How to create and apply patch files, skipping over file versions, if needed?

Question

First, an example to clarify my question. I create a file called file1, where the digit indicate the file revision. The file contains only one line:

~$ cat file1
test1

I modify that file, and add a line containing test2. I save that file under new revision, calling it file2. Calling cat on that file yields:

~$ cat file2
test1
test2

I then make a third change, again saving that file under new revision file3, with contents:

~$ cat file3
test1
test2
test3

So my question is this: If I wish to create a patch that would apply changes made in file3 on file1, skipping over revision file2, how can I do that? Is it possible?

Basically I need a patch that updates contents of file1 from:

~$ cat file1
test1

To

~$ cat file1
test1
test3

Is this doable? Thank you.

Answer 1

What you are doing looks like a poor man's VCS, with each file representing a revision. The differences between two consecutive files (revisions) represent a changeset. It sounds like you want to create a patch for a particular changeset:

$ diff file2 file3 > changes.patch

and apply it to an earlier revision:

$ patch file1 < changes.patch
$ cat file1
test1
test3

Or alternatively, perhaps you want to create a patch to reverse a particular changeset:

$ diff file2 file1 > changes.patch

and apply it to your latest revision:

$ patch file3 < changes.patch
$ cat file3
test1
test3

For this particular example, the first approach fails if you use a contextual or unified diff. Neither approach is particularly robust in a more general case. For example, if you need to patch an entire source tree instead of a single file, the names of the files to be updated will be pulled out of the patch file itself.

You should use a real version control system so you don't have to track revisions manually. svn merge allows you to "patch" a working copy using a specific changeset. You can do this to "rollback" one or more changesets. Say your repo contains a single file, file, and you've committed three revisions:

# file at r1:
test1

# file at r2:
test1
test2

# file at r3:
test1
test2
test3

You want to reverse the second changeset. First check out the file at r3 (HEAD):

$ svn checkout svn://path/to/repo
A    repo/file
Checked out revision 3.

Next do a reverse merge of the changes from changeset 2 (i.e. the changes made between revision 1 and revision 2) into your working copy:

$ cd repo
$ svn merge -c -2 svn://path/to/repo

In this particular case, you will get a conflict which you will have to manually resolve. In real-world repositories, you may or may not get conflicts when you try to reverse earlier changesets.