[PATCH] User Manual: add a chapter for submodules

[Miklos Vajna <vmiklos@frugalware.org>]

Signed-off-by: Michael Smith <msmith@cbnco.com>
Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
---

On Tue, Sep 18, 2007 at 02:12:17PM -0400, Michael Smith <msmith@cbnco.com> wrote:
> On Tue, 18 Sep 2007, Miklos Vajna wrote:
>
> > Michael, i think the wiki version is better as my example does not
> > contain any extra to the wiki version. is it ok if i would send a patch
> > to include your work in the official docs?
>
> Thanks, that would be great.

here it is. this version is a bit shorter than the wiki one, but i think it
does not contain less useful info

 Documentation/user-manual.txt |  175 +++++++++++++++++++++++++++++++++++++++++
 1 files changed, 175 insertions(+), 0 deletions(-)

diff --git a/Documentation/user-manual.txt b/Documentation/user-manual.txt
index ecb2bf9..ce0cf38 100644
--- a/Documentation/user-manual.txt
+++ b/Documentation/user-manual.txt
@@ -3155,6 +3155,181 @@ a tree which you are in the process of working on.
 If you blow the index away entirely, you generally haven't lost any
 information as long as you have the name of the tree that it described.
 
+[[submodules]]
+Submodules
+==========
+
+This tutorial explains how to create and publish a repository with submodules
+using the gitlink:git-submodule[1] command.
+
+Submodules maintain their own identity; the submodule support just stores the
+submodule repository location and commit ID, so other developers who clone the
+superproject can easily clone all the submodules at the same revision.
+
+To see how submodule support works, create (for example) four example
+repository that can be used later as a submodule:
+
+-------------------------------------------------
+$ mkdir ~/git
+$ cd ~/git
+$ for i in a b c d
+do
+	mkdir $i
+	cd $i
+	git init
+	echo "module $i" > $i.txt
+	git add $i.txt
+	git commit -m "Initial commit, submodule $mod"
+	cd ..
+done
+-------------------------------------------------
+
+Now create the superproject and add all the submodules:
+
+-------------------------------------------------
+$ mkdir super
+$ cd super
+$ git init
+$ echo hi > super.txt
+$ git add super.txt
+$ git commit -m "Initial commit of empty superproject"
+$ for i in a b c d
+do
+	git submodule add ~/git/$i
+done
+-------------------------------------------------
+
+See what files `git submodule` created:
+
+-------------------------------------------------
+$ ls -a
+.  ..  .git  .gitmodules  a  b  c  d  super.txt
+-------------------------------------------------
+
+The `git submodule add` command does a couple of things:
+
+- It clones the submodule under the current directory and by default checks out
+  the master branch.
+- It adds the submodule's clone path to the `.gitmodules` file and adds this
+  file to the index, ready to be committed.
+- It adds the submodule's current commit ID to the index, ready to be
+  committed.
+
+Commit the superproject:
+
+-------------------------------------------------
+$ git commit -m "Add submodules a, b, c, d."
+-------------------------------------------------
+
+Now clone the superproject:
+
+-------------------------------------------------
+$ cd ..
+$ git clone super cloned
+$ cd cloned
+-------------------------------------------------
+
+The submodule directories are there, but they're empty:
+
+-------------------------------------------------
+$ ls -a a
+.  ..
+$ git submodule status
+-d266b9873ad50488163457f025db7cdd9683d88b a
+-e81d457da15309b4fef4249aba9b50187999670d b
+-c1536a972b9affea0f16e0680ba87332dc059146 c
+-d96249ff5d57de5de093e6baff9e0aafa5276a74 d
+-------------------------------------------------
+
+Pulling down the submodules is a two-step process. First run `git submodule
+init` to add the submodule repository URLs to `.git/config`:
+
+-------------------------------------------------
+$ git submodule init
+-------------------------------------------------
+
+Now use `git submodule update` to clone the repositories and check out the
+commits specified in the superproject:
+
+-------------------------------------------------
+$ git submodule update
+$ cd a
+$ ls -a
+.  ..  .git  a.txt
+-------------------------------------------------
+
+One major difference between `git submodule update` and `git submodule add` is
+that `git submodule update` checks out a specific commit, rather than the tip
+of a branch. It's like checking out a tag: the head is detached, so you're not
+working on a branch.
+
+-------------------------------------------------
+$ git branch
+* (no branch)
+  master
+-------------------------------------------------
+
+If you want to make a change within a submodule, you should first check out a
+branch, make your changes, publish the change within the submodule, and then
+update the superproject to reference the new commit:
+
+-------------------------------------------------
+$ git branch
+* (no branch)
+  master
+$ git checkout master
+$ echo "adding a line again" >> a.txt
+$ git commit -a -m "Updated the submodule from within the superproject."
+$ git push
+$ cd ..
+$ git add a
+$ git commit -m "Updated submodule a."
+$ git push
+-------------------------------------------------
+
+NOTE: This means that you have to run `git submodule update` after `git pull`
+if you want to update the subprojects, too.
+
+Problems with submodules
+------------------------
+
+Always publish the submodule change before publishing the change to the
+superproject that references it. If you forget to publish the submodule change,
+others won't be able to clone the repository:
+
+-------------------------------------------------
+$ echo i added another line to this file >> a.txt
+$ git commit -a -m "doing it wrong this time"
+$ cd ..
+$ git add a
+$ git commit -m "Updated submodule a again."
+$ git push
+$ cd ~/git/cloned
+$ git pull
+$ git submodule update
+error: pathspec '261dfac35cb99d380eb966e102c1197139f7fa24' did not match any file(s) known to git.
+Did you forget to 'git add'?
+Unable to checkout '261dfac35cb99d380eb966e102c1197139f7fa24' in submodule path 'a'
+-------------------------------------------------
+
+It's not safe to run `git submodule update` if you've made changes within a
+submodule. They will be silently overwritten:
+
+-------------------------------------------------
+$ cat a.txt
+module a
+$ echo line added from private2 >> a.txt
+$ git commit -a -m "line added inside private2"
+$ cd ..
+$ git submodule update
+Submodule path 'a': checked out 'd266b9873ad50488163457f025db7cdd9683d88b'
+$ cd a
+$ cat a.txt
+module a
+-------------------------------------------------
+
+NOTE: The changes are still visible in the submodule's reflog.
+
 [[low-level-operations]]
 Low-level git operations
 ========================
-- 
1.5.3.1.1.g1e61-dirty