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.
Recovering from Massive Errors

If the time comes when you would rather simply revert to an older version, choose the version to which you wish to revert (call it 1.x) and run these commands:

$ ci -l foo
$ co -r1.x foo
RCS/foo,v  -->  foo
revision 1.x
writable foo exists; remove it? [ny](n): y
$ chmod +w foo

At this point, you have reverted to version 1.x of your file. From this point, you can continue making changes as if you had simply edited the latest version until it was identical to version 1.x.

Note that you should almost never have to do this; many people never have.

Looking for Old Changes

How do you figure out which versions to look at, when you are looking for one particular change? One command provides an instant summary of the changes that have been made since revision 1.1. This is where providing descriptions for important changes comes in handy. To see the log of all the changes, use the rlog command:

$ rlog bar
RCS file: RCS/bar,v
Working file: bar
head: 1.3
locks: strict
        johnsonm: 1.3
access list:
symbolic names:
keyword substitution: kv
total revisions: 3;     selected revisions: 3
revision 1.3    locked by: johnsonm;
date: 1996/02/01 02:40:16;  author: johnsonm;
    state: Exp;  lines: +1 -0
Added different text.
revision 1.2
date: 1996/02/01 02:39:59;  author: johnsonm;
    state: Exp;  lines: +1 -0
Added some text.
revision 1.1
date: 1996/01/31 21:22:36;  author: johnsonm;
    state: Exp;
Initial revision

The most important parts for you to understand are the revision descriptions at the end. For example, under the revision 1.2 section, there is a comment Added some text. This is the comment that you are allowed to type in after every revision. If you choose not to enter a comment, it says instead:

*** empty log message ***

which isn't particularly helpful when you are looking for a change you made.

On the other hand, if you find entering a comment to be so much work that you are tempted not to use RCS, you are better off when things go wrong to be able to go looking for the version you want than to have no information whatever. Don't avoid using RCS because you feel you ought to describe every revision...

These logs become very long, very quickly. In order to look at the log one screen at a time, use a pager program:

$ rlog bar | less

Again, you can send the log to a file or print it, if you prefer.


There is a lot more to RCS than can be found in this article, since this is a tutorial intended to make it easy for you to use RCS. If you are interested, RCS comes with a full complement of manual pages, as well as papers about how to use RCS in a development environment. In addition, Linux Journal had an earlier article on RCS, aimed more at developers, in the February 1995 issue. But don't think that you have to know everything about RCS in order to use it effectively.

Michael K. Johnson (XXXXXXXXXXXXXX) is the Editor of Linux Journal, and uses RCS in the way described in this article to keep track of all the changes that he makes while editing articles for Linux Journal.


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