Unix as IDE: Revisions

Version control is now seen as an indispensable part of professional software development, and GUI IDEs like Eclipse and Visual Studio have embraced it and included support for industry standard version control systems in their products. Modern version control systems trace their lineage back to Unix concepts from programs such as diff and patch however, and there are plenty of people who will insist that the best way to use a version control system is still at a shell prompt.

In this last article in the Unix as an IDE series, I’ll follow the evolution of common open-source version control systems from the basic concepts of diff and patch, among the very first version control tools.

diff, patch, and RCS

A central concept for version control systems has been that of the unified diff, a file expressing in human and computer readable terms a set of changes made to a file or files. The diff command was first released by Douglas McIlroy in 1974 for the 5th Edition of Unix, so it’s one of the oldest commands still in regular use on modern systems.

A unified diff, the most common and interoperable format, can be generated by comparing two versions of a file with the following syntax:

$ diff -u example.{1,2}.c
--- example.c.1    2012-02-15 20:15:37.000000000 +1300
+++ example.c.2    2012-02-15 20:15:57.000000000 +1300
@@ -1,8 +1,9 @@
 #include <stdio.h>
+#include <stdlib.h> 

 int main (int argc, char* argv[]) { printf("Hello, world!\n");
-    return 0;
+    return EXIT_SUCCESS; }

In this example, the second file has a header file added, and the call to return changed to use the standard EXIT_SUCCESS rather than a literal 0 as the return value for main(). Note that the output for diff also includes metadata such as the filename that was changed and the last modification time of each of the files.

A primitive form of version control for larger code bases was thus for developers to trade diff output, called patches in this context, so that they could be applied to one another’s code bases with the patch tool. We could save the output from diff above as a patch like so:

$ diff -u example.{1,2}.c > example.patch

We could then send this patch to a developer who still had the old version of the file, and they could automatically apply it with:

$ patch example.1.c < example.patch

A patch can include diff output from more than one file, including within subdirectories, so this provides a very workable way to apply changes to a source tree.

The operations involved in using diff output to track changes were sufficiently regular that for keeping in-place history of a file, the Source Code Control System and the Revision Control System that has pretty much replaced it were developed. RCS enabled “locking” files so that they could not be edited by anyone else while “checked out” of the system, paving the way for other concepts in more developed version control systems.

RCS retains the advantage of being very simple to use. To place an existing file under version control, one need only type ci <filename> and provide an appropriate description for the file:

$ ci example.c
example.c,v  <--  example.c
enter description, terminated with single '.' or end of file:
NOTE: This is NOT the log message!
>> example file
>> .
initial revision: 1.1
done

This creates a file in the same directory, example.c,v, that will track the changes. To make changes to the file, you check it out, make the changes, then check it back in:

$ co -l example.c
example.c,v  -->  example.c
revision 1.1 (locked)
done
$ vim example.c
$ ci -u example.c
example.c,v  <--  example.c
new revision: 1.2; previous revision: 1.1
enter log message, terminated with single '.' or end of file:
>> added a line
>> .
done

You can then view the history of a project with rlog:

$ rlog example.c

RCS file: example.c,v
Working file: example.c
head: 1.2
branch:
locks: strict
access list:
symbolic names:
keyword substitution: kv
total revisions: 2; selected revisions: 2
description:
example file
----------------------------
revision 1.2
date: 2012/02/15 07:39:16;  author: tom;  state: Exp;  lines: +1 -0
added a line
----------------------------
revision 1.1
date: 2012/02/15 07:36:23;  author: tom;  state: Exp;
Initial revision
=============================================================================

And get a patch in unified diff format between two revisions with rcsdiff -u:

$ rcsdiff -u -r1.1 -r1.2 ./example.c 
===================================================================
RCS file: ./example.c,v
retrieving revision 1.1
retrieving revision 1.2
diff -u -r1.1 -r1.2
--- ./example.c 2012/02/15 07:36:23 1.1
+++ ./example.c 2012/02/15 07:39:16 1.2
@@ -4,6 +4,7 @@
 int main (int argc, char* argv[])
 {
     printf("Hello, world!\n");
+    printf("Extra line!\n");
     return EXIT_SUCCESS;
 }

It would be misleading to imply that simple patches were now in disuse as a method of version control; they are still very commonly used in the forms above, and also figure prominently in both centralised and decentralised version control systems.

CVS and Subversion

To handle the problem of resolving changes made to a code base by multiple developers, centralized version systems were developed, with the Concurrent Versions System (CVS) developed first and the slightly more advanced Subversion later on. The central feature of these systems are using a central server that contains the repository, from which authoritative versions of the codebase at any particular time or revision can be retrieved. These are termed working copies of the code.

For these systems, the basic unit of the systems remained the changeset, and the most common way to represent these to the user was in the archetypal diff format used in earlier systems. Both systems work by keeping records of these changesets, rather than the actual files themselves from state to state.

Other concepts introduced by this generation of systems were of branching projects so that separate instances of the same project could be worked on concurrently, and then merged into the mainline, or trunk with appropriate testing and review. Similarly, the concept of tagging was introduced to flag certain revisions as representing the state of a codebase at the time of a release of the software. The concept of the merge was also introduced; reconciling conflicting changes made to a file manually.

Git and Mercurial

The next generation of version control systems are distributed or decentralized systems, in which working copies of the code themselves contain a complete history of the project, and are hence not reliant on a central server to contribute to the project. In the open source, Unix-friendly environment, the standout systems are Git and Mercurial, with their client programs git and hg.

For both of these systems, the concept of communicating changesets is done with the operations push, pull and merge; changes from one repository are accepted by another. This decentralized system allows for a very complex but tightly controlled ecosystem of development; Git was originally developed by Linus Torvalds to provide an open-source DVCS capable of managing development for the Linux kernel.

Both Git and Mercurial differ from CVS and Subversion in that the basic unit for their operations is not changesets, but complete files (blobs) saved using compression. This makes finding the log history of a single file or the differences between two revisions of a file slightly more expensive, but the output of git log --patch still retains the familiar unified diff output for each revision, some forty years after diff was first being used:

commit c1e5559ddb09f8d02b989596b0f4100ad1aab422
Author: Tom Ryder <tom@sanctum.geek.nz>
Date:   Thu Feb 2 01:14:21 2012

Changed my mind about this one.

diff --git a/vim/vimrc b/vim/vimrc index cfbe8e0..65a3143 100644
--- a/vim/vimrc
+++ b/vim/vimrc
@@ -47,10 +47,6 @@
 set shiftwidth=4
 set softtabstop=4
 set tabstop=4

-" Heresy
-inoremap <C-a> <Home>
-inoremap <C-e> <End>
-
 " History
 set history=1000

The two systems have considerable overlap in functionality and even in command set, and the question of which to use provokes considerable debate. The best introductions I’ve seen to each are Pro Git by Scott Chacon, and Hg Init by Joel Spolsky.

Conclusion

This is the last post in the Unix as IDE series; I’ve tried to offer a rapid survey of the basic tools available just within a shell on Linux for all of the basic functionality afforded by professional IDEs. At points I’ve had to be not quite as thorough as I’d like in explaining certain features, but to those unfamiliar to development on Linux machines this will all have hopefully given some idea of how comprehensive a development environment the humble shell can be, and all with free, highly mature, and standard software tools.

This entry is part 7 of 7 in the series Unix as IDE.

Committing part of a file

One of the advantages that Git has over Subversion and CVS is the use of its index as a staging area, which turns out to be a much more flexible model than Subversion. One of the things that always annoyed me about Subversion was that there seemed to be no elegant way to only commit only some of your changes to a particular tracked file. Subversion deals only in files in the working copy, and if you want to commit changes to a file, you have to commit all the changes in that file, even if they’re not related.

Where Subversion falls short

As an example, suppose you’re making changes to a working copy of a Subversion repository called myproject, and you’ve made a few changes to the main file, myproject.php; on one line, you’ve fixed a bug caused by getting the parameters for htmlentities() in the wrong order. On another, near the head of the file, you’ve changed a php.ini setting to allow the script to run for a long time. Here’s what the output of svn status and svn diff might look like in this case:

$ svn status
M myproject.php

$ svn diff
Index: myproject.php
================================================================
--- myproject.php (revision 2)
+++ myproject.php (working copy)
@@ -1,5 +1,7 @@
 <?php
+ini_set("max_execution_time", 300);
+
 /**
  * Open main class.
  */
 @@ -120,7 +122,7 @@
 public function dumpvalue($value)
 {
-    print htmlentities($value, "UTF-8", ENT_COMPAT);
+    print htmlentities($value, ENT_COMPAT, "UTF-8");
 }

Under Subversion, unless you move files around, you can’t commit only one of these changes; you need to commit both. This isn’t really the end of the world, since you could include a commit message describing both things you changed:

$ svn commit -m "Allowed longer runtime, fixed parameter order bug"
Transmitting file data .
Committed revision 3.

But if you’re finicky like me, and you’d prefer to think of commits as grouping semantically related changes as much as possible, it would be much better to be able to commit these two changes separately, and this is where Git’s use of an index shines.

Git’s method

Let’s work with the same project again, but this time as a Git repository. We’ll make the same changes again, and view the output of git status and git diff:

$ git status
# On branch master
# Changes not staged for commit:
#
# modified: myproject.php
#
no changes added to commit

$ git diff
diff --git a/myproject.php b/myproject.php
index 7c20f21..c149190 100644
--- a/myproject.php
+++ b/myproject.php
@@ -1,5 +1,7 @@
 <?php
+ini_set("max_execution_time", 300);
+
 /**
 * Open main class.
 */
 @@ -120,7 +122,7 @@ class MyProject
 public function dumpvalue($value)
 {
-    print htmlentities($value, "UTF-8", ENT_COMPAT);
+    print htmlentities($value, ENT_COMPAT, "UTF-8");
 }

So far, so good. Now when we run git add myproject.php to stage the changes in the index ready for commit, by default it does the same thing Subversion does, putting all of the changes in that file into the staging area. That’s probably fine in most cases, but today we want to commit one change, and then the other. The most basic way to do this is using Git’s --patch option.

The --patch option can be added to git add, and to some other Git commands concerned with manipulating the index as well, to explicitly prompt you about staging or not staging different sections of the file, that it terms hunks. In our case, the process of including only the first change would look something like this:

$ git add --patch myproject.php
diff --git a/myproject.php b/myproject.php
index 7c20f21..c149190 100644
--- a/myproject.php
+++ b/myproject.php
@@ -1,5 +1,7 @@
 <?php
+ini_set("max_execution_time", 300);
+
 /**
 * Open main class.
 */
 Stage this hunk [y,n,q,a,d,/,j,J,g,e,?]? y
 @@ -120,7 +122,7 @@ class MyProject
 public function dumpvalue($value)
 {
-    print htmlentities($value, "UTF-8", ENT_COMPAT);
+    print htmlentities($value, ENT_COMPAT, "UTF-8");
 }
Stage this hunk [y,n,q,a,d,/,K,g,e,?]? n

This done, if you compare the output of git diff --staged and git diff, you’ll notice that there are changes staged ready for commit in the file, and also changes that are not staged that we can commit separately later:

$ git diff --staged
diff --git a/myproject.php b/myproject.php
index 7c20f21..4bb2362 100644
--- a/myproject.php
+++ b/myproject.php
@@ -1,5 +1,7 @@
 <?php
+ini_set("max_execution_time", 300);
+
 /**
 * Open main class.
 */

$ git diff
diff --git a/myproject.php b/myproject.php
index 4bb2362..c149190 100644
--- a/myproject.php
+++ b/myproject.php
@@ -122,7 +122,7 @@ class MyProject
 public function dumpvalue($value)
 {
-    print htmlentities($value, "UTF-8", ENT_COMPAT);
+    print htmlentities($value, ENT_COMPAT, "UTF-8");
 }

So your staging area is all ready with just that one change in it, and all you need to do is type git commit with an appropriate message:

$ git commit -m "Allowed longer runtime"
[master 19d9068] Allowed longer runtime
1 files changed, 2 insertions(+), 0 deletions(-)

And the other change you made is still there, waiting to be staged and committed whenever you see fit:

$ git diff
diff --git a/myproject.php b/myproject.php
index 4bb2362..c149190 100644
--- a/myproject.php
+++ b/myproject.php
@@ -122,7 +122,7 @@ class MyProject
 public function dumpvalue($value)
 {
-    print htmlentities($value, "UTF-8", ENT_COMPAT);
+    print htmlentities($value, ENT_COMPAT, "UTF-8");
 }

Other methods

Because Git’s index can be manipulated with its lower-level tools very easily, you can treat the differences between your changes and the index like any other diff task. This means more advanced tools like Fugitive for Vim can be even better for seeing changesets in individual files as you stage them for commit. Check out Drew Neil’s Vimcast series on Fugitive if you’re interested in doing this; it’s quite an in-depth series of videos, but very much worth watching if you’re a Vim user who wants to understand and use Git to its fullest, and you really value precision and clarity in your commits.