git read-tree [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>] [-u [--exclude-per-directory=<gitignore>] | -i]] [--index-output=<file>] [--no-sparse-checkout] (--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])git-checkout-index(1)) Optionally, it can merge a tree into the index, perform a fast-forward (i.e. 2-way) merge, or a 3-way merge, with the -m flag. When used with -m, the -u flag causes it to also update the files in the work tree with the result of the merge. Trivial merges are done by git read-tree itself. Only conflicting paths will be in unmerged state when git read-tree returns.
Perform a merge, not just a read. The command will refuse to run if your index file has unmerged entries, indicating that you have not finished previous merge you started.--reset
Same as -m, except that unmerged entries are discarded instead of failing.-u
After a successful merge, update the files in the work tree with the result of the merge.-i
Usually a merge requires the index file as well as the files in the working tree to be up to date with the current head commit, in order not to lose local changes. This flag disables the check with the working tree and is meant to be used when creating a merge of trees that are not directly related to the current working tree status into a temporary index file.-n, --dry-run
Check if the command would error out, without updating the index or the files in the working tree for real.-v
Show the progress of checking files out.--trivial
Restrict three-way merge by git read-tree to happen only if there is no file-level merging required, instead of resolving merge for trivial cases and leaving conflicting files unresolved in the index.--aggressive
Usually a three-way merge by git read-tree resolves the merge for really trivial cases and leaves other cases unresolved in the index, so that porcelains can implement different merge policies. This flag makes the command resolve a few more cases internally:--prefix=<prefix>
•when one side removes a path and the other side leaves the path unmodified. The resolution is to remove that path.
•when both sides remove a path. The resolution is to remove that path.
•when both sides add a path identically. The resolution is to add that path.
Keep the current index contents, and read the contents of the named tree-ish under the directory at <prefix>. The command will refuse to overwrite entries that already existed in the original index file.--exclude-per-directory=<gitignore>
When running the command with -u and -m options, the merge result may need to overwrite paths that are not tracked in the current branch. The command usually refuses to proceed with the merge to avoid losing such a path. However this safety valve sometimes gets in the way. For example, it often happens that the other branch added a file that used to be a generated file in your branch, and the safety valve triggers when you try to switch to that branch after you ran make but before running make clean to remove the generated file. This option tells the command to read per-directory exclude file (usually .gitignore) and allows such an untracked but explicitly ignored file to be overwritten.--index-output=<file>
Instead of writing the results out to $GIT_INDEX_FILE, write the resulting index in the named file. While the command is operating, the original index file is locked with the same mechanism as usual. The file must allow to be rename(2)ed into from a temporary file that is created next to the usual index file; typically this means it needs to be on the same filesystem as the index file itself, and you need write permission to the directories the index file and index output file are located in.--[no-]recurse-submodules
Using --recurse-submodules will update the content of all initialized submodules according to the commit recorded in the superproject by calling read-tree recursively, also setting the submodules HEAD to be detached at that commit.--no-sparse-checkout
Disable sparse checkout support even if core.sparseCheckout is true.--empty
Instead of reading tree object(s) into the index, just empty it.<tree-ish#>
The id of the tree object(s) to be read/merged.
1.The current index and work tree is derived from $H, but the user may have local changes in them since $H.
2.The user wants to fast-forward to $M.In this case, the git read-tree -m $H $M command makes sure that no local change is lost as the result of this "merge". Here are the "carry forward" rules, where "I" denotes the index, "clean" means that index and work tree coincide, and "exists"/"nothing" refer to the presence of a path in the specified commit:
I H M Result ------------------------------------------------------- 0 nothing nothing nothing (does not happen) 1 nothing nothing exists use M 2 nothing exists nothing remove path from index 3 nothing exists exists, use M if "initial checkout", H == M keep index otherwise exists, fail H != M clean I==H I==M ------------------ 4 yes N/A N/A nothing nothing keep index 5 no N/A N/A nothing nothing keep index 6 yes N/A yes nothing exists keep index 7 no N/A yes nothing exists keep index 8 yes N/A no nothing exists fail 9 no N/A no nothing exists fail 10 yes yes N/A exists nothing remove path from index 11 no yes N/A exists nothing fail 12 yes no N/A exists nothing fail 13 no no N/A exists nothing fail clean (H==M) ------ 14 yes exists exists keep index 15 no exists exists keep index clean I==H I==M (H!=M) ------------------ 16 yes no no exists exists fail 17 no no no exists exists fail 18 yes no yes exists exists keep index 19 no no yes exists exists keep index 20 yes yes no exists exists use M 21 no yes no exists exists fail
$ git read-tree -m <tree1> <tree2> <tree3>
•stage 2 and 3 are the same; take one or the other (it makes no difference - the same work has been done on our branch in stage 2 and their branch in stage 3)
•stage 1 and stage 2 are the same and stage 3 is different; take stage 3 (our branch in stage 2 did not do anything since the ancestor in stage 1 while their branch in stage 3 worked on it)
•stage 1 and stage 3 are the same and stage 2 is different take stage 2 (we did something while they did nothing)The git write-tree command refuses to write a nonsensical tree, and it will complain about unmerged entries if it sees a single entry that is not stage 0. OK, this all sounds like a collection of totally nonsensical rules, but it’s actually exactly what you want in order to do a fast merge. The different stages represent the "result tree" (stage 0, aka "merged"), the original tree (stage 1, aka "orig"), and the two trees you are trying to merge (stage 2 and 3 respectively). The order of stages 1, 2 and 3 (hence the order of three <tree-ish> command-line arguments) are significant when you start a 3-way merge with an index file that is already populated. Here is an outline of how the algorithm works:
•if a file exists in identical format in all three trees, it will automatically collapse to "merged" state by git read-tree.
•a file that has any difference what-so-ever in the three trees will stay as separate entries in the index. It’s up to "porcelain policy" to determine how to remove the non-0 stages, and insert a merged version.
•the index file saves and restores with all this information, so you can merge things incrementally, but as long as it has entries in stages 1/2/3 (i.e., "unmerged entries") you can’t write the result. So now the merge algorithm ends up being really simple:You would normally use git merge-index with supplied git merge-one-file to do this last step. The script updates the files in the working tree as it merges each path and at the end of a successful merge. When you start a 3-way merge with an index file that is already populated, it is assumed that it represents the state of the files in your work tree, and you can even have files with changes unrecorded in the index file. It is further assumed that this state is "derived" from the stage 2 tree. The 3-way merge refuses to run if it finds an entry in the original index file that does not match stage 2. This is done to prevent you from losing your work-in-progress changes, and mixing your random changes in an unrelated merge commit. To illustrate, suppose you start from what has been committed last to your repository:
•you walk the index in order, and ignore all entries of stage 0, since they’ve already been done.
•if you find a "stage1", but no matching "stage2" or "stage3", you know it’s been removed from both trees (it only existed in the original tree), and you remove that entry.
•if you find a matching "stage2" and "stage3" tree, you remove one of them, and turn the other into a "stage0" entry. Remove any matching "stage1" entry if it exists too. .. all the normal trivial rules ..
$ JC=`git rev-parse --verify "HEAD^0"` $ git checkout-index -f -u -a $JC
$ git fetch git://.... linus $ LT=`git rev-parse FETCH_HEAD`
$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT $ git merge-index git-merge-one-file -a $ echo "Merge with Linus" | \ git commit-tree `git write-tree` -p $JC -p $LT