CVS allows you to isolate changes onto a separate line of development, known as a branch. When you change files on a branch, those changes do not appear on the main trunk or other branches.
Later you can move changes from one branch to another
branch (or the main trunk) by merging. Merging
involves first running cvs update -j, to merge
the changes into the working directory.
You can then commit that revision, and thus effectively
copy the changes onto another branch.
Suppose that release 1.0 of tc has been made. You are continuing to develop tc, planning to create release 1.1 in a couple of months. After a while your customers start to complain about a fatal bug. You check out release 1.0 (see section Tags--Symbolic revisions) and find the bug (which turns out to have a trivial fix). However, the current revision of the sources are in a state of flux and are not expected to be stable for at least another month. There is no way to make a bugfix release based on the newest sources.
The thing to do in a situation like this is to create a branch on the revision trees for all the files that make up release 1.0 of tc. You can then make modifications to the branch without disturbing the main trunk. When the modifications are finished you can elect to either incorporate them on the main trunk, or leave them on the branch.
You can create a branch with tag -b; for
example, assuming you're in a working copy:
$ cvs tag -b rel-1-0-patches
This splits off a branch based on the current revisions in the working copy, assigning that branch the name `rel-1-0-patches'.
It is important to understand that branches get created in the repository, not in the working copy. Creating a branch based on current revisions, as the above example does, will not automatically switch the working copy to be on the new branch. For information on how to do that, see section Accessing branches.
You can also create a branch without reference to any
working copy, by using rtag:
$ cvs rtag -b -r rel-1-0 rel-1-0-patches tc
`-r rel-1-0' says that this branch should be rooted at the revision that corresponds to the tag `rel-1-0'. It need not be the most recent revision -- it's often useful to split a branch off an old revision (for example, when fixing a bug in a past release otherwise known to be stable).
As with `tag', the `-b' flag tells
rtag to create a branch (rather than just a
symbolic revision name). Note that the numeric
revision number that matches `rel-1-0' will
probably be different from file to file.
So, the full effect of the command is to create a new branch -- named `rel-1-0-patches' -- in module `tc', rooted in the revision tree at the point tagged by `rel-1-0'.
You can retrieve a branch in one of two ways: by checking it out fresh from the repository, or by switching an existing working copy over to the branch.
To check out a branch from the repository, invoke `checkout' with the `-r' flag, followed by the tag name of the branch (see section Creating a branch):
$ cvs checkout -r rel-1-0-patches tc
Or, if you already have a working copy, you can switch it to a given branch with `update -r':
$ cvs update -r rel-1-0-patches tc
or equivalently:
$ cd tc $ cvs update -r rel-1-0-patches
It does not matter if the working copy was originally on the main trunk or on some other branch -- the above command will switch it to the named branch. And similarly to a regular `update' command, `update -r' merges any changes you have made, notifying you of conflicts where they occur.
Once you have a working copy tied to a particular branch, it remains there until you tell it otherwise. This means that changes checked in from the working copy will add new revisions on that branch, while leaving the main trunk and other branches unaffected.
To find out what branch a working copy is on, you can use the `status' command. In its output, look for the field named `Sticky tag' (see section Sticky tags) -- that's CVS's way of telling you the branch, if any, of the current working files:
$ cvs status -v driver.c backend.c
===================================================================
File: driver.c Status: Up-to-date
Version: 1.7 Sat Dec 5 18:25:54 1992
RCS Version: 1.7 /u/cvsroot/yoyodyne/tc/driver.c,v
Sticky Tag: rel-1-0-patches (branch: 1.7.2)
Sticky Date: (none)
Sticky Options: (none)
Existing Tags:
rel-1-0-patches (branch: 1.7.2)
rel-1-0 (revision: 1.7)
===================================================================
File: backend.c Status: Up-to-date
Version: 1.4 Tue Dec 1 14:39:01 1992
RCS Version: 1.4 /u/cvsroot/yoyodyne/tc/backend.c,v
Sticky Tag: rel-1-0-patches (branch: 1.4.2)
Sticky Date: (none)
Sticky Options: (none)
Existing Tags:
rel-1-0-patches (branch: 1.4.2)
rel-1-0 (revision: 1.4)
rel-0-4 (revision: 1.4)
Don't be confused by the fact that the branch numbers for each file are different (`1.7.2' and `1.4.2' respectively). The branch tag is the same, `rel-1-0-patches', and the files are indeed on the same branch. The numbers simply reflect the point in each file's revision history at which the branch was made. In the above example, one can deduce that `driver.c' had been through more changes than `backend.c' before this branch was created.
See section Branches and revisions for details about how branch numbers are constructed.
Ordinarily, a file's revision history is a linear series of increments (see section Revision numbers):
+-----+ +-----+ +-----+ +-----+ +-----+
! 1.1 !----! 1.2 !----! 1.3 !----! 1.4 !----! 1.5 !
+-----+ +-----+ +-----+ +-----+ +-----+
However, CVS is not limited to linear development. The revision tree can be split into branches, where each branch is a self-maintained line of development. Changes made on one branch can easily be moved back to the main trunk.
Each branch has a branch number, consisting of an odd number of period-separated decimal integers. The branch number is created by appending an integer to the revision number where the corresponding branch forked off. Having branch numbers allows more than one branch to be forked off from a certain revision.
All revisions on a branch have revision numbers formed by appending an ordinal number to the branch number. The following figure illustrates branching with an example.
+-------------+
Branch 1.2.2.3.2 -> ! 1.2.2.3.2.1 !
/ +-------------+
/
/
+---------+ +---------+ +---------+
Branch 1.2.2 -> _! 1.2.2.1 !----! 1.2.2.2 !----! 1.2.2.3 !
/ +---------+ +---------+ +---------+
/
/
+-----+ +-----+ +-----+ +-----+ +-----+
! 1.1 !----! 1.2 !----! 1.3 !----! 1.4 !----! 1.5 ! <- The main trunk
+-----+ +-----+ +-----+ +-----+ +-----+
!
!
! +---------+ +---------+ +---------+
Branch 1.2.4 -> +---! 1.2.4.1 !----! 1.2.4.2 !----! 1.2.4.3 !
+---------+ +---------+ +---------+
The exact details of how the branch number is constructed is not something you normally need to be concerned about, but here is how it works: When CVS creates a branch number it picks the first unused even integer, starting with 2. So when you want to create a branch from revision 6.4 it will be numbered 6.4.2. All branch numbers ending in a zero (such as 6.4.0) are used internally by CVS (see section Magic branch numbers). The branch 1.1.1 has a special meaning. See section Tracking third-party sources.
This section describes a CVS feature called magic branches. For most purposes, you need not worry about magic branches; CVS handles them for you. However, they are visible to you in certain circumstances, so it may be useful to have some idea of how it works.
Externally, branch numbers consist of an odd number of dot-separated decimal integers. See section Revision numbers. That is not the whole truth, however. For efficiency reasons CVS sometimes inserts an extra 0 in the second rightmost position (1.2.4 becomes 1.2.0.4, 8.9.10.11.12 becomes 8.9.10.11.0.12 and so on).
CVS does a pretty good job at hiding these so called magic branches, but in a few places the hiding is incomplete:
cvs log.
cvs
admin.
You can use the admin command to reassign a
symbolic name to a branch the way RCS expects it
to be. If R4patches is assigned to the branch
1.4.2 (magic branch number 1.4.0.2) in file
`numbers.c' you can do this:
$ cvs admin -NR4patches:1.4.2 numbers.c
It only works if at least one revision is already committed on the branch. Be very careful so that you do not assign the tag to the wrong number. (There is no way to see how the tag was assigned yesterday).
You can merge changes made on a branch into your working copy by giving
the `-j branch' flag to the update command. With one
`-j branch' option it merges the changes made between the
point where the branch forked and newest revision on that branch (into
your working copy).
+-----+ +-----+ +-----+ +-----+
! 1.1 !----! 1.2 !----! 1.3 !----! 1.4 ! <- The main trunk
+-----+ +-----+ +-----+ +-----+
!
!
! +---------+ +---------+
Branch R1fix -> +---! 1.2.2.1 !----! 1.2.2.2 !
+---------+ +---------+
The branch 1.2.2 has been given the tag (symbolic name) `R1fix'. The following example assumes that the module `mod' contains only one file, `m.c'.
$ cvs checkout mod # Retrieve the latest revision, 1.4
$ cvs update -j R1fix m.c # Merge all changes made on the branch,
# i.e. the changes between revision 1.2
# and 1.2.2.2, into your working copy
# of the file.
$ cvs commit -m "Included R1fix" # Create revision 1.5.
A conflict can result from a merge operation. If that happens, you should resolve it before committing the new revision. See section Conflicts example.
If your source files contain keywords (see section Keyword substitution), you might be getting more conflicts than strictly necessary. See section Merging and keywords, for information on how to avoid this.
The checkout command also supports the `-j branch' flag. The
same effect as above could be achieved with this:
$ cvs checkout -j R1fix mod $ cvs commit -m "Included R1fix"
Continuing our example, the revision tree now looks like this:
+-----+ +-----+ +-----+ +-----+ +-----+
! 1.1 !----! 1.2 !----! 1.3 !----! 1.4 !----! 1.5 ! <- The main trunk
+-----+ +-----+ +-----+ +-----+ +-----+
! *
! *
! +---------+ +---------+
Branch R1fix -> +---! 1.2.2.1 !----! 1.2.2.2 !
+---------+ +---------+
where the starred line represents the merge from the `R1fix' branch to the main trunk, as just discussed.
Now suppose that development continues on the `R1fix' branch:
+-----+ +-----+ +-----+ +-----+ +-----+
! 1.1 !----! 1.2 !----! 1.3 !----! 1.4 !----! 1.5 ! <- The main trunk
+-----+ +-----+ +-----+ +-----+ +-----+
! *
! *
! +---------+ +---------+ +---------+
Branch R1fix -> +---! 1.2.2.1 !----! 1.2.2.2 !----! 1.2.2.3 !
+---------+ +---------+ +---------+
and then you want to merge those new changes onto the
main trunk. If you just use the cvs update -j
R1fix m.c command again, CVS will attempt to
merge again the changes which you have already merged,
which can have undesirable side effects.
So instead you need to specify that you only want to merge the changes on the branch which have not yet been merged into the trunk. To do that you specify two `-j' options, and CVS merges the changes from the first revision to the second revision. For example, in this case the simplest way would be
cvs update -j 1.2.2.2 -j R1fix m.c # Merge changes from 1.2.2.2 to the
# head of the R1fix branch
The problem with this is that you need to specify the 1.2.2.2 revision manually. A slightly better approach might be to use the date the last merge was done:
cvs update -j R1fix:yesterday -j R1fix m.c
Better yet, tag the R1fix branch after every merge into the trunk, and then use that tag for subsequent merges:
cvs update -j merged_from_R1fix_to_trunk -j R1fix m.c
With two `-j revision' flags, the update
(and checkout) command can merge the differences
between any two revisions into your working file.
$ cvs update -j 1.5 -j 1.3 backend.c
will undo all changes made between revision 1.3 and 1.5. Note the order of the revisions!
If you try to use this option when operating on multiple files, remember that the numeric revisions will probably be very different between the various files. You almost always use symbolic tags rather than revision numbers when operating on multiple files.
Specifying two `-j' options can also undo file removals or additions. For example, suppose you have a file named `file1' which existed as revision 1.1, and you then removed it (thus adding a dead revision 1.2). Now suppose you want to add it again, with the same contents it had previously. Here is how to do it:
$ cvs update -j 1.2 -j 1.1 file1 U file1 $ cvs commit -m test Checking in file1; /tmp/cvs-sanity/cvsroot/first-dir/file1,v <-- file1 new revision: 1.3; previous revision: 1.2 done $
If the changes which you are merging involve removing
or adding some files, update -j will reflect
such additions or removals.
For example:
cvs update -A touch a b c cvs add a b c ; cvs ci -m "added" a b c cvs tag -b branchtag cvs update -r branchtag touch d ; cvs add d rm a ; cvs rm a cvs ci -m "added d, removed a" cvs update -A cvs update -jbranchtag
After these commands are executed and a `cvs commit' is done, file `a' will be removed and file `d' added in the main branch.
If you merge files containing keywords (see section Keyword substitution), you will normally get numerous conflicts during the merge, because the keywords are expanded differently in the revisions which you are merging.
Therefore, you will often want to specify the `-kk' (see section Substitution modes) switch to the merge command line. By substituting just the name of the keyword, not the expanded value of that keyword, this option ensures that the revisions which you are merging will be the same as each other, and avoid spurious conflicts.
For example, suppose you have a file like this:
+---------+
_! 1.1.2.1 ! <- br1
/ +---------+
/
/
+-----+ +-----+
! 1.1 !----! 1.2 !
+-----+ +-----+
and your working directory is currently on the trunk (revision 1.2). Then you might get the following results from a merge:
$ cat file1 key $Revision: 1.1.1.1 $ . . . $ cvs update -j br1 U file1 RCS file: /cvsroot/first-dir/file1,v retrieving revision 1.1 retrieving revision 1.1.2.1 Merging differences between 1.1 and 1.1.2.1 into file1 rcsmerge: warning: conflicts during merge $ cat file1 <<<<<<< file1 key $Revision: 1.1.1.1 $ ======= key $Revision: 1.1.1.1 $ >>>>>>> 1.1.2.1 . . .
What happened was that the merge tried to merge the
differences between 1.1 and 1.1.2.1 into your working
directory. So, since the keyword changed from
Revision: 1.1 to Revision: 1.1.2.1,
CVS tried to merge that change into your working
directory, which conflicted with the fact that your
working directory had contained Revision: 1.2.
Here is what happens if you had used `-kk':
$ cat file1 key $Revision: 1.1.1.1 $ . . . $ cvs update -kk -j br1 U file1 RCS file: /cvsroot/first-dir/file1,v retrieving revision 1.1 retrieving revision 1.1.2.1 Merging differences between 1.1 and 1.1.2.1 into file1 $ cat file1 key $Revision: 1.1.1.1 $ . . .
What is going on here is that revision 1.1 and 1.1.2.1
both expand as plain Revision, and therefore
merging the changes between them into the working
directory need not change anything. Therefore, there
is no conflict.
There is, however, one major caveat with using `-kk' on merges. Namely, it overrides whatever keyword expansion mode CVS would normally have used. In particular, this is a problem if the mode had been `-kb' for a binary file. Therefore, if your repository contains binary files, you will need to deal with the conflicts rather than using `-kk'.
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