Documentation: talk about guts of merge in tutorial.
While discussing Jon's ASCII art on merge operations with him, I realized that the tutorial stops talking about the plumbing details halfway. So fill in the gory details, and update the examples to use 'git-merge', not 'git-resolve'. Signed-off-by: Junio C Hamano <junkio@cox.net>
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@ -301,7 +301,7 @@ all with a sequence of simple shell commands:
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------------------------------------------------
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tree=$(git-write-tree)
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commit=$(echo 'Initial commit' | git-commit-tree $tree)
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git-update-ref HEAD $(commit)
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git-update-ref HEAD $commit
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------------------------------------------------
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which will say:
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@ -836,14 +836,14 @@ source.
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Anyway, let's exit `gitk` (`^Q` or the File menu), and decide that we want
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to merge the work we did on the `mybranch` branch into the `master`
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branch (which is currently our `HEAD` too). To do that, there's a nice
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script called `git resolve`, which wants to know which branches you want
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script called `git merge`, which wants to know which branches you want
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to resolve and what the merge is all about:
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------------
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git resolve HEAD mybranch "Merge work in mybranch"
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git merge "Merge work in mybranch" HEAD mybranch
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------------
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where the third argument is going to be used as the commit message if
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where the first argument is going to be used as the commit message if
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the merge can be resolved automatically.
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Now, in this case we've intentionally created a situation where the
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@ -851,12 +851,14 @@ merge will need to be fixed up by hand, though, so git will do as much
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of it as it can automatically (which in this case is just merge the `example`
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file, which had no differences in the `mybranch` branch), and say:
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Simple merge failed, trying Automatic merge
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Auto-merging hello.
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Trying really trivial in-index merge...
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fatal: Merge requires file-level merging
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Nope.
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...
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merge: warning: conflicts during merge
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ERROR: Merge conflict in hello.
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fatal: merge program failed
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Automatic merge failed, fix up by hand
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Automatic merge failed/prevented; fix up by hand
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which is way too verbose, but it basically tells you that it failed the
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really trivial merge ("Simple merge") and did an "Automatic merge"
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@ -928,7 +930,7 @@ resolve to get the "upstream changes" back to your branch.
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------------
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git checkout mybranch
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git resolve HEAD master "Merge upstream changes."
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git merge "Merge upstream changes." HEAD master
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------------
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This outputs something like this (the actual commit object names
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@ -1103,6 +1105,155 @@ the above are equivalent to:
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. `git pull http://www.kernel.org/pub/.../jgarzik/netdev-2.6.git e100`
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How does the merge work?
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------------------------
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We said this tutorial shows what plumbing does to help you cope
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with the porcelain that isn't flushing, but we so far did not
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talk about how the merge really works. If you are following
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this tutorial the first time, I'd suggest to skip to "Publishing
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your work" section and come back here later.
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OK, still with me? To give us an example to look at, let's go
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back to the earlier repository with "hello" and "example" file,
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and bring ourselves back to the pre-merge state:
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------------
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$ git show-branch --more=3 master mybranch
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! [master] Merge work in mybranch
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* [mybranch] Merge work in mybranch
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--
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++ [master] Merge work in mybranch
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++ [master^2] Some work.
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++ [master^] Some fun.
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------------
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Remember, before running `git merge`, our `master` head was at
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"Some fun." commit, while our `mybranch` head was at "Some
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work." commit.
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------------
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$ git checkout mybranch
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$ git reset --hard master^2
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$ git checkout master
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$ git reset --hard master^
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------------
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After rewinding, the commit structure should look like this:
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------------
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$ git show-branch
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* [master] Some fun.
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! [mybranch] Some work.
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--
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+ [mybranch] Some work.
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+ [master] Some fun.
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++ [mybranch^] New day.
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------------
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Now we are ready to experiment with the merge by hand.
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`git merge` command, when merging two branches, uses 3-way merge
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algorithm. First, it finds the common ancestor between them.
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The command it uses is `git-merge-base`:
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------------
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$ mb=$(git-merge-base HEAD mybranch)
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------------
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The command writes the commit object name of the common ancestor
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to the standard output, so we captured its output to a variable,
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because we will be using it in the next step. BTW, the common
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ancestor commit is the "New day." commit in this case. You can
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tell it by:
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------------
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$ git-name-rev $mb
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my-first-tag
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------------
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After finding out a common ancestor commit, the second step is
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this:
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------------
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$ git-read-tree -m -u $mb HEAD mybranch
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------------
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This is the same `git-read-tree` command we have already seen,
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but it takes three trees, unlike previous examples. This reads
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the contents of each tree into different 'stage' in the index
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file (the first tree goes to stage 1, the second stage 2,
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etc.). After reading three trees into three stages, the paths
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that are the same in all three stages are 'collapsed' into stage
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0. Also paths that are the same in two of three stages are
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collapsed into stage 0, taking the SHA1 from either stage 2 or
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stage 3, whichever is different from stage 1 (i.e. only one side
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changed from the common ancestor).
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After 'collapsing' operation, paths that are different in three
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trees are left in non-zero stages. At this point, you can
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inspect the index file with this command:
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------------
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$ git-ls-files --stage
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100644 7f8b141b65fdcee47321e399a2598a235a032422 0 example
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100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello
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100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello
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100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
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------------
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In our example of only two files, we did not have unchanged
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files so only 'example' resulted in collapsing, but in real-life
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large projects, only small number of files change in one commit,
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and this 'collapsing' tends to trivially merge most of the paths
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fairly quickly, leaving only the real changes in non-zero stages.
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To look at only non-zero stages, use `\--unmerged` flag:
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------------
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$ git-ls-files --unmerged
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100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello
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100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello
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100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello
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------------
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The next step of merging is to merge these three versions of the
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file, using 3-way merge. This is done by giving
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`git-merge-one-file` command as one of the arguments to
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`git-merge-index` command:
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------------
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$ git-merge-index git-merge-one-file hello
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Auto-merging hello.
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merge: warning: conflicts during merge
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ERROR: Merge conflict in hello.
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fatal: merge program failed
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------------
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`git-merge-one-file` script is called with parameters to
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describe those three versions, and is responsible to leave the
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merge results in the working tree and register it in the index
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file. It is a fairly straightforward shell script, and
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eventually calls `merge` program from RCS suite to perform the
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file-level 3-way merge. In this case, `merge` detects
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conflicts, and the merge result with conflict marks is left in
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the working tree, while the index file is updated with the
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version from the current branch (this is to make `git diff`
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useful after this step). This can be seen if you run `ls-files
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--stage` again at this point:
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------------
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$ git-ls-files --stage
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100644 7f8b141b65fdcee47321e399a2598a235a032422 0 example
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100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 0 hello
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------------
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As you can see, there is no unmerged paths in the index file.
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This is the state of the index file and the working file after
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`git merge` returns control back to you, leaving the conflicting
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merge for you to resolve.
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Publishing your work
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--------------------
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