Keeping Track of Change

Have you ever wanted to turn back time after making a mistake and irrevocably damaging a file you were editing? You can do so with minimal effort after reading this article.
How Does It Work?

RCS doesn't keep a whole new copy of the file each time you check in changes. Instead, it records only the lines with changes in them, along with the descriptions of the changes (if you choose to provide them). It does this in a separate file. Changes to the file filename are kept in the file filename,v. If you find this to be too much clutter, you can tell RCS to stash the filename,v file out of the way, in a subdirectory called RCS. Simply create the subdirectory, and RCS will automatically use it.

Remember what it looked like when the foo file was checked in, above? Let's create a bar file whose corresponding bar,v file is kept out of sight in an RCS directory:

$ mkdir RCS
$ vi bar
$ ci -l bar
RCS/bar,v  <--bar
enter description, terminated with single '.'
  or end of file:
NOTE: This is NOT the log message!
$ vi bar
$ ci -l bar
RCS/bar,v  <--  bar
new revision: 1.2; previous revision: 1.1
enter log message, terminated with single '.'
  or end of file:

If you did not originally create an RCS directory, and you get tired of seeing the ,v files, you can use these commands to get them out of the way safely:

$ mkdir RCS
$ mv *,v RCS

RCS will know where to search for them.

Turning Back the Clock

So what do you do when you screw up? You have carefully kept copies of your changes, but how do you retrieve yesterday's version, or last year's version, or the second most recent version?

RCS keeps track of versions by version numbers. The first version you check in is assigned the number 1.1, the second is assigned the number 1.2, the third 1.3, and so on.

When you find that you have made a mistake, you can compare what you currently have with previous revisions. To compare against the previous revision, use this command:

$ rcsdiff -u filename

The -u tells rcsdiff to use the “unified diff” format to show you the changes, and it compares the current version of the file filename with the most recent version that was checked in. Here's an example. The previously checked in version of the file foo, version 1.3, consisted of:

This is a test of the emergency
RCS system.  This is only a test.

I have since edited the current version to read:

This is a test of the emergency
RCS version control system.
This is only a test.

Before checking this new version in, I can check the differences between the current contents of the file and the previous version that was checked in, giving me this:

$ rcsdiff -u foo
RCS file: foo,v
retrieving revision 1.3
diff -u -r1.3 foo
--- foo 1996/02/01 00:34:15     1.3
+++ foo 1996/02/01 00:34:31
@@ -1,2 +1,3 @@
 This is a test of the emergency
-RCS system.  This is only a test.
+RCS version control system.
+This is only a test.

After showing what versions are being compared, the differences are shown. Lines that have not been changed are printed with a single space in front of them. Lines that have been removed in the newer version have a - in front of them, and lines that have been added have a + in front of them. As you can see, lines that are changed are considered to be deleted from the older version, and changed replacements added to the new version. Around any section with changed line, up to 3 lines of unchanged “context” will be shown to help you understand where in the file the change has been made.

This mechanism can be used to compare any two versions. After making a few more changes to the file, I can compare revision 1.6 to revision 1.3:

$ rcsdiff -u -r1.3 -r1.6 foo
RCS file: foo,v
retrieving revision 1.3
retrieving revision 1.6
diff -u -r1.3 -r1.6
-- foo 1996/02/01 00:34:15     1.3
+++ foo 1996/02/01 01:05:28     1.6
@@ -1,2 +1,6 @@
 This is a test of the emergency
-RCS system.  This is only a test.
+RCS version control system.
+This is only a test.
+I'm now adding a few lines for
+the next version.

Note that it is good to list the earlier revision first. Otherwise, the sense of + and - are reversed.

Your changes are likely to be much more significant than these examples, and may take up more than a screen listing. This is not a problem; use a pager to view your output one screenful at a time:

$ rcsdiff -u -r1.3 -r1.6 foo | less

Once you see the changes you have made, you can usually figure out where you made your mistake and fix it by hand. Sometimes you might have accidentally done damage that is large and confusing, but more often you have changed a phrase or a paragraph, are dissatisfied with your change, and simply can't remember exactly what it used to look like.

If you want to, you can store a copy of the changes in a file with a command like:

$ rcsdiff -u -r1.3 -r1.6 foo > filename

or print the changes like:

$ rcsdiff -u -r1.3 -r1.6 foo | lpr

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