branches/my-calc-branch
--- Merging differences between repository URLs into '.':
U    src/real.c
U    src/main.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        Makefile
Sending        src/main.c
Sending        src/real.c
Transmitting file data ...
Committed revision 380.

Congratulations, your branch-specific changes have now been merged back into the main line of development. Notice that the automatic reintegrate merge did 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 341:351 from /calc/trunk to /calc/branches/my-calc-branch; later on, it continues by merging the next contiguously available range, 351:361. When doing the final sync, it merges the range 361:378.

When merging /calc/branches/my-calc-branch back to the /calc/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 using an automatic merge, 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 changes!)

Keep in mind that the automatic reintegrate merges only support the use case described above. Because of this narrow focus, in addition to the requirements previously mentioned (up-to-date working copy ^[36] with no mixed-revisions, switched paths or local changes) 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: --accept, --dry-run, --diff3-cmd, --extensions, or --quiet.

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 r381."
…

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 /calc/branches directory, its existence is still an immutable part of the repository's history. A simple svn log command on the /calc/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”).

If you choose not to delete your branch after reintegrating it to the trunk you may continue to perform sync merges from the trunk and then reintegrate the branch again^[37]. If you do this, only the changes made on your branch after the first reintegrate are merged to the trunk.

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 the 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 mergeinfo property, just like any other versioned property:

$ cd my-calc-branch

$ svn pg svn:mergeinfo -v
Properties on '.':
  svn:mergeinfo
    /calc/trunk:341-378

The 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 /calc/trunk and the directory which has received the changes is /calc/branches/my-calc-branch. 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 changed the svn:mergeinfo property. In Subversion 1.7 and later this is no longer true as there are several 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 the merge relationships between two branches; specifically which changesets a directory has absorbed or which changesets it's still eligible to receive. The latter gives a sort of preview of which changes a subsequent svn merge operation would replicate to your branch. By default, svn mergeinfo gives an graphical overview of the relationship between to branches. Returning to our earlier example, we use the subcommand to analyze the relationship between /calc/trunk and /calc/branches/my-calc-branch:

$ cd my-calc-branch

$ svn mergeinfo ^/calc/trunk
    youngest common ancestor
    |         last full merge
    |         |        tip of branch
    |         |        |         repository path

    340                382
    |                  |
  -------| |------------         calc/trunk
     \          /
      \        /
       --| |------------         calc/branches/my-calc-branch
              |        |
              379      382

The diagram shows that /calc/branches/my-calc-branch was copied from /calc/trunk@340 and that most recent automatic merge was the reintegrate merge we made from the branch to the trunk in r380. Notice that the diagram does not show the four automatic sync merges we made in revisions 352, 362, 372, and 379. Only the most recent automatic merge, in either direction^[38], is shown. This default output is useful for obtaining an overview of the merges between two branches, but to see the specific revisions which were merged we use the --show-revs=merged option:

$ svn mergeinfo ^/calc/trunk --show-revs merged
r344
r345
r346
…
r366
r367
r368

Likewise, to see which changes are eligible to merge from the trunk to the branch we can use the --show-revs=eligible option:

$ svn mergeinfo ^/calc/trunk --show-revs eligible
r380
r381
r382

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 /calc/branches/my-calc-branch from the specified trunk URL.

Since 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 --recursive or --depth options, while non-inheritable mergeinfo is considered by default.

# What explicit mergeinfo exists on a branch?
$ svn propget svn:mergeinfo ^/branches/proj-X --recursive
/trunk:651-652

# What children does proj-X have?
$ svn list --recursive ^/branches/proj-X
doc/
doc/INSTALL
README
src/main.c

# Ask what revs were merged to a file with no explicit mergeinfo
$ svn mergeinfo ^/trunk/src/main.c ^/branches/proj-X/src/main.c \
                --show-revs merged
651
652

# The '*' decorator indicates non-inheritable mergeinfo
$ svn propget svn:mergeinfo ^/branches/proj-X
/trunk:651-652,758*

# Revision 758 is non-inheritable, but still applies to the path it is
# set on. Here the '*' decorator signals that r758 is only partially
# merged from trunk. 
$ svn mergeinfo ^/trunk ^/branches/proj-X --show-revs merged
651
652
758*

# Revision 758 is not reported as merged because it is non-inheritable
# and applies only to ^/trunk
$ svn mergeinfo ^/trunk/src/main.c ^/branches/proj-X/src/main.c \
                --show-revs merged
651
652

Let's say we have a branch with both subtree and non-inheritable mergeinfo:

$ svn pg svn:mergeinfo -vR
# Non-inheritable mergeinfo
Properties on '.':
  svn:mergeinfo
    /calc/trunk:354,385-388*
# Subtree mergeinfo
Properties on 'Makefile':
  svn:mergeinfo
    /calc/trunk/Makefile:354,380

From the above mergeinfo we see that r385-388 has only been merged into the root of the branch, but not any of the root's children. We also see that r380 has only been merged to Makefile. When we use svn mergeinfo with the --recursive option to see what has been merged from /calc/trunk to this branch, we see three revisions are flagged with the * marker:

$ svn mergeinfo -R --show-revs=merged ^/calc/trunk .
r354
r380*
r385
r386
r387*
r388*

The * 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 r380, r387, or r388 from ^/trunk then more changes would result. Likewise, because r354, r385 and r386 are not flagged with a *, we know that re-merging those revisions would have no result. ^[39]

Another way to get a more precise preview of a merge operation is to use the --dry-run option:

$ svn merge ^/paint/trunk paint-feature-branch --dry-run
--- Merging r290 through r383 into 'paint-feature-branch':
U    paint-feature-branch/src/palettes.c
U    paint-feature-branch/src/brushes.c
U    paint-feature-branch/Makefile

$ svn status
#  nothing printed, working copy is still unchanged.

The --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 detail.

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, simply run 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 back in revision 392, which changed several code files, 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 specifying --revision 392:391, or by an equivalent --change -392.)

$ svn merge ^/calc/trunk . -c-392
--- Reverse-merging r392 into '.':
U    src/real.c
U    src/main.c
U    src/button.c
U    src/integer.c
--- Recording mergeinfo for reverse merge of r392 into '.':
 U   .

$ svn st
M       src/button.c
M       src/integer.c
M       src/main.c
M       src/real.c

$ svn diff
…
# verify that the change is removed
…

$ svn commit -m "Undoing erroneous change committed in r392."
Sending        src/button.c
Sending        src/integer.c
Sending        src/main.c
Sending        src/real.c
Transmitting file data ....
Committed revision 399.

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 r392 to our working copy backward.

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 in the HEAD revision.

Again, you may be thinking: well, that really didn't undo the commit, did it? The change still exists in revision 392. If somebody checks out a version of the calc project between revisions 392 and 398, 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.^[40]

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 HEAD 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 --verbose (-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). If you know that the item in question was recently deleted you might also use the --limit option to keep the log output brief enough to examine manually.

$ cd calc/trunk

$ svn log -v --limit 3
------------------------------------------------------------------------
r401 | sally | 2013-02-19 23:15:44 -0500 (Tue, 19 Feb 2013) | 1 line
Changed paths:
   M /calc/trunk/src/main.c

Follow-up to r400: Fix typos in help text.
------------------------------------------------------------------------
r400 | bill | 2013-02-19 20:55:08 -0500 (Tue, 19 Feb 2013) | 4 lines
Changed paths:
   M /calc/trunk/src/main.c
   D /calc/trunk/src/real.c

* calc/trunk/src/main.c: Update help text.

* calc/trunk/src/real.c: Remove this file, none of the APIs
  implemented here are used anymore.
------------------------------------------------------------------------
r399 | sally | 2013-02-19 20:05:14 -0500 (Tue, 19 Feb 2013) | 1 line
Changed paths:
   M /calc/trunk/src/button.c
   M /calc/trunk/src/integer.c
   M /calc/trunk/src/main.c
   M /calc/trunk/src/real.c

Undoing erroneous change committed in r392.
------------------------------------------------------------------------

In the example, we're assuming that you're looking for a deleted file real.c. By looking through the logs of a parent directory, you've spotted that this file was deleted in revision 400. 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 399.

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 400 “in reverse.” (We already discussed how to undo changes in the section called “Undoing Changes”.) This would have the effect of re-adding real.c as a local modification. The file would be scheduled for addition, and after a commit, the file would again exist in HEAD.

In this particular example, however, this is probably not the best strategy. Reverse-applying revision 400 would not only schedule real.c for addition, but the log message indicates that it would also undo certain changes to main.c, which you don't want. Certainly, you could reverse-merge revision 400 and then svn revert the local modifications to main.c, but this technique doesn't scale well. What if 90 files were changed in revision 400?

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/src/real.c@399 ./real.c
A         real.c

$ svn st
A  +    real.c

# Commit the resurrection.
…

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 399. 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/src/real.c@399 > ./real.c

$ svn add real.c
A         real.c

# Commit the resurrection.
…

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/src/real.c@399 ^/calc/trunk/src/real.c \
           -m "Resurrect real.c from revision 399."

Committed revision 402.

$ svn up
Updating '.':
A    real.c
Updated to revision 402.