This is intro.info, produced by makeinfo version 4.3 from
../book/intro.texi.

   These are the notes for Cyclic Software's two-day CVS class.  They
were written by Jim Blandy.

   Copyright (C) 1996 Jim Blandy.  All rights reserved.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the entire resulting derived work is distributed under the terms
of a permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Free Software Foundation.


File: intro.info,  Node: Top,  Next: What is CVS for?,  Prev: (dir),  Up: (dir)

Introduction to CVS
*******************

   These are the notes for Cyclic Software's two-day CVS class.  The CVS
Versioning System is a program for maintaining and consulting the
history of a software project, or any other directory tree of text
files.

* Menu:

* What is CVS for?::
* How to use CVS --- First Sketch::


File: intro.info,  Node: What is CVS for?,  Next: How to use CVS --- First Sketch,  Prev: Top,  Up: Top

What is CVS for?
****************

   CVS maintains a history of a source tree, in terms of a series of
changes.  It stamps each change with the time it was made and the user
name of the person who made it.  Usually, the person provides a bit of
text describing why they made the change as well.  Given that
information, CVS can help developers answer questions like:
   * Who made a given change?

   * When did they make it?

   * Why did they make it?

   * What other changes did they make at the same time?


File: intro.info,  Node: How to use CVS --- First Sketch,  Prev: What is CVS for?,  Up: Top

How to use CVS -- First Sketch
******************************

   Before discussing too many vague terms and concepts, let's look over
the essential CVS commands.

* Menu:

* Setting your repository::
* Checking out a working directory::
* Making changes to files::
* Merging your changes::
* Committing your changes::
* Adding and deleting files::
* Examining changes::
* Writing good log entries::
* Handling conflicts::


File: intro.info,  Node: Setting your repository,  Next: Checking out a working directory,  Prev: How to use CVS --- First Sketch,  Up: How to use CVS --- First Sketch

Setting your repository
=======================

   CVS records everyone's changes to a given project in a directory tree
called a "repository".  Before you can use CVS, you need to set the
`CVSROOT' environment variable to the repository's path.  Whoever is in
charge of your project's configuration management will know what this
is; perhaps they've made a global definition for `CVSROOT' somewhere.

   In any case, on our system, the CVS repository is `/u/src/master'.
In that case, you would need to enter the commands
     setenv CVSROOT /u/src/master

if your shell is `csh' or one of its descendents, or
     CVSROOT=/u/src/master
     export CVSROOT

if your shell is Bash or some other Bourne shell variant.

   If you forget to do this, CVS will complain when you try to use it:
     $ cvs checkout httpc
     cvs checkout: No CVSROOT specified!  Please use the `-d' option
     cvs [checkout aborted]: or set the CVSROOT environment variable.
     $


File: intro.info,  Node: Checking out a working directory,  Next: Making changes to files,  Prev: Setting your repository,  Up: How to use CVS --- First Sketch

Checking out a working directory
================================

   CVS doesn't work on ordinary directory trees; you need to work
within a directory that CVS created for you.  Just as you check out a
book from a library before taking it home to read it, you use the `cvs
checkout' command to get a directory tree from CVS before working on it.
For example, suppose you are working on a project named `httpc', a
trivial HTTP client:
     $ cd
     $ cvs checkout httpc
     cvs checkout: Updating httpc
     U httpc/.cvsignore
     U httpc/Makefile
     U httpc/httpc.c
     U httpc/poll-server
     $

   The command `cvs checkout httpc' means, "Check out the source tree
called `httpc' from the repository specified by the `CVSROOT'
environment variable."

   CVS puts the tree in a subdirectory named `httpc':
     $ cd httpc
     $ ls -l
     total 8
     drwxr-xr-x  2 jimb          512 Oct 31 11:04 CVS
     -rw-r--r--  1 jimb           89 Oct 31 10:42 Makefile
     -rw-r--r--  1 jimb         4432 Oct 31 10:45 httpc.c
     -rwxr-xr-x  1 jimb          460 Oct 30 10:21 poll-server

   Most of these files are your working copies of the `httpc' sources.
However, the subdirectory called `CVS' (at the top) is different.  CVS
uses it to record extra information about each of the files in that
directory, to help it determine what changes you've made since you
checked it out.


File: intro.info,  Node: Making changes to files,  Next: Merging your changes,  Prev: Checking out a working directory,  Up: How to use CVS --- First Sketch

Making changes to files
=======================

   Once CVS has created a working directory tree, you can edit, compile
and test the files it contains in the usual way -- they're just files.

   For example, suppose we try compiling the package we just checked
out:

     $ make
     gcc -g -Wall  -lnsl -lsocket  httpc.c   -o httpc
     httpc.c: In function `tcp_connection':
     httpc.c:48: warning: passing arg 2 of `connect' from incompatible pointer type
     $

   It seems that `httpc.c' hasn't been ported to this operating system
yet.  We need to cast one of the arguments to `connect'.  To fix that,
line 48 must change from this:
         if (connect (sock, &name, sizeof (name)) >= 0)
    to this:
         if (connect (sock, (struct sockaddr *) &name, sizeof (name)) >= 0)

   Now it should compile:
     $ make
     gcc -g -Wall  -lnsl -lsocket  httpc.c   -o httpc
     $ httpc GET http://www.cyclic.com
     ... HTML TEXT FOR CYCLIC SOFTWARE'S HOME PAGE FOLLOWS ...
     $


File: intro.info,  Node: Merging your changes,  Next: Committing your changes,  Prev: Making changes to files,  Up: How to use CVS --- First Sketch

Merging your changes
====================

   Since each developer uses their own working directory, the changes
you make to your working directory aren't automatically visible to the
other developers on your team.  CVS doesn't publish your changes until
you're ready.  When you're done testing your changes, you must "commit"
them to the repository to make them available to the rest of the group.
We'll describe the `cvs commit' command below.

   However, what if another developer has changed the same files you
have, or the same lines?  Whose changes should prevail?  It's generally
impossible to answer this question automatically; CVS certainly isn't
competent to make that judgment.

   Thus, before you can commit your changes, CVS requires your sources
to be in sync with any changes committed by the other team members.  The
`cvs update' command takes care of this:
     $ cvs update
     cvs update: Updating .
     U Makefile
     RCS file: /u/src/master/httpc/httpc.c,v
     retrieving revision 1.6
     retrieving revision 1.7
     Merging differences between 1.6 and 1.7 into httpc.c
     M httpc.c
     $

   Let's look at this line-by-line:
`U Makefile'
     A line of the form `U FILE' means that FILE was simply "Updated";
     someone else had made a change to the file, and CVS copied the
     modified file into your home directory.

`RCS file: ...'
`retrieving revision 1.6'
`retrieving revision 1.7'
`Merging differences between 1.6 and 1.7 into httpc.c'
     These messages indicate that someone else has changed `httpc.c';
     CVS merged their changes with yours, and did not find any textual
     conflicts.  The numbers `1.6' and `1.7' are "revision numbers",
     used to identify a specific point in a file's history.

     Note that CVS merges changes into your working copy only; the
     repository and the other developers' working directories are left
     undisturbed.  It is up to you to test the merged text, and make
     sure it's valid.

`M httpc.c'
     A line of the form `M FILE' means that FILE has been "Modified" by
     you, and contains changes that are not yet visible to the other
     developers.  These are changes you need to commit.

     In this case, `httpc.c' now contains both your modifications and
     those of the other user.

   Since CVS has merged someone else's changes into your source, it's
best to make sure things still work:
     $ make
     gcc -g -Wall -Wmissing-prototypes  -lnsl -lsocket  httpc.c   -o httpc
     $ httpc GET http://www.cyclic.com
     ... HTML TEXT FOR CYCLIC SOFTWARE'S HOME PAGE FOLLOWS ...
     $

   It seems to still work.


File: intro.info,  Node: Committing your changes,  Next: Examining changes,  Prev: Merging your changes,  Up: How to use CVS --- First Sketch

Committing your changes
=======================

   Now that you have brought your sources up to date with the rest of
the group and tested them, you are ready to commit your changes to the
repository and make them visible to the rest of the group.  The only
file you've modified is `httpc.c', but it's always safe to run `cvs
update' to get a list of the modified files from CVS:
     $ cvs update
     cvs update: Updating .
     M httpc.c
     $

As expected, the only file CVS mentions is `httpc.c'; it says it
contains changes which you have not yet committed.  You can commit them
like so:
     $ cvs commit httpc.c

At this point, CVS will start up your favorite editor and prompt you for
a log message describing the change.  When you exit the editor, CVS will
commit your change:
     Checking in httpc.c;
     /u/src/master/httpc/httpc.c,v  <--  httpc.c
     new revision: 1.8; previous revision: 1.7
     $

   Now that you have committed your changes, they are visible to the
rest of the group.  When another developer runs `cvs update', CVS will
merge your changes to `httpc.c' into their working directory.


File: intro.info,  Node: Examining changes,  Next: Adding and deleting files,  Prev: Committing your changes,  Up: How to use CVS --- First Sketch

Examining changes
=================

   At this point, you might well be curious what changes the other
developer made to `httpc.c'.  To look at the log entries for a given
file, you can use the `cvs log' command:
     $ cvs log httpc.c
     
     RCS file: /u/src/master/httpc/httpc.c,v
     Working file: httpc.c
     head: 1.8
     branch:
     locks: strict
     access list:
     symbolic names:
     keyword substitution: kv
     total revisions: 8;     selected revisions: 8
     description:
     The one and only source file for the trivial HTTP client
     ----------------------------
     revision 1.8
     date: 1996/10/31 20:11:14;  author: jimb;  state: Exp;  lines: +1 -1
     (tcp_connection): Cast address structure when calling connect.
     ----------------------------
     revision 1.7
     date: 1996/10/31 19:18:45;  author: fred;  state: Exp;  lines: +6 -2
     (match_header): Make this test case-insensitive.
     ----------------------------
     revision 1.6
     date: 1996/10/31 19:15:23;  author: jimb;  state: Exp;  lines: +2 -6
     ...
     $

   Most of the text here you can ignore; the portion to look at
carefully is the series of log entries after the first line of hyphens.
The log entries appear in reverse chronological order, under the
assumption that more recent changes are usually more interesting.  Each
entry describes one change to the file, and may be parsed as follows:
`revision 1.8'
     Each version of a file has a unique "revision number".  Revision
     numbers look like `1.1', `1.2', `1.3.2.2' or even `1.3.2.2.4.5'.
     By default revision 1.1 is the first revision of a file.  Each
     successive revision is given a new number by increasing the
     rightmost number by one.

`date: 1996/10/31 20:11:14;  author: jimb; ...'
     This line gives the date of the change, and the username of the
     person who committed it; the remainder of the line is not very
     interesting.

`(tcp_connection): Cast ...'
     This (pretty obviously) is the log entry describing that change.

   The `cvs log' command can select log entries by date range, or by
revision number; see the manual for details on this.

   If you would actually like to see the change in question, you can use
the `cvs diff' command.  For example, if you would like to see the
changes Fred committed as revision 1.7, you can use the following
command:
     $ cvs diff -c -r 1.6 -r 1.7 httpc.c

   Before we look at the output from this command, let's look at what
the various parts mean:
`-c'
     This requests that `cvs diff' use a more human-readable format for
     its output.  (I'm not sure why it isn't the default.)

`-r 1.6 -r 1.7'
     This tells CVS to display the changes needed to turn revision 1.6
     of httpc.c into revision 1.7.  You can request a wider range of
     revisions if you like; for example, `-r 1.6 -r 1.8' would display
     both fred's changes and your most recent change.  (You can even
     request that changes be displayed backwards -- as if they were
     being undone -- by specifying the revisions backwards: `-r 1.7 -r
     1.6'.  This sounds odd, but it is useful sometimes.)

`httpc.c'
     This is the name of the file to inspect.  If you don't give it
     specific files to report on, CVS will produce a report for the
     entire directory.

   Here is the output from the command:

     Index: httpc.c
     ===================================================================
     RCS file: /u/src/master/httpc/httpc.c,v
     retrieving revision 1.6
     retrieving revision 1.7
     diff -c -r1.6 -r1.7
     *** httpc.c     1996/10/31 19:15:23     1.6
     --- httpc.c     1996/10/31 19:18:45     1.7
     ***************
     *** 62,68 ****
       }
     
     
     ! /* Return non-zero iff HEADER is a prefix of TEXT.  HEADER should be
          null-terminated; LEN is the length of TEXT.  */
       static int
       match_header (char *header, char *text, size_t len)
     --- 62,69 ----
       }
     
     
     ! /* Return non-zero iff HEADER is a prefix of TEXT, ignoring
     !    differences in case.  HEADER should be lower-case, and
          null-terminated; LEN is the length of TEXT.  */
       static int
       match_header (char *header, char *text, size_t len)
     ***************
     *** 76,81 ****
     --- 77,84 ----
         for (i = 0; i < header_len; i++)
           {
             char t = text[i];
     +       if ('A' <= t && t <= 'Z')
     +         t += 'a' - 'A';
             if (header[i] != t)
               return 0;
           }
     $

   This output takes a bit of effort to get used to, but it is
definitely worth understanding.

   The interesting portion starts with the first two lines beginning
with `***' and `---'; those describe the older and newer files
compared.  The remainder consists of two "hunks", each of which starts
with a line of asterisks.  Here is the first hunk:

     ***************
     *** 62,68 ****
       }
     
     
     ! /* Return non-zero iff HEADER is a prefix of TEXT.  HEADER should be
          null-terminated; LEN is the length of TEXT.  */
       static int
       match_header (char *header, char *text, size_t len)
     --- 62,69 ----
       }
     
     
     ! /* Return non-zero iff HEADER is a prefix of TEXT, ignoring
     !    differences in case.  HEADER should be lower-case, and
          null-terminated; LEN is the length of TEXT.  */
       static int
       match_header (char *header, char *text, size_t len)

   Text from the older version appears after the `*** 62,68 ***' line;
text from the new appears after the `--- 62,69 ---' line.  The pair of
numbers in each indicates the range of lines shown.  CVS provides
context around the change, and marks the actual lines affected with `!'
characters.  Thus, one can see that the single line in the top half was
replaced with the two lines in the bottom half.

   Here is the second hunk:

     ***************
     *** 76,81 ****
     --- 77,84 ----
         for (i = 0; i < header_len; i++)
           {
             char t = text[i];
     +       if ('A' <= t && t <= 'Z')
     +         t += 'a' - 'A';
             if (header[i] != t)
             return 0;
           }

   This hunk describes the insertion of two lines, marked with `+'
characters.  CVS omits the old text in this case, because it would be
redundant.  CVS uses a similar hunk format to describe deletions.

   Like the Unix `diff' command, output from `cvs diff' is usually
called a "patch", because developers have traditionally used the format
to distribute bug fixes or small new features.  While reasonably
readable to humans, a patch contains enough information for a program
to apply the changes it describes to an unmodified text file.  In fact,
the Unix `patch' command does exactly this, given a patch as input.


File: intro.info,  Node: Adding and deleting files,  Next: Writing good log entries,  Prev: Examining changes,  Up: How to use CVS --- First Sketch

Adding and deleting files
=========================

   CVS treats file creation and deletion like other changes, recording
such events in the files' histories.  One way to look at this is to say
that CVS records the history of directories as well as the files they
contain.

   CVS doesn't assume that newly created files should be placed under
its control; this would do the wrong thing in many circumstances.  For
example, one needn't record changes to object files and executables,
since their contents can always be recreated from the source files (one
hopes).  Instead, if you create a new file, `cvs update' will flag it
with a `?' character until you tell CVS what you intend to do with it.

   To add a file to a project, you must first create the file, and then
use the `cvs add' command to mark it for addition.  Then, the next call
to `cvs commit' will add the file to the repository.  For example,
here's how you might add a README file to the httpc project:
     $ ls
     CVS          Makefile     httpc.c      poll-server
     $ vi README
     ... ENTER A DESCRIPTION OF HTTPC ...
     $ ls
     CVS          Makefile     README       httpc.c      poll-server
     $ cvs update
     cvs update: Updating .
     ? README -- CVS DOESN'T KNOW ABOUT THIS FILE YET.
     $ cvs add README
     cvs add: scheduling file `README' for addition
     cvs add: use 'cvs commit' to add this file permanently
     $ cvs update -- NOW WHAT DOES CVS THINK?
     cvs update: Updating .
     A README -- THE FILE IS MARKED FOR ADDITION.
     $ cvs commit README
     ... CVS PROMPTS YOU FOR A LOG ENTRY ...
     RCS file: /u/jimb/cvs-class/rep/httpc/README,v
     done
     Checking in README;
     /u/src/master/httpc/README,v  <--  README
     initial revision: 1.1
     done
     $

   CVS treats deleted files similarly.  If you delete a file and then
run `cvs update', CVS doesn't assume you intend the file for deletion.
Instead, it does something benign -- it recreates the file with its
last recorded contents, and flags it with a `U' character, as for any
other update.  (This means that if you want to undo the changes you've
made to a file in your working directory, you can simply delete the
files, and then let `cvs update' recreate them.)

   To remove a file from a project, you must first delete the file, and
then use the `cvs rm' command to mark it for deletion.  Then, the next
call to `cvs commit' will delete the file from the repository.

   Committing a file marked with `cvs rm' does not destroy the file's
history.  It simply adds a new revision, which is marked as
"non-existent."  The repository still has records of the file's prior
contents, and can recall them as needed -- for example, by `cvs diff'
or `cvs log'.

   There are several strategies for renaming files; the simplest is to
simply rename the file in your working directory, and run `cvs rm' on
the old name, and `cvs add' on the new name.  The disadvantage of this
approach is that the log entries for the old file's content do not
carry over to the new file.  Other strategies avoid this quirk, but
have other, stranger problems.

   You can add directories just as you would ordinary files;


File: intro.info,  Node: Writing good log entries,  Next: Handling conflicts,  Prev: Adding and deleting files,  Up: How to use CVS --- First Sketch

Writing good log entries
========================

   If one can use `cvs diff' to retrieve the actual text of a change,
why should one bother writing a log entry?  Obviously, log entries can
be shorter than a patch, and allow the reader to get a general
understanding of the change without delving into its details.

   However, a good log entry describes the _reason_ the developer made
the change.  For example, a bad log entry for revision 1.7 shown above
might say, "Convert t to lower-case."  This would be accurate, but
completely useless; `cvs diff' provides all the same information, more
clearly.  A better log entry would be, "Make this test
case-insensitive," because it makes the purpose clear to anyone with a
general understanding of the code: HTTP clients should ignore case
differences when parsing reply headers.


File: intro.info,  Node: Handling conflicts,  Prev: Writing good log entries,  Up: How to use CVS --- First Sketch

Handling conflicts
==================

   As mentioned above, the `cvs update' command incorporates changes
made by other developers into your working directory.  If both you and
another developer have modified the same file, CVS merges their changes
with yours.

   It's straightforward to imagine how this works when the changes
apply to distant regions of a file, but what happens when you and
another developer have changed the same line?  CVS calls this situation
a "conflict", and leaves it up to you to resolve it.

   For example, suppose that you have just added some error checking to
the host name lookup code.  Before you commit your change, you must run
`cvs update', to bring your sources into sync:
     $ cvs update
     cvs update: Updating .
     RCS file: /u/src/master/httpc/httpc.c,v
     retrieving revision 1.8
     retrieving revision 1.9
     Merging differences between 1.8 and 1.9 into httpc.c
     rcsmerge: warning: conflicts during merge
     cvs update: conflicts found in httpc.c
     C httpc.c
     $

In this case, another developer has changed the same region of the file
you have, so CVS complains about a conflict.  Instead of printing `M
httpc.c', as it usually does, it prints `C httpc.c', to indicate that a
conflict has occurred in that file.

   To resolve the conflict, bring up the file in your editor.  CVS marks
the conflicting text this way:

       /* Look up the IP address of the host.  */
       host_info = gethostbyname (hostname);
     <<<<<<< httpc.c
       if (! host_info)
         {
           fprintf (stderr, "%s: host not found: %s\n", progname, hostname);
           exit (1);
         }
     =======
       if (! host_info)
         {
           printf ("httpc: no host");
           exit (1);
         }
     >>>>>>> 1.9
     
       sock = socket (PF_INET, SOCK_STREAM, 0);

   The text from your working file appears at the top, after the `<<<'
characters; below it is the conflicting text from the other developer.
The revision number `1.9' indicates that the conflicting change was
introduced in version 1.9 of the file, making it easier for you to
check the logs, or examine the entire change with `cvs diff'.

   Once you've decided how the conflict should be resolved, remove the
markers from the code, and put it in its proper state.  In this case,
since your error handling code is clearly superior, you could simply
throw out the other developer's change, leaving the text like this:

       /* Look up the IP address of the host.  */
       host_info = gethostbyname (hostname);
       if (! host_info)
         {
           fprintf (stderr, "%s: host not found: %s\n", progname, hostname);
           exit (1);
         }
     
       sock = socket (PF_INET, SOCK_STREAM, 0);

   Once this is done, you can test your change and commit the file:
     $ make
     gcc -g -Wall -Wmissing-prototypes  -lnsl -lsocket  httpc.c   -o httpc
     $ httpc GET http://www.cyclic.com
     HTTP/1.0 200 Document follows
     Date: Thu, 31 Oct 1996 23:04:06 GMT
     ...
     $ httpc GET http://www.frobnitz.com
     httpc: host not found: www.frobnitz.com
     $ cvs commit httpc.c

   It's important to understand what CVS does and doesn't consider a
conflict.  CVS does not understand the semantics of your program; it
simply treats its source code as a tree of text files.  If one developer
adds a new argument to a function and fixes its callers, while another
developer simultaneously adds a new call to that function, and does not
pass the new argument, that is certainly a conflict -- the two changes
are incompatible -- but CVS will not report it.  CVS's understanding of
conflicts is strictly textual.

   In practice, fortunately, conflicts are rare.  Usually, they seem to
result from two developers attempting to address the same problem, a
lack of communication between developers, or disagreement about the
design of the program.  Allocating tasks to developers in a reasonable
way reduces the likelihood of conflicts.

   Many version control systems allow a developer to "lock" a file,
preventing others from making changes to it until she has committed her
changes.  While locking is appropriate in some situations, it is not
clearly a better solution than the approach CVS takes.  Changes can
usually be merged correctly, and developers occasionally forget to
release locks; in both cases, explicit locking causes unnecessary
delays.  Furthermore, locks prevent only textual conflicts; they do not
prevent semantic conflicts of the sort described above, if the two
developers make their changes to different files.



Tag Table:
Node: Top914
Node: What is CVS for?1319
Node: How to use CVS --- First Sketch1939
Node: Setting your repository2458
Node: Checking out a working directory3592
Node: Making changes to files5140
Node: Merging your changes6291
Node: Committing your changes9066
Node: Examining changes10333
Node: Adding and deleting files17309
Node: Writing good log entries20645
Node: Handling conflicts21629

End Tag Table
