GIT\-READ\-TREESection: Git Manual (1)
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NAMEgit-read-tree - Reads tree information into the index
SYNOPSISgit-read-tree (<tree-ish> | [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>] [-u | -i]] [--exclude-per-directory=<gitignore>] [--index-output=<file>] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
DESCRIPTIONReads the tree information given by <tree-ish> into the index, but does not actually update any of the files it "caches". (see: 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.
- 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.
- Same as -m, except that unmerged entries are discarded instead of failing.
- After a successful merge, update the files in the work tree with the result of the merge.
- Usually a merge requires the index file as well as the files in the working tree are 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.
- 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.
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 to resolve a few more cases internally:
- *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 adds a path identically. The resolution is to add that path.
- Keep the current index contents, and read the contents of named tree-ish under directory at <prefix>. The original index file cannot have anything at the path <prefix> itself, and have nothing in <prefix>/ directory. Note that the <prefix>/ value must end with a slash.
- 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.
- 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.
- The id of the tree object(s) to be read/merged.
MERGINGIf -m is specified, git-read-tree can perform 3 kinds of merge, a single tree merge if only 1 tree is given, a fast-forward merge with 2 trees, or a 3-way merge if 3 trees are provided.
Single Tree MergeIf only 1 tree is specified, git-read-tree operates as if the user did not specify -m, except that if the original index has an entry for a given pathname, and the contents of the path matches with the tree being read, the stat info from the index is used. (In other words, the index's stat()s take precedence over the merged tree's).
That means that if you do a git-read-tree -m <newtree> followed by a git-checkout-index -f -u -a, the git-checkout-index only checks out the stuff that really changed.
Two Tree MergeTypically, this is invoked as git-read-tree -m $H $M, where $H is the head commit of the current repository, and $M is the head of a foreign tree, which is simply ahead of $H (i.e. we are in a fast forward situation).
When two trees are specified, the user is telling git-read-tree the following:
- 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.
I (index) 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
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
When this form of git-read-tree returns successfully, you can see what "local changes" you made are carried forward by running git-diff-index --cached $M. Note that this does not necessarily match git-diff-index --cached $H would have produced before such a two tree merge. This is because of cases 18 and 19 --- if you already had the changes in $M (e.g. maybe you picked it up via e-mail in a patch form), git-diff-index --cached $H would have told you about the change before this merge, but it would not show in git-diff-index --cached $M output after two-tree merge.
3-Way MergeEach "index" entry has two bits worth of "stage" state. stage 0 is the normal one, and is the only one you'd see in any kind of normal use.
However, when you do git-read-tree with three trees, the "stage" starts out at 1.
This means that you can do
.ft C $ git-read-tree -m <tree1> <tree2> <tree3> .ft
Furthermore, git-read-tree has special-case logic that says: if you see a file that matches in all respects in the following states, it "collapses" back to "stage0":
- *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)
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 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 ..
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:
.ft C $ JC=`git-rev-parse --verify "HEAD^0"` $ git-checkout-index -f -u -a $JC .ft
.ft C $ git-fetch git://.... linus $ LT=`cat .git/FETCH_HEAD` .ft
.ft C $ 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 .ft
However, if you have local changes in the working tree that would be overwritten by this merge,git-read-tree will refuse to run to prevent your changes from being lost.
In other words, there is no need to worry about what exists only in the working tree. When you have local changes in a part of the project that is not involved in the merge, your changes do not interfere with the merge, and are kept intact. When they do interfere, the merge does not even start (git-read-tree complains loudly and fails without modifying anything). In such a case, you can simply continue doing what you were in the middle of doing, and when your working tree is ready (i.e. you have finished your work-in-progress), attempt the merge again.
SEE ALSOgit-write-tree(1); git-ls-files(1); gitignore(5)
AUTHORWritten by Linus Torvalds <email@example.com>
DOCUMENTATIONDocumentation by David Greaves, Junio C Hamano and the git-list <firstname.lastname@example.org>.
GITPart of the git(7) suite
- SEE ALSO